string_ref: a non-owning reference to a string, revision 2 (original) (raw)

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Overview

This paper updates N3334 (string_ref + array_ref) and N3442 (string_ref for a TS) with wording for the draft C++14 standard. Note that we still aren't sure whether we're aiming for a TS or C++14. The most recent version of this paper is maintained on GitHub.

References to strings are very common in C++ programs, but often the callee doesn't care about the exact type of the object that owns the data. 3 things generally happen in this case:

1. The callee takes const std::string& and insists that callers copy the data if it was originally owned by another type.
2. The callee takes two parameters—a char* and a length (or just char* and assumes 0-termination)—and reduces the readability and safety of calls and loses any helper functions the original type provided.
3. The callee is rewritten as a template and its implementation is moved to a header file. This can increase flexibility if the author takes the time to code to a weaker iterator concept, but it can also increase compile time and code size, and can even introduce bugs if the author misses an assumption that the argument's contents are contiguous.

Google, LLVM, and Bloomberg have independently implemented a string-reference type to encapsulate this kind of argument. string_ref is implicitly constructible from const char* and std::string. It provides most of the const member operations fromstd::string to ease conversion. This paper follows Chromium and Bloomberg in extending string_ref to [basic_string_ref](#basic.string.ref)<charT>, and further extends it to include a traits parameter to matchbasic_string. We provide typedefs to parallel the 4basic_string typedefs.

Both Google's and LLVM's string_ref types (but not Bloomberg's) extend the interface from std::string to provide some helpful utility functions:

• starts_with
• ends_with
• remove_prefix
• remove_suffix
• split
• trim
• consume_prefix
• count

Versions of std::string operations that takestring_ref instead also give the standard a way to provide in-place operations on non-null-terminated byte/character sequences:

• hash, as requested by c++std-lib-31935
• numeric conversions

Inventions in this paper

Google's StringPiece provides as_string andToString methods to convert to std::string. LLVM'sStringRef provides both a str() explicit conversion and an implicit operator std::string(). Since this paper builds on top of C++11, we provide an explicit conversion constructor as well as a less verbose to_string function.

Google's and LLVM's string_ref types provide a subset ofstd::string's searching operations, but they do providepos arguments to specify where to start the search. Because string_ref::substr is much cheaper than string::substr, this paper removes thepos argument entirely.

None of the existing classes have constexpr methods.

Bikeshed!

Consensus seems to be developing around renaming this class tostring_view. If that keeps up, the next revision of this paper will incorporate that change. Other options include:

• basic_string_range (meaning a templated iterator range)
• char_range
• const_string_facade
• ext_string
• external_string
• str_ref
• string_cref (and string_ref for non-const)
• string_piece
• string_range
• string_ref
• string_view
• sub_string

Modifications vs std::string

The interface of string_ref is similar to, but not exactly the same as the interface of std::string. In general, we want to minimize differences between std::string andstring_ref so that users can go back and forth between the two often. This section justifies the differences whose utility we think overcomes that general rule.

Additions

• [remove_prefix()](#string.ref.modifiers) and[remove_suffix()](#string.ref.modifiers) make it easy to parse strings using string_ref. They could both be implemented as non-member functions (e.g. str.remove_prefix(n)=== str = str.substr(n)), but it seems useful to provide the simplest mutators as member functions. Note that other traversal primitives need to be non-members so that they're extensible, which may argue for pulling these out too.
• starts_with and ends_with are common queries on strings. The non-member equivalents produce calls that are somewhat ambiguous between starts_with(haystack, needle) vsstarts_with(needle, haystack), whilehaystack.starts_with(needle) is the only English reading of the member version. These queries apply equally well tobasic_string, so I've added them there too.

Removals

• The copy method has been removed fromstring_ref. std:🧵:copy is copy_out_, not in. It's not well named. Users can easily usestd::copy instead.
• pos and n parameters to methods have been removed from string_ref. std::string needs these parameters because std:🧵:substring is an expensive (copying and sometimes allocating) operation. However, these are always integral parameters, so the compiler can't check that their order is correct, and readers often have a hard time. Because [string_ref::substr](#string.ref.ops) is cheap, we insist users call it instead of passing its arguments to other functions.

Modifications to the rest of the standard library

This paper makeschar_traits::{length,find,compare}constexpr so that basic_string_ref can be constructed from string literals at compile time and so thatbasic_string_ref::{find,compare,operator==, etc.} can be constexpr. This implements the proposed resolution for LWG Issue 2232. An alternate way to get compile-time string_refs would be to define a user-defined literal operator a'la N3468. Arguably,basic_string_ref is a better candidate for the ""s suffix than basic_string since basic_string isn't a literal type.

When deciding which functions to add basic_string_ref overloads to, I followed the following rules:

• If the function already had overloads for const char* andbasic_string, I added a basic_string_ref overload.
• If the function had a basic_string overload but not aconst char* overload, and const char* arguments could not be passed with an implicit conversion (generally because template argument deduction would fail), I added abasic_string_ref overload.
• If the function had a basic_string overload but not aconst char* overload, and const char* arguments_could_ be passed with an implicit conversion, then I did not add abasic_string_ref overload, on the theory that copies are already expected and cheap enough, and because adding abasic_string_ref overload would require also adding aconst char* overload to avoid breaking user code.

I didn't add basic_string_ref overloads to a few functions where this would be hard to implement:

• The locale _byname facets are implemented on Unix by forwarding to a C library function that takes a '\0'-terminated string, with no option to pass a length. Since accepting astring_ref would require a copy regardless, users may as well pass a string.
• Several locale facet methods take basic_string, and forward the argument on to a virtual method. Defining a version takingbasic_string_ref, even with a default implementation forwarding to the string version, would break user subclasses because of a common compiler warning that fires when a virtual method is hidden. This propagates to the put_money I/O manipulator.

I also omitted operator+(basic_string, basic_string_ref) because LLVM returns a lightweight object from this overload and only performs the concatenation lazily. If we define this overload, we'll have a hard time introducing that lightweight concatenation later.

Following the basic_string and const char* overloads of regex_replace(), thestring_ref overloads don't let the user specify an explicit allocator. This should be fixed with the resolution to issue 2216. I believe you have enough to read in this paper as it is and that it would be unwise to jam a fix into here.

Why not change ?

I haven't taken every suggestion to change string_ref. This section explains the rationales.

Remove the find*() methods

Many people have asked why we aren't removing all of thefind* methods, since they're widely considered a wart on std::string. First, we'd like to make it as easy as possible to convert code to use string_ref, so it's useful to keep the interface as similar as reasonable to std::string. Second, replacing these these methods with uses of the standard algorithms library requires switching from indices to iterators, writing somewhat-complicated conversion code, and/or passing custom lambdas tofind_if. Let's look at the replacement code for each of the remaining methods:

haystack.find(needle)

Replaced by:

auto iter = std::search(haystack.begin(), haystack.end(),
                        needle.begin(), needle.end());
return iter == haystack.end() ? std:🧵:npos : iter - haystack.begin();

haystack.rfind(needle)

Replaced by:

auto iter = std::find_end(haystack.begin(), haystack.end(),
                          needle.begin(), needle.end());
return iter == haystack.end() ? std:🧵:npos : iter - haystack.begin();

haystack.find_first_of(needles)

Replaced by:

auto iter = std::find_first_of(haystack.begin(), haystack.end(),
                               needles.begin(), needles.end());
return iter == haystack.end() ? std:🧵:npos : iter - haystack.begin();

haystack.find_last_of(needles)

Replaced by:

auto iter = std::find_first_of(haystack.rbegin(), haystack.rend(),
                               needles.begin(), needles.end());
return iter == haystack.rend() ? std:🧵:npos : iter.base() - 1 - haystack.begin();

haystack.find_first_not_of(straw)

Replaced by:

auto iter = std::find_if(haystack.begin(), haystack.end(), [&](char c) {
  return std::find(straw.begin(), straw.end(), c) == straw.end();
});
return iter == haystack.end() ? std:🧵:npos : iter - haystack.begin();

haystack.find_last_not_of(straw)

Replaced by:

auto iter = std::find_if(haystack.rbegin(), haystack.rend(), [&](char c) {
  return std::find(straw.begin(), straw.end(), c) == straw.end();
});
return iter == haystack.rend() ? std:🧵:npos : iter.base() - 1 - haystack.begin();

find, rfind, and find_first_of are straightforward, although the conversion from indices to iterators would prevent many users from switching even to them.find_last_of, find_first_not_of, andfind_last_not_of get progressively worse to handle even in an iterator-based function.

