C++ named requirements: LiteralType (since C++11) (original) (raw)

Specifies that a type is a literal type. Literal types are the types of constexpr variables and they can be constructed, manipulated, and returned from constexpr functions.

Note: the standard doesn't define a named requirement with this name. This is a type category defined by the core language. It is included here as a named requirement only for consistency.

[edit] Requirements

A literal type is any of the following:

• scalar type;

• reference type;

• an array of literal type;

• possibly cv-qualified class type that has all of the following properties:

• has a trivial(until C++20)constexpr(since C++20) destructor,

• all of its non-static non-variant data members and base classes are of non-volatile literal types, and

• is one of

• an aggregate union type that

• has no variant members, or

• has at least one variant member of non-volatile literal type,

• a non-union aggregate type, and each of its anonymous union members

• has no variant members, or

• has at least one variant member of non-volatile literal type,

• a type with at least one constexpr (possibly template) constructor that is not a copy or move constructor.

[edit] Notes

A type can be literal even if all of its constexpr constructors are deleted, inaccessible, or cannot participate in overload resolution.

struct A { constexpr A(int) = delete; char c; }; // A is a literal type constexpr A v = std::bit_cast('0'); // OK in C++20 // v has literal type and thus can be constexpr

[edit] Example

Literal type that extends string literals:

#include #include #include   class conststr // conststr is a literal type { const char* p; std::size_t sz; public: template<std::size_t N> constexpr conststr(const char(&a)[N]) : p(a), sz(N - 1) {}   constexpr char operator n) const { return n < sz ? p[n] : throw std::out_of_range(""); }   constexpr std::size_t size() const { return sz; } };   constexpr std::size_t count_lower(conststr s) { std::size_t c{}; for (std::size_t n{}; n != s.size(); ++n) if ('a' <= s[n] && s[n] <= 'z') ++c; return c; }   // An output function that requires a compile-time constant N, for testing