Make basic_string_ref<char> mutable

… and use basic_string_ref<const char> for the constant case. The constant case is enough more common than the mutable case that it needs to be the default. Making the mutable case the default would prevent passing string literals into string_ref parameters, which would defeat a significant use case for string_ref. In a somewhat analogous sitation, LLVM defined an ArrayRef class in Feb 2011, and didn't find a need for the matchingMutableArrayRef until Jan 2012. They still haven't needed a mutable version of StringRef. One possible reason for this is that most uses that need to modify a string also need to be able to change its length, and that's impossible through even a mutable version of string_ref.

We could use typedef basic_string_ref<const char> string_ref to make the immutable case the default while still supporting the mutable case using the same template. I haven't gone this way because it would complicate the template's definition without significantly helping users.

Add an explicit operator bool

This would be an abbreviation for !empty(), usable for initialization in if statements. I didn't add this because it would be inconsistent with the rest of the containers library, but if another proposal adds it to the rest of the containers, that proposal should also add it to string_ref.

Avoid strlen("string literal")

With a constructor of the form:

template<size_t N>
basic_string_ref(const charT (&str)[N]);

we could avoid a strlen() call when abasic_string_ref is constructed from a string literal. Unfortunately, this constructor does completely the wrong thing when called like:

char space[PATH_MAX];
snprintf(space, sizeof(space), "some string");
string_ref str(space);

I don't know any way to distinguish string literals from local arrays, so the only way to be safe is to call strlen() on array arguments. Some people have suggested astring_ref::from_literal method, but I consider that too verbose.

Even the original worry is obsolete given modern optimizers: both gcc and clang optimize strlen("Literal") into a constant, making the safe code as efficient as the template. Other implementations should provide the same optimization as a QoI issue.

Define comparison on begin/end instead of the elements

Operations on string_ref apply to the characters in the string, and not the pointers that refer to the characters. This introduces the possibility that the underlying characters might change while a string_ref referring to them is in an associative container, which would break the container, but we believe this risk is worthwhile because it matches existing practice and matches user intentions more often.

Wait for contiguous_range<charT>

contiguous_range<T> along with anis_contiguous<IteratorOrRange> trait would be useful for many purposes. However, a reference class that's specifically for strings provides a couple extra benefits:

• string_ref can have an implicit conversion fromconst char*, while it would be a surprising special case to provide that on contiguous_range<const char*>.
• We can provide a subset of basic_string's interface to ease transitions to and from ownership, while such methods would be very strange on contiguous_range.
• basic_string_ref takes a char_traits argument allowing customization of comparison.contiguous_range likely wouldn't.
• We compare and hash string_refs using the elements they refer to. There's a stronger argument to compare acontiguous_range using the pointers inside it, meaning twocontiguous_range<char>s of the same characters might compare unequal.
• The notion of a "string" is different from the notion of a range of characters, which is one reason we have std::string in addition to std::vector<char>. Users benefit from saying which they mean in interfaces.

Make string_ref null-terminated

Doing this naively makes substr impossible to implement without a copy. We could imagine inventing a more complex interface that records whether the input string was null-terminated, giving the user the option to use that string when trying to pass a string_ref to a legacy or C function expecting a null-terminated const char*. This proposal doesn't include such an interface because it would make string_ref bigger or more expensive, and because there's no existing practice to guide us toward the right interface.

Another option would be to define a separate zstring_ref class to represent null-terminated strings and let it decay tostring_ref when necessary. That's plausible but not part of this proposal.

s/remove_prefix/pop_front/, etc.

In Kona 2012, I proposed a range<> class withpop_front, etc. members that adjusted the bounds of the range. Discussion there indicated that committee members were uncomfortable using the same names for lightweight range operations as container operations. Existing practice doesn't agree on a name for this operation, so I've kept the name used by Google'sStringPiece.

Allow implicit conversion from more types.

Beman Dawes suggested defining std::string_ref_{begin,end} and allowing users to add overloads within std. Using ADL is a slight variant. We could also allow conversion from any type with.data() and .size() members returning the right types.

Ultimately, I think we want to allow this conversion based on detecting contiguous ranges. Any constructor we add to work around that is going to look like a wart in a couple years. I think we'll be better off making users explicitly convert when they can't add an appropriate conversion operator, and then we can add the optimal constructor when contiguous iterators make it into the library.

Open Questions

How does this interact with N3456? Several basic_string_ref overloads would be handled by theRange&& arguments that N3456 adds, and several more (e.g. in<regex>) would be handled by extending those template arguments to the rest of the library. In non-deducing contexts,basic_string_ref can serve as an implicit conversion target, so we want it anyway, but in deducing context, I think the Range&& argument is strictly better.

Wording

Wording changes are being maintained at https://github.com/google/cxx-std-draft/compare/string-ref and a snapshot of the changes is copied below. A very early implementation is at https://github.com/google/libcxx/compare/string-ref. Patches and pull requests are welcome against both.

Clause 17, Library introduction

Modify the note in [defns.component]

For example, the class template templatesbasic_string and basic_string_ref and the non-member function templates that operate on strings are referred to as thestring component.

Clause 20, General utilities library

Add a constructor to class bitset in [template.bitset]

template<class charT, class traits>
  explicit bitset(
    basic_string_ref<charT,traits> str,
    charT zero = charT(’0’), charT one = charT(’1’));

Modify the constructor definitions in [bitset.cons]

template <class charT, class traits, class Allocator>
explicit
bitset(const basic_string<charT, traits, Allocator>& str,
       typename basic_string<charT, traits, Allocator>::size_type pos = 0,
       typename basic_string<charT, traits, Allocator>::size_type n =
         basic_string<charT, traits, Allocator>::npos,
       charT zero = charT(’0’), charT one = charT(’1’));

Effects: Equivalent to bitset(basic_string_ref<charT, traits>(str).substr(pos, n), zero, one)

template <class charT, class traits>
explicit
bitset(basic_string_ref<charT, traits> str,
       charT zero = charT(’0’), charT one = charT(’1’));

Requires: pos <= str.size().

Throws: out_of_range if pos > str.size().

Effects: Determines the effective length rlen of the initializing string as the smaller of n and str.size() - pos.

The function then throws Throws: invalid_argument if any of the rlen characters instr beginning at position pos is other than zero or one. The function uses traits::eq() to compare the character values.

Otherwise, the Effects: function constructs an object of class bitset<N>, initializing the first M bit positions to values determined from the corresponding characters in the stringstr. M is the smaller of N and~~rlen~~str.size().

An element of the constructed string has value zero if the corresponding character in str, beginning at position pos, is 0zero. Otherwise, the element has the value 1. Character position pos ~~+ M~~ - 1 corresponds to bit position zero. Subsequent decreasing character positions correspond to increasing bit positions.

If M < N, remaining bit positions are initialized to zero.

template <class charT>
  explicit bitset(
    const charT* str,
    typename basic_string<charT>::size_type n = basic_string<charT>::npos,
    charT zero = charT(’0’), charT one = charT(’1’));

Effects: Constructs an object of class bitset as if by

bitset(
  n == basic_string<charT>::npos
    ? basic_string_ref<charT>(str)
    : basic_string_ref<charT>(str, n),
  0, n, zero, one)

Clause 21, Strings library

Modify [char.traits.specializations.char], [char.traits.specializations.char16_t], [char.traits.specializations.char32_t], and [char.traits.specializations.wchar.t]

This implements the proposed resolution of LWG issue ####. I could take it out at the cost of makingbasic_string_ref(const charT*) and all of the comparison operators and methods non-constexpr.

static constexpr int compare(const char_type* s1, const char_type* s2, size_t n);
static constexpr size_t length(const char_type* s);
static constexpr const char_type* find(const char_type* s, size_t n,
                                       const char_type& a);

The header defines the basic_string class template for manipulating varying-length sequences of char-like objects and four typedefs, string, u16string, u32string, and wstring, that name the specializations basic_string, basic_string<char16_t>, basic_string<char32_t>, and basic_string<wchar_t>, respectively.

also defines the basic_string_ref class template for referring to constant sequences of char-like objects and four typedefs, string_ref, u16string_ref,u32string_ref, and wstring_ref, that name the specializations basic_string_ref<char>,basic_string_ref<char16_t>,basic_string_ref<char32_t>, andbasic_string_ref<wchar_t>, respectively.

Add to the appropriate places within "Header synopsis"

  // [basic.string.ref], basic_string_ref:
  template<class charT, class traits = char_traits<charT>>
      class basic_string_ref;

  // [string.ref.comparison], non-member basic_string_ref comparison functions
  template<typename charT, typename traits>
  bool operator==(basic_string_ref<charT, traits> x, basic_string_ref<charT, traits> y);
  template<typename charT, typename traits>
  bool operator!=(basic_string_ref<charT, traits> x, basic_string_ref<charT, traits> y);
  template<typename charT, typename traits>
  bool operator< (basic_string_ref<charT, traits> x, basic_string_ref<charT, traits> y);
  template<typename charT, typename traits>
  bool operator> (basic_string_ref<charT, traits> x, basic_string_ref<charT, traits> y);
  template<typename charT, typename traits>
  bool operator<=(basic_string_ref<charT, traits> x, basic_string_ref<charT, traits> y);
  template<typename charT, typename traits>
  bool operator>=(basic_string_ref<charT, traits> x, basic_string_ref<charT, traits> y);
  // Sufficient additional overloads to allow mixed comparisons with any type
  // that has an implicit conversion to basic_string_ref<charT, traits>.

  // [string.ref.nonmem], other non-member basic_string_ref functions
  template<class charT, class traits = char_traits<charT>,
           class Allocator = allocator<charT> >
    basic_string<charT, traits, Allocator> to_string(
      basic_string_ref<charT, traits>,
      const Allocator& a = Allocator());

  template<class charT, class traits>
    basic_ostream<charT, traits>&
      operator<<(basic_ostream<charT, traits>& os,
                 basic_string_ref<charT,traits> str);

  // basic_string_ref typedef names
  typedef basic_string_ref<char> string_ref;
  typedef basic_string_ref<char16_t> u16string_ref;
  typedef basic_string_ref<char32_t> u32string_ref;
  typedef basic_string_ref<wchar_t> wstring_ref;

It would be nice to remove the string overloads for the numeric conversion functions, but that could break code that relies on an implicit conversion to string. Instead, we need to add const char* overloads to avoid ambiguity.

I'd also like to add optional<T> stox_consume(string_ref&, int base=10) if/whenoptional becomes available.

  int stoi(string_ref str, size_t* idx = 0, int base = 10);
  int stoi(const char* str, size_t* idx = 0, int base = 10);
  long stol(string_ref str, size_t* idx = 0, int base = 10);
  long stol(const char* str, size_t* idx = 0, int base = 10);
  unsigned long stoul(string_ref str, size_t* idx = 0, int base = 10);
  unsigned long stoul(const char* str, size_t* idx = 0, int base = 10);
  long long stoll(string_ref str, size_t* idx = 0, int base = 10);
  long long stoll(const char* str, size_t* idx = 0, int base = 10);
  unsigned long long stoull(string_ref str, size_t* idx = 0, int base = 10);
  unsigned long long stoull(const char* str, size_t* idx = 0, int base = 10);
  float stof(string_ref str, size_t* idx = 0);
  float stof(const char* str, size_t* idx = 0);
  double stod(string_ref str, size_t* idx = 0);
  double stod(const char* str, size_t* idx = 0);
  long double stold(string_ref str, size_t* idx = 0);
  long double stold(const char* str, size_t* idx = 0);

  int stoi(wstring_ref str, size_t* idx = 0, int base = 10);
  int stoi(const wchar_t* str, size_t* idx = 0, int base = 10);
  long stol(wstring_ref str, size_t* idx = 0, int base = 10);
  long stol(const wchar_t* str, size_t* idx = 0, int base = 10);
  unsigned long stoul(wstring_ref str, size_t* idx = 0, int base = 10);
  unsigned long stoul(const wchar_t* str, size_t* idx = 0, int base = 10);
  long long stoll(wstring_ref str, size_t* idx = 0, int base = 10);
  long long stoll(const wchar_t* str, size_t* idx = 0, int base = 10);
  unsigned long long stoull(wstring_ref str, size_t* idx = 0, int base = 10);
  unsigned long long stoull(const wchar_t* str, size_t* idx = 0, int base = 10);
  float stof(wstring_ref str, size_t* idx = 0);
  float stof(const wchar_t* str, size_t* idx = 0);
  double stod(wstring_ref str, size_t* idx = 0);
  double stod(const wchar_t* str, size_t* idx = 0);
  long double stold(wstring_ref str, size_t* idx = 0);
  long double stold(const wchar_t* str, size_t* idx = 0);

  template <> struct hash<string_ref>;
  template <> struct hash<u16string_ref>;
  template <> struct hash<u32string_ref>;
  template <> struct hash<wstring_ref>;

Add a subclause "x.y Class template basic_string_ref [basic.string.ref]"

The class template basic_string_ref describes objects that can refer to a constant sequence of arbitrary char-like objects with the first element of the sequence at position zero. In the rest of this Clause, the type of the char-like objects held in a basic_string_ref object is designated by charT.

[Note: The library provides implicit conversions from const charT* and std::basic_string<charT, ...> to std::basic_string_ref<charT, ...> so that user code can accept just std::basic_string_ref as a parameter wherever a sequence of characters is expected. User-defined types should define their own implicit conversions to std::basic_string_ref in order to interoperate with these functions. — end note ]

The complexity of member functions is O(1) unless otherwise specified.

namespace std {
  template<typename charT, typename traits = char_traits<charT>>
  class basic_string_ref {
    public:
    // types
    typedef charT value_type;
    typedef const charT* pointer;
    typedef const charT* const_pointer;
    typedef const charT& reference;
    typedef const charT& const_reference;
    typedef implementation-defined const_iterator; // See [string.ref.iterators]
    typedef const_iterator iterator;  // [Footnote: Because basic_string_ref refers to a constant sequence, iterator and const_iterator are the same type. --end footnote]
    typedef std::reverse_iterator<const_iterator> const_reverse_iterator; typedef const_reverse_iterator reverse_iterator;
    typedef size_t size_type;
    typedef ptrdiff_t difference_type;
    static constexpr size_type npos = size_type(-1);

    // [string.ref.cons], construct/copy
    constexpr basic_string_ref() noexcept;
    constexpr basic_string_ref(const basic_string_ref&) noexcept = default;
    basic_string_ref& operator=(const basic_string_ref&) noexcept = default;
    constexpr basic_string_ref(const charT* str);
    constexpr basic_string_ref(const charT* str, size_type len);

No initializer_list constructor because [dcl.init.list]p6 says it would likely store a dangling reference into the string_ref.

    // [string.ref.iterators], iterators
    constexpr const_iterator begin() const noexcept;
    constexpr const_iterator end() const noexcept;
    constexpr const_iterator cbegin() const noexcept;
    constexpr const_iterator cend() const noexcept;

reverse_iterator methods aren’t constexpr because reverse_iterator isn’t a literal type. See LWG Issue 2208.

    const_reverse_iterator rbegin() const noexcept;
    const_reverse_iterator rend() const noexcept;
    const_reverse_iterator crbegin() const noexcept;
    const_reverse_iterator crend() const noexcept;

    // [string.ref.capacity], capacity
    constexpr size_type size() const noexcept;
    constexpr size_type length() const noexcept;
    constexpr size_type max_size() const noexcept;
    constexpr bool empty() const noexcept;

    // [string.ref.access], element access
    constexpr const charT& operator[](size_type pos) const;
    const charT& at(size_t pos) const;
    constexpr const charT& front() const;
    constexpr const charT& back() const;
    constexpr const charT* data() const noexcept;

    // [string.ref.modifiers], modifiers:
    void clear() noexcept;
    void remove_prefix(size_type n);
    void remove_suffix(size_type n);

    // [string.ref.ops], string operations:
    constexpr basic_string_ref substr(size_type pos, size_type n=npos) const;
    constexpr int compare(basic_string_ref s) const noexcept;
    constexpr int compare(const charT* s) const noexcept;
    constexpr bool starts_with(basic_string_ref s) const noexcept;
    constexpr bool starts_with(charT c) const noexcept;
    constexpr bool starts_with(const charT* s) const noexcept;
    constexpr bool ends_with(basic_string_ref s) const noexcept;
    constexpr bool ends_with(charT c) const noexcept;
    constexpr bool ends_with(const charT* s) const noexcept;
    size_type find(basic_string_ref s) const noexcept;
    size_type find(charT c) const noexcept;
    size_type find(const charT* s) const noexcept;
    size_type rfind(basic_string_ref s) const noexcept;
    size_type rfind(charT c) const noexcept;
    size_type rfind(const charT* s) const noexcept;
    size_type find_first_of(basic_string_ref s) const noexcept;
    size_type find_first_of(charT c) const noexcept;
    size_type find_first_of(const charT* s) const noexcept;
    size_type find_last_of(basic_string_ref s) const noexcept;
    size_type find_last_of(charT c) const noexcept;
    size_type find_last_of(const charT* s) const noexcept;
    size_type find_first_not_of(basic_string_ref s) const noexcept;
    size_type find_first_not_of(charT c) const noexcept;
    size_type find_first_not_of(const charT* s) const noexcept;
    size_type find_last_not_of(basic_string_ref s) const noexcept;
    size_type find_last_not_of(charT c) const noexcept;
    size_type find_last_not_of(const charT* s) const noexcept;
};

Each member function of the form

rt fx1(const charT* s); // such as compare(), find()

is equivalent to fx1(basic_string_ref(s)).

Each member function of the form

rt fx2(charT c); // such as starts_with(), find()

is equivalent to fx2(basic_string_ref(&c, 1)).

Add a sub-subclause "x.y.1 basic_string_ref constructors and assignment operators [string.ref.cons]"

constexpr basic_string_ref();

Effects: Constructs an empty basic_string_ref.

Postcondition: empty() == true and [data(),data()) is a valid range.

basic_string_ref(const charT* str);

Requires: [str,str + traits::length(str)) is a valid range.

Effects: Constructs a basic_string_ref referring to the same string as str, with the postconditions in Table [tab:string.ref.ctr.1]

Table [tab:string.ref.ctr.1] — basic_string_ref(const charT*) effects

Complexity: O(size())

constexpr basic_string_ref(const charT* str, size_type len);

Requires: str is not a null pointer and [str,str + len) is a valid range.

Effects: Constructs a basic_string_ref, with the postconditions in Table [tab:string.ref.ctr.2]

Table [tab:string.ref.ctr.2] — basic_string_ref(const charT*, size_type) effects

Add a sub-subclause "x.y.2 basic_string_ref iterator support [string.ref.iterators]"

typedef implementation-defined const_iterator;

A random-access, contiguous iterator type.

For a basic_string_ref str, any operation that invalidates a pointer in the range [str.data(), str.data()+str.size()) invalidates pointers and iterators returned from str’s methods.

constexpr const_iterator begin() const noexcept;
constexpr const_iterator cbegin() const noexcept;

Returns: An iterator referring to the first character in the string.

constexpr const_iterator end() const noexcept;
constexpr const_iterator cend() const noexcept;

Returns: An iterator which is the past-the-end value.

const_reverse_iterator rbegin() const noexcept;
const_reverse_iterator crbegin() const noexcept;

Returns: An iterator which is semantically equivalent to reverse_iterator(end()).

const_reverse_iterator rend() const noexcept;
const_reverse_iterator crend() const noexcept;

Returns: An iterator which is semantically equivalent to reverse_iterator(begin()).

Add a sub-subclause "x.y.3 basic_string_ref capacity [string.ref.capacity]"

size_type size() const noexcept;

Returns: A count of the number of char-like objects referred to by the string_ref.

Complexity: constant time.

size_type length() const noexcept;

Returns: size().

size_type max_size() const noexcept;

Returns: The size of the largest possible string_ref.

bool empty() const noexcept;

Returns: size() == 0.

Add a sub-subclause "x.y.4 basic_string_ref element access [string.ref.access]"

constexpr const_reference operator[](size_type pos) const;

Requires: pos < size().

Returns: *(begin() + pos)

Throws: Nothing.

[ Note: Unlike basic_string::operator[], basic_string_ref::operator[](size()) has undefined behavior instead of returning charT(). — end note ]

constexpr const_reference at(size_type pos) const;

Throws: out_of_range if pos >= size().

Returns: operator[](pos).

constexpr const charT& front() const;

Requires: !empty()

Effects: Equivalent to operator[](0).

constexpr const charT& back() const;

Requires: !empty()

Effects: Equivalent to operator[](size() - 1).

const charT* data() const noexcept;

Returns: A non-null pointer p such that p + i == &operator[](i) for each i in [0,size()).

[ Note: Unlike std:🧵:data() and string literals, data() may return a pointer to a buffer that is not null-terminated. Therefore it is typically a mistake to pass data() to a routine that takes just a const charT* and expects a null-terminated string. — end note ]

Add a sub-subclause "x.y.5 basic_string_ref modifiers [string.ref.modifiers]"

void clear() noexcept;

Effects: Equivalent to *this = basic_string_ref()

void remove_prefix(size_type n);

Requires: n <= size()

Effects: Equivalent to *this = substr(n, npos)

void remove_suffix(size_type n);

Requires: n <= size()

Effects: Equivalent to *this = substr(0, size() - n)

Add a sub-subclause "x.y.6 basic_string_ref string operations [string.ref.ops]"

[ Note: Unlike std::basic_string, std::basic_string_ref provides no whole-string methods with posi- tion or length parameters. Instead, users should use the substr() method to create the character sequence they’re actually interested in, and use that. — end note ]

basic_string_ref substr(size_type pos, size_type n = npos) const;

Throws: out_of_range if pos > size().

Effects: Determines the effective length rlen of the string to reference as the smaller of n and size() - pos.

Returns: basic_string_ref(data()+pos, rlen).

int compare(const basic_string_ref& str) const noexcept;

Effects: Determines the effective length rlen of the strings to compare as the smallest of size() and str.size(). The function then compares the two strings by calling traits::compare(data(), str.data(), rlen).

Complexity: O(rlen)

Returns: The nonzero result if the result of the comparison is nonzero. Otherwise, returns a value as indicated in Table [tab:string.ref.compare].

Table [tab:string.ref.compare] — compare() results

bool starts_with(const basic_string_ref& prefix) const noexcept;

Complexity: O(min(size(), prefix.size()))

Returns: size() >= prefix.size() && substr(0, prefix.size()) == prefix

bool ends_with(const basic_string_ref& suffix) const noexcept;

Complexity: O(min(size(), suffix.size()))

Returns: size() >= suffix.size() && substr(size() - suffix.size(), npos) == suffix

Add a sub3clause "x.y.6.1 Searching basic_string_ref [string.ref.find]"

Member functions in this section have complexity O(size() * argument.size()) at worst, although implementations are encouraged to do better.

size_type find(const basic_string_ref& str) const noexcept;

Effects: Determines the lowest position xpos, if possible, such that both of the following conditions obtain:

• xpos + str.size() <= size();
• traits::eq(at(xpos+I), str.at(I)) for all elements I of the string referenced by str.

Returns: xpos if the function can determine such a value for xpos. Otherwise, returns npos.

Remarks: Uses traits::eq().

size_type rfind(const basic_string_ref& str) const noexcept;

Effects: Determines the highest position xpos, if possible, such that both of the following conditions obtain:

• xpos + str.size() <= size();
• traits::eq(at(xpos+I), str.at(I)) for all elements I of the string referenced by str.

Returns: xpos if the function can determine such a value for xpos. Otherwise, returns npos.

Remarks: Uses traits::eq().

size_type find_first_of(const basic_string_ref& str) const noexcept;

Effects: Determines the lowest position xpos, if possible, such that both of the following conditions obtain:

• xpos < size();
• traits::eq(at(xpos), str.at(I)) for some element I of the string referenced by str.

Returns: xpos if the function can determine such a value for xpos. Otherwise, returns npos.

Remarks: Uses traits::eq().

size_type find_last_of(const basic_string_ref& str) const noexcept;

Effects: Determines the highest position xpos, if possible, such that both of the following conditions obtain:

• xpos < size();
• traits::eq(at(xpos), str.at(I)) for some element I of the string referenced by str.

Returns: xpos if the function can determine such a value for xpos. Otherwise, returns npos.

Remarks: Uses traits::eq().

size_type find_first_not_of(const basic_string_ref& str) const noexcept;

Effects: Determines the lowest position xpos, if possible, such that both of the following conditions obtain:

• xpos < size();
• traits::eq(at(xpos), str.at(I)) for no element I of the string referenced by str.

Returns: xpos if the function can determine such a value for xpos. Otherwise, returns npos.

Remarks: Uses traits::eq().

size_type find_last_not_of(const basic_string_ref& str) const noexcept;

Effects: Determines the highest position xpos, if possible, such that both of the following conditions obtain:

• xpos < size();
• traits::eq(at(xpos), str.at(I)) for no element I of the string referenced by str.

Returns: xpos if the function can determine such a value for xpos. Otherwise, returns npos.

Remarks: Uses traits::eq().

Add a sub-subclause "x.y.7 basic_string_ref non-member comparison functions [string.ref.comparison]"

Implementations shall provide sufficient additional overloads so that an object t with an implicit conversion tobasic_string_ref<charT, traits> can be compared according to Table [tab:string.ref.comparison.overloads], wheresp is an instance ofbasic_string_ref<charT, traits>.

Table [tab:string.ref.comparison.overloads] — Additional basic_string_ref comparison overloads

[ Example: A sample conforming implementation for operator== would be:

  template<typename T> struct __identity { typedef T type; };
  template<typename charT, typename traits>
  bool operator==(basic_string_ref<charT, traits> lhs,
                  basic_string_ref<charT, traits> rhs) {
    return lhs.compare(rhs) == 0;
  }
  template<typename charT, typename traits>
  bool operator==(basic_string_ref<charT, traits> lhs,
                  typename __identity<basic_string_ref<charT, traits>>::type rhs) {
    return lhs.compare(rhs) == 0;
  }
  template<typename charT, typename traits>
  bool operator==(typename __identity<basic_string_ref<charT, traits>>::type lhs,
                  basic_string_ref<charT, traits> rhs) {
    return lhs.compare(rhs) == 0;
}

— end example ]

template<class charT, class traits>
  bool operator==(basic_string_ref<charT,traits> lhs,
                  basic_string_ref<charT,traits> rhs) noexcept;

Returns: lhs.compare(rhs) == 0.

template<class charT, class traits>
  bool operator!=(basic_string_ref<charT,traits> lhs,
                  basic_string_ref<charT,traits> rhs) noexcept;

Returns: !(lhs == rhs).

template<class charT, class traits>
  bool operator< (basic_string_ref<charT,traits> lhs,
                  basic_string_ref<charT,traits> rhs) noexcept;

Returns: lhs.compare(rhs) < 0.

template<class charT, class traits>
  bool operator> (basic_string_ref<charT,traits> lhs,
                  basic_string_ref<charT,traits> rhs) noexcept;

Returns: lhs.compare(rhs) > 0.

template<class charT, class traits>
  bool operator<=(basic_string_ref<charT,traits> lhs,
                  basic_string_ref<charT,traits> rhs) noexcept;

Returns: lhs.compare(rhs) <= 0.

template<class charT, class traits>
  bool operator>=(basic_string_ref<charT,traits> lhs,
                  basic_string_ref<charT,traits> rhs) noexcept;

Returns: lhs.compare(rhs) >= 0.

Add a sub-subclause "x.y.8 Other basic_string_ref non-member functions [string.ref.nonmem]"

template<class charT, class traits = char_traits<charT>,
         class Allocator = allocator<charT> >
  basic_string<charT, traits, Allocator> to_string(
    basic_string_ref<charT, traits> str,
    const Allocator& a = Allocator());

Complexity: O(str.size())

Returns: basic_string<charT, traits, Allocator>(str, a).

Add to the appropriate places within the class definition of [basic.string]

    explicit // Footnote: This conversion is explicit to avoid accidental O(N) operations on type mismatches. --end footnote
    basic_string(basic_string_ref<charT, traits>,
                 const Allocator& = Allocator());
    basic_string& operator=(basic_string_ref<charT, traits>);
    basic_string& operator+=(basic_string_ref<charT, traits>);
    basic_string& append(basic_string_ref<charT, traits>);
    basic_string& assign(basic_string_ref<charT, traits>);
    basic_string& replace(const_iterator, const_iterator, basic_string_ref<charT, traits>);
    operator basic_string_ref<charT, traits>() const noexcept;
    bool starts_with(basic_string_ref<charT, traits> s) const noexcept;
    bool starts_with(charT c) const noexcept;
    bool starts_with(const charT* s) const noexcept;
    bool ends_with(basic_string_ref<charT, traits> s) const noexcept;
    bool ends_with(charT c) const noexcept;
    bool ends_with(const charT* s) const noexcept;

Add to [string.cons]

explicit basic_string(basic_string_ref<charT, traits> str, const Allocator& a = Allocator());

Effects: Same as basic_string(str.begin(), str.end(), a).

This uses "Same as" instead of "Equivalent to" to be consistent with the rest of the basic_string specification.

Add to [string.accessors]

operator basic_string_ref<charT, traits>() const noexcept;

Returns: basic_string_ref<charT, traits>(data(), size()).

Complexity: constant time.

Requires: The program shall not alter any of the values stored in the character array.

Add a section "basic_string::starts_with and ends_with [string.starts.ends.with]" inside [string.ops]

bool starts_with(const basic_string_ref<charT, traits>& prefix) const noexcept;
bool starts_with(charT prefix) const noexcept;
bool starts_with(const charT* prefix) const noexcept;

Effects: Equivalent to basic_string_ref<charT, traits>(*this).starts_with(prefix)

bool ends_with(const basic_string_ref<charT, traits>& suffix) const noexcept;
bool ends_with(charT suffix) const noexcept;
bool ends_with(const charT* suffix) const noexcept;

Effects: Equivalent to basic_string_ref<charT, traits>(*this).ends_with(suffix)

Move [string.io] directly under [strings] and modify it

template<class charT, class traits, class Allocator>
  basic_ostream<charT, traits>&
    operator<<(basic_ostream<charT, traits>& os,
               const basic_string<charT,traits,Allocator>& str);
template<class charT, class traits, class Allocator>
  basic_ostream<charT, traits>&
    operator<<(basic_ostream<charT, traits>& os,
               basic_string_ref<charT,traits> str);

Effects: Behaves as a formatted output function ([ostream.formatted.reqmts]) of os. Forms a character sequence seq, initially consisting of the elements defined by the range [str.begin(), str.end()). Determines padding forseq as described in [ostream.formatted.reqmts]. Then insertsseq as if by calling os.rdbuf()->sputn(seq, n), where n is the larger of os.width() andstr.size(); then calls os.width(0).

Returns: os

Add to [string.conversions]

int stoi(string_ref str, size_t *idx = 0, int base = 10);
long stol(string_ref str, size_t *idx = 0, int base = 10);
unsigned long stoul(string_ref str, size_t *idx = 0, int base = 10);
long long stoll(string_ref str, size_t *idx = 0, int base = 10);
unsigned long long stoull(string_ref str, size_t *idx = 0, int base = 10);
float stof(string_ref str, size_t *idx = 0);
double stod(string_ref str, size_t *idx = 0);
long double stold(string_ref str, size_t *idx = 0);

int stoi(const char* str, size_t *idx = 0, int base = 10);
long stol(const char* str, size_t *idx = 0, int base = 10);
unsigned long stoul(const char* str, size_t *idx = 0, int base = 10);
long long stoll(const char* str, size_t *idx = 0, int base = 10);
unsigned long long stoull(const char* str, size_t *idx = 0, int base = 10);
float stof(const char* str, size_t *idx = 0);
double stod(const char* str, size_t *idx = 0);
long double stold(const char* str, size_t *idx = 0);

Each function of the forms

  rt fn(string_ref str, size_t *idx = 0, int base = 10);
  rt fn(const char* str, size_t *idx = 0, int base = 10);

is equivalent to fn(string(str), idx, base).

Each function of the forms

  rt fn(string_ref str, size_t *idx = 0);
  rt fn(const char* str, size_t *idx = 0);

is equivalent to fn(string(str), idx).

int stoi(wstring_ref str, size_t *idx = 0, int base = 10);
long stol(wstring_ref str, size_t *idx = 0, int base = 10);
unsigned long stoul(wstring_ref str, size_t *idx = 0, int base = 10);
long long stoll(wstring_ref str, size_t *idx = 0, int base = 10);
unsigned long long stoull(wstring_ref str, size_t *idx = 0, int base = 10);
float stof(wstring_ref str, size_t *idx = 0);
double stod(wstring_ref str, size_t *idx = 0);
long double stold(wstring_ref str, size_t *idx = 0);

int stoi(const wchar_t* str, size_t *idx = 0, int base = 10);
long stol(const wchar_t* str, size_t *idx = 0, int base = 10);
unsigned long stoul(const wchar_t* str, size_t *idx = 0, int base = 10);
long long stoll(const wchar_t* str, size_t *idx = 0, int base = 10);
unsigned long long stoull(const wchar_t* str, size_t *idx = 0, int base = 10);
float stof(const wchar_t* str, size_t *idx = 0);
double stod(const wchar_t* str, size_t *idx = 0);
long double stold(const wchar_t* str, size_t *idx = 0);

Each function of the forms

  rt fn(wstring_ref str, size_t *idx = 0, int base = 10);
  rt fn(const wchar_t* str, size_t *idx = 0, int base = 10);

is equivalent to fn(wstring(str), idx, base).

Each function of the forms

  rt fn(wstring_ref str, size_t *idx = 0);
  rt fn(const wchar_t* str, size_t *idx = 0);

is equivalent to fn(wstring(str), idx).

Modify [basic.string.hash]

template <> struct hash<string_ref>;
template <> struct hash<u16string_ref>;
template <> struct hash<u32string_ref>;
template <> struct hash<wstring_ref>;
template <> struct hash<string>;
template <> struct hash<u16string>;
template <> struct hash<u32string>;
template <> struct hash<wstring>;

Requires: the template specializations shall meet the requirements of class template hash ([unord.hash]).

Clause 22, Localization library

Modify a note in [locale]

[ Note: All locale semantics are accessed via use_facet<> and has_facet<>, except that:

• A member operator template Two member operator templates operator()(const basic_string<C, T, A>&, const basic_string<C, T, A>&) is and operator()(const basic_string_ref<C, T>&, const basic_string_ref<C, T>&) areprovided so that a locale may be used as a predicate argument to the standard collections, to collate strings.
• Convenient global interfaces are provided for traditional ctype functions such as isdigit() and isspace(), so that given a locale object loc a C++ program can call isspace(c,loc). (This eases upgrading existing extractors (27.7.2.2).) — end note ]

Modify [locale.operators]

template <class charT, class traits, class Allocator>
  bool operator()(const basic_string<charT,traits,Allocator>& s1,
                  const basic_string<charT,traits,Allocator>& s2) const;
template <class charT, class traits>
  bool operator()(const basic_string_ref<charT,traits>& s1,
                  const basic_string_ref<charT,traits>& s2) const;

Effects: Compares two strings according to the collate facet.

Remarks: This member operator template (and therefore locale itself) satisfies requirements for a comparator predicate template argument (Clause 25) applied to strings.

Returns: The result of the following expression:use_facet< collate<charT> >(*this).compare (s1.data(), s1.data()+s1.size(), s2.data(), s2.data()+s2.size()) < 0;

Modify [conversions.string]

namespace std {
template<class Codecvt, class Elem = wchar_t,
    class Wide_alloc = std::allocator<Elem>,
    class Byte_alloc = std::allocator<char> > class wstring_convert {
    // ...
    wide_string from_bytes(const basic_string_ref<char>& str);
    byte_string to_bytes(const basic_string_ref<Elem>& wstr);
    // ...
  };
}

wide_string from_bytes(char byte);
wide_string from_bytes(const char *ptr);
wide_string from_bytes(const byte_string& str);
wide_string from_bytes(const basic_string_ref<char>& str);
wide_string from_bytes(const char *first, const char *last);

Effects: The first member function shall convert the single-element sequence byte to a wide string. The second member function shall convert the null-terminated sequence beginning at ptr to a wide string. The third member function and fourth member functions shall convert the sequence stored in str to a wide string. The fourth fifth member function shall convert the sequence defined by the range [first,last) to a wide string.

byte_string to_bytes(Elem wchar);
byte_string to_bytes(const Elem *wptr);
byte_string to_bytes(const wide_string& wstr);
byte_string to_bytes(const basic_string_ref<Elem>& wstr);
byte_string to_bytes(const Elem *first, const Elem *last);

Effects: The first member function shall convert the single-element sequence wchar to a byte string. The second member function shall convert the null-terminated sequence beginning at wptr to a byte string. The third member function and fourth member functions shall convert the sequence stored in wstr to a byte string. The fourth fifth member function shall convert the sequence defined by the range [first,last) to a byte string.

Clause 28, Regular expressions library

Add to the appropriate places within [re.syn]

template <class BiIter, class ST>
  bool operator==(
    basic_string_ref<
      typename iterator_traits<BiIter>::value_type, ST> lhs,
    const sub_match<BiIter>& rhs);
template <class BiIter, class ST>
  bool operator!=(
    basic_string_ref<
      typename iterator_traits<BiIter>::value_type, ST> lhs,
    const sub_match<BiIter>& rhs);
template <class BiIter, class ST>
  bool operator<(
    basic_string_ref<
      typename iterator_traits<BiIter>::value_type, ST> lhs,
    const sub_match<BiIter>& rhs);
template <class BiIter, class ST>
  bool operator>(
    basic_string_ref<
      typename iterator_traits<BiIter>::value_type, ST> lhs,
    const sub_match<BiIter>& rhs);
template <class BiIter, class ST>
  bool operator>=(
    basic_string_ref<
      typename iterator_traits<BiIter>::value_type, ST> lhs,
    const sub_match<BiIter>& rhs);
template <class BiIter, class ST>
  bool operator<=(
    basic_string_ref<
      typename iterator_traits<BiIter>::value_type, ST> lhs,
    const sub_match<BiIter>& rhs);

template <class BiIter, class ST>
  bool operator==(
    const sub_match<BiIter>& lhs,
    basic_string_ref<
      typename iterator_traits<BiIter>::value_type, ST> rhs);
template <class BiIter, class ST>
bool operator!=(
       const sub_match<BiIter>& lhs,
        basic_string_ref<
          typename iterator_traits<BiIter>::value_type, ST> rhs);
template <class BiIter, class ST>
  bool operator<(
    const sub_match<BiIter>& lhs,
    basic_string_ref<
      typename iterator_traits<BiIter>::value_type, ST> rhs);
template <class BiIter, class ST>
  bool operator>(
    const sub_match<BiIter>& lhs,
    basic_string_ref<
      typename iterator_traits<BiIter>::value_type, ST> rhs);
template <class BiIter, class ST>
  bool operator>=(
    const sub_match<BiIter>& lhs,
    basic_string_ref<
      typename iterator_traits<BiIter>::value_type, ST> rhs);
template <class BiIter, class ST>
  bool operator<=(
    const sub_match<BiIter>& lhs,
    basic_string_ref<
      typename iterator_traits<BiIter>::value_type, ST> rhs);

template <class ST, class Allocator, class charT, class traits>
  bool regex_match(basic_string_ref<charT, ST> s,
                   match_results<
                     typename basic_string_ref<charT, ST>::const_iterator,
                     Allocator>& m,
                   const basic_regex<charT, traits>& e,
                   regex_constants::match_flag_type flags =
                     regex_constants::match_default);
template <class ST, class charT, class traits>
  bool regex_match(basic_string_ref<charT, ST> s,
                   const basic_regex<charT, traits>& e,
                   regex_constants::match_flag_type flags =
                     regex_constants::match_default);

template <class ST, class charT, class traits>
  bool regex_search(basic_string_ref<charT, ST> s,
                    const basic_regex<charT, traits>& e,
                    regex_constants::match_flag_type flags =
                      regex_constants::match_default);
template <class ST, class Allocator, class charT, class traits>
  bool regex_search(basic_string_ref<charT, ST> s,
                    match_results<
                      typename basic_string_ref<charT, ST>::const_iterator,
                      Allocator>& m,
                    const basic_regex<charT, traits>& e,
                    regex_constants::match_flag_type flags =
                      regex_constants::match_default);

template <class OutputIterator, class BidirectionalIterator,
    class traits, class charT, class ST>
  OutputIterator
  regex_replace(OutputIterator out,
                BidirectionalIterator first, BidirectionalIterator last,
                const basic_regex<charT, traits>& e,
                basic_string_ref<charT, ST> fmt,
                regex_constants::match_flag_type flags =
                  regex_constants::match_default);

This wording does not include a fix for LWG issue 2216. That is, the new overloads always use the default allocator.

template <class traits, class charT, class ST, class FST>
  basic_string<charT, ST>
  regex_replace(basic_string_ref<charT, ST> s,
                const basic_regex<charT, traits>& e,
                basic_string_ref<charT, FST> fmt,
                regex_constants::match_flag_type flags =
                  regex_constants::match_default);
template <class traits, class charT, class ST>
  basic_string<charT, ST>
  regex_replace(basic_string_ref<charT, ST> s,
                const basic_regex<charT, traits>& e,
                const charT* fmt,
                regex_constants::match_flag_type flags =
                  regex_constants::match_default);
template <class traits, class charT, class ST>
  basic_string<charT>
  regex_replace(const charT* s,
                const basic_regex<charT, traits>& e,
                basic_string_ref<charT, ST> fmt,
                regex_constants::match_flag_type flags =
                  regex_constants::match_default);
template <class traits, class charT, class ST, class SA,
          class FST>
  basic_string<charT, ST, SA>
  regex_replace(const basic_string<charT, ST, SA>& s,
                const basic_regex<charT, traits>& e,
                basic_string_ref<charT, FST> fmt,
                regex_constants::match_flag_type flags =
                  regex_constants::match_default);
template <class traits, class charT, class ST,
          class FST, class FSA>
  basic_string<charT, ST>
  regex_replace(basic_string_ref<charT, ST> s,
                const basic_regex<charT, traits>& e,
                const basic_string<charT, FST, FSA>& fmt,
                regex_constants::match_flag_type flags =
                  regex_constants::match_default);

Add to the appropriate places within [re.regex]

template <class ST>
  explicit basic_regex(basic_string_ref<charT, ST> p,
                       flag_type f = regex_constants::ECMAScript);
template <class ST>
  basic_regex& operator=(basic_string_ref<charT, ST> p);
template <class string_traits>
  basic_regex& assign(basic_string_ref<charT, string_traits> s,
                      flag_type f = regex_constants::ECMAScript);

Modify [re.regex.construct]

template <class ST>
  basic_regex(basic_string_ref<charT, ST> s,
              flag_type f = regex_constants::ECMAScript);
template <class ST, class SA>
basic_regex(const basic_string<charT, ST, SA>& s,
            flag_type f = regex_constants::ECMAScript);

Throws: regex_error if s is not a valid regular expression.

Effects: Constructs an object of class basic_regex; the object’s internal finite state machine is con- structed from the regular expression contained in the string s, and interpreted according to the flags specified in f.

Postconditions: flags() returns f. mark_count() returns the number of marked sub-expressions within the expression.

Modify [re.regex.assign]

template <class ST>
  basic_regex& operator=(basic_string_ref<charT, ST> p);
template <class ST, class SA>
  basic_regex& operator=(const basic_string<charT, ST, SA>& p);

Effects: returns assign(p).

template <class string_traits>
  basic_regex& assign(basic_string_ref<charT, string_traits> s,
                      flag_type f = regex_constants::ECMAScript);
template <class string_traits, class A>
  basic_regex& assign(const basic_string<charT, string_traits, A>& s,
                      flag_type f = regex_constants::ECMAScript);

Throws: regex_error if s is not a valid regular expression.

Returns: *this.

Effects: Assigns the regular expression contained in the string s, interpreted according the flags specified in f. If an exception is thrown, *this is unchanged.

Postconditions: If no exception is thrown, flags() returns f and mark_count() returns the number of marked sub-expressions within the expression.

Add to [re.submatch]

int compare(basic_string_ref<value_type> s) const;

Add to [re.submatch.members]

int compare(basic_string_ref<value_type> s) const;

Returns: str().compare(s).

Add to [re.submatch.op]

template <class BiIter, class ST>
  bool operator==(
    basic_string_ref<
      typename iterator_traits<BiIter>::value_type, ST> lhs,
    const sub_match<BiIter>& rhs);

Returns: rhs.compare(basic_string_ref<typename iterator_traits::value_type>(lhs.data(), lhs.size())) == 0.

[ Footnote: This and the other comparison operators ignore the basic_string_ref's traits. -- end footnote]

template <class BiIter, class ST>
  bool operator!=(
basic_string_ref<
  typename iterator_traits<BiIter>::value_type, ST> lhs,
const sub_match<BiIter>& rhs);

Returns: !(lhs == rhs).

template <class BiIter, class ST>
  bool operator<(
basic_string_ref<
  typename iterator_traits<BiIter>::value_type, ST> lhs,
const sub_match<BiIter>& rhs);

Returns: rhs.compare(basic_string_ref<typename iterator_traits::value_type>(lhs.data(), lhs.size())) > 0.

template <class BiIter, class ST>
  bool operator>(
basic_string_ref<
  typename iterator_traits<BiIter>::value_type, ST> lhs,
const sub_match<BiIter>& rhs);

Returns: rhs < lhs.

template <class BiIter, class ST>
  bool operator>=(
basic_string_ref<
  typename iterator_traits<BiIter>::value_type, ST> lhs,
const sub_match<BiIter>& rhs);

Returns: !(lhs < rhs).

template <class BiIter, class ST>
  bool operator<=(
basic_string_ref<
  typename iterator_traits<BiIter>::value_type, ST> lhs,
const sub_match<BiIter>& rhs);

Returns: !(rhs < lhs).

template <class BiIter, class ST>
  bool operator==(const sub_match<BiIter>& lhs,
             basic_string_ref<
               typename iterator_traits<BiIter>::value_type, ST> rhs);

Returns: lhs.compare(basic_string_ref<typename iterator_traits::value_type>(rhs.data(), rhs.size())) == 0.

template <class BiIter, class ST>
  bool operator!=(const sub_match<BiIter>& lhs,
                  basic_string_ref<
                    typename iterator_traits<BiIter>::value_type, ST> rhs);

Returns: !(lhs == rhs).

template <class BiIter, class ST>
  bool operator<(const sub_match<BiIter>& lhs,
            basic_string_ref<
              typename iterator_traits<BiIter>::value_type, ST> rhs);

Returns: lhs.compare(basic_string_ref<typename iterator_traits::value_type>(rhs.data(), rhs.size())) < 0.

template <class BiIter, class ST>
  bool operator>(const sub_match<BiIter>& lhs,
            basic_string_ref<
              typename iterator_traits<BiIter>::value_type, ST> rhs);

Returns: rhs < lhs.

template <class BiIter, class ST>
  bool operator>=(const sub_match<BiIter>& lhs,
             basic_string_ref<
               typename iterator_traits<BiIter>::value_type, ST> rhs);

Returns: !(lhs < rhs).

template <class BiIter, class ST>
  bool operator<=(const sub_match<BiIter>& lhs,
             basic_string_ref<
               typename iterator_traits<BiIter>::value_type, ST> rhs);

Returns: !(rhs < lhs).

Add to the appropriate places within [re.results]

template <class OutputIter, class ST>
       OutputIter
       format(OutputIter out,
              basic_string_ref<char_type, ST> fmt,
              regex_constants::match_flag_type flags =
                regex_constants::format_default) const;
template <class ST>
      basic_string<char_type, ST>
      format(basic_string_ref<char_type, ST> fmt,
             regex_constants::match_flag_type flags =
               regex_constants::format_default) const;

Modify [re.results.form]

template <class OutputIter, class ST>
  OutputIter format(OutputIter out,
                    basic_string_ref<char_type, ST> fmt,
                    regex_constants::match_flag_type flags =
                      regex_constants::format_default) const;
template <class OutputIter, class ST, class SA>
  OutputIter format(OutputIter out,
                    const basic_string<char_type, ST, SA>& fmt,
                    regex_constants::match_flag_type flags =
                      regex_constants::format_default) const;

Effects: Equivalent to return format(out, fmt.data(), fmt.data() + fmt.size(), flags).

template <class ST>
  basic_string<char_type, ST>
  format(basic_string_ref<char_type, ST>& fmt,
    regex_constants::match_flag_type flags =
      regex_constants::format_default) const;

Requires: ready() == true.

Effects: Constructs an empty string result of type basic_string<char_type, ST> and callsformat(back_inserter(result), fmt, flags).

Returns: result.

Modify [re.alg.match]

template <class ST, class Allocator, class charT, class traits>
  bool regex_match(basic_string_ref<charT, ST> s,
                   match_results<
                     typename basic_string_ref<charT, ST>::const_iterator,
                     Allocator>& m,
                   const basic_regex<charT, traits>& e,
                   regex_constants::match_flag_type flags =
                     regex_constants::match_default);
template <class ST, class SA, class Allocator, class charT, class traits>
  bool regex_match(const basic_string<charT, ST, SA>& s,
                   match_results<
                     typename basic_string<charT, ST, SA>::const_iterator,
                     Allocator>& m,
                   const basic_regex<charT, traits>& e,
                   regex_constants::match_flag_type flags =
                     regex_constants::match_default);

Returns: regex_match(s.begin(), s.end(), m, e, flags).

template <class ST, class charT, class traits>
  bool regex_match(basic_string_ref<charT, ST> s,
                   const basic_regex<charT, traits>& e,
                   regex_constants::match_flag_type flags =
                     regex_constants::match_default);
template <class ST, class SA, class charT, class traits>
  bool regex_match(const basic_string<charT, ST, SA>& s,
                   const basic_regex<charT, traits>& e,
                   regex_constants::match_flag_type flags =
                     regex_constants::match_default);

Returns: regex_match(s.begin(), s.end(), e, flags).

Modify [re.alg.search]

template <class ST, class Allocator, class charT, class traits>
  bool regex_search(basic_string_ref<charT, ST> s,
                    match_results<
                      typename basic_string_ref<charT, ST>::const_iterator,
                      Allocator>& m,
                    const basic_regex<charT, traits>& e,
                    regex_constants::match_flag_type flags =
                      regex_constants::match_default);
template <class ST, class SA, class Allocator, class charT, class traits>
  bool regex_search(const basic_string<charT, ST, SA>& s,
                    match_results<
                      typename basic_string<charT, ST, SA>::const_iterator,
                      Allocator>& m,
                    const basic_regex<charT, traits>& e,
                    regex_constants::match_flag_type flags =
                      regex_constants::match_default);

Returns: The result of regex_search(s.begin(), s.end(), m, e, flags).

template <class ST, class SA, class charT, class traits>
  bool regex_search(basic_string_ref<charT, ST>& s,
                    const basic_regex<charT, traits>& e,
                    regex_constants::match_flag_type flags =
                      regex_constants::match_default);
template <class ST, class SA, class charT, class traits>
  bool regex_search(const basic_string<charT, ST, SA>& s,
                    const basic_regex<charT, traits>& e,
                    regex_constants::match_flag_type flags =
                      regex_constants::match_default);

Returns: regex_search(s.begin(), s.end(), e, flags).

Modify [re.alg.replace]

template <class OutputIterator, class BidirectionalIterator,
    class traits, class charT, class ST, class SA>
  OutputIterator
  regex_replace(OutputIterator out,
                BidirectionalIterator first, BidirectionalIterator last,
                const basic_regex<charT, traits>& e,
                const basic_string<charT, ST, SA>& fmt,
                regex_constants::match_flag_type flags =
                  regex_constants::match_default);
template <class OutputIterator, class BidirectionalIterator,
    class traits, class charT, class ST>
  OutputIterator
  regex_replace(OutputIterator out,
                BidirectionalIterator first, BidirectionalIterator last,
                const basic_regex<charT, traits>& e,
                basic_string_ref<charT, ST> fmt,
                regex_constants::match_flag_type flags =
                  regex_constants::match_default);
template <class OutputIterator, class BidirectionalIterator,
    class traits, class charT>
  OutputIterator
  regex_replace(OutputIterator out,
                BidirectionalIterator first, BidirectionalIterator last,
                const basic_regex<charT, traits>& e,
                const charT* fmt,
                regex_constants::match_flag_type flags =
                  regex_constants::match_default);

Effects: Constructs a regex_iterator object i as if by regex_iterator<BidirectionalIterator, charT, traits> i(first, last, e, flags), and uses i to enumerate through all of the matches m of type match_results that occur within the sequence [first,last ). If no such matches are found and !(flags & regex_constants ::format_no_copy) then calls std::copy(first, last, out). If any matches are found then, for each such match, if !(flags & regex_constants::format_no_copy), calls std::copy(m.prefix().first, m.prefix().second, out), and then calls m.format(out, fmt, flags) for the first form and second forms of the function and m.format(out, fmt, fmt + char_traits::length(fmt), flags) for the second third. Finally, if such a match is found and !(flags & regex_constants ::format_no_copy), calls std:: copy(last_m.suffix().first, last_m.suffix().second, out) where last_m is a copy of the last match found. If flags & regex_constants::format_first_only is non-zero then only the first match found is replaced.

Returns: out.

template <class traits, class charT, class ST, class FST, class FSA>
  basic_string<charT, ST>
  regex_replace(basic_string_ref<charT, ST> s,
                const basic_regex<charT, traits>& e,
                const basic_string<charT, FST, FSA>& fmt,
                regex_constants::match_flag_type flags =
                  regex_constants::match_default);
template <class traits, class charT, class ST, class FST>
  basic_string<charT, ST>
  regex_replace(basic_string_ref<charT, ST> s,
                const basic_regex<charT, traits>& e,
                basic_string_ref<charT, FST> fmt,
                regex_constants::match_flag_type flags =
                  regex_constants::match_default);
template <class traits, class charT, class ST>
  basic_string<charT, ST>
  regex_replace(basic_string_ref<charT, ST> s,
  const basic_regex<charT, traits>& e,
                const charT* fmt,
                regex_constants::match_flag_type flags =
                  regex_constants::match_default);

Effects: Constructs an empty string result of type basic_string<charT, ST> and calls regex_replace(back_inserter(result), s.begin(), s.end(), e, fmt, flags).

Returns: result.

template <class traits, class charT, class ST, class SA, class FST, class FSA>
  basic_string<charT, ST, SA>
  regex_replace(const basic_string<charT, ST, SA>& s,
                const basic_regex<charT, traits>& e,
                const basic_string<charT, FST, FSA>& fmt,
                regex_constants::match_flag_type flags =
                  regex_constants::match_default);
template <class traits, class charT, class ST, class SA, class FST>
  basic_string<charT, ST, SA>
  regex_replace(const basic_string<charT, ST, SA>& s,
                const basic_regex<charT, traits>& e,
                basic_string_ref<charT, FST> fmt,
                regex_constants::match_flag_type flags =
                  regex_constants::match_default);
template <class traits, class charT, class ST, class SA>
basic_string<charT, ST, SA>
regex_replace(const basic_string<charT, ST, SA>& s,
              const basic_regex<charT, traits>& e,
              const charT* fmt,
              regex_constants::match_flag_type flags =
                regex_constants::match_default);

Effects: Constructs an empty string result of type basic_string<charT, ST, SA> and calls regex_replace(back_inserter(result), s.begin(), s.end(), e, fmt, flags).

Returns: result.

template <class traits, class charT, class ST, class SA>
  basic_string<charT>
  regex_replace(const charT* s,
                const basic_regex<charT, traits>& e,
                const basic_string<charT, ST, SA>& fmt,
                regex_constants::match_flag_type flags =
                  regex_constants::match_default);
template <class traits, class charT, class ST>
  basic_string<charT>
  regex_replace(const charT* s,
                const basic_regex<charT, traits>& e,
                basic_string_ref<charT, ST> fmt,
                   regex_constants::match_flag_type flags =
                     regex_constants::match_default);
template <class traits, class charT>
  basic_string<charT>
  regex_replace(const charT* s,
                const basic_regex<charT, traits>& e,
                const charT* fmt,
                regex_constants::match_flag_type flags =
                  regex_constants::match_default);

Effects: Constructs an empty string result of type basic_string and calls regex_replace( back_inserter(result), s, s + char_traits::length(s), e, fmt, flags).

Returns: result.

Acknowledgements

I'd like to thank Marshall Clow, Olaf van der Spek, the Boost and std-proposals mailing lists, Chandler Carruth, Beman Dawes, Daniel Krügler, and Alisdair Meredith for help, advice, and wording in this paper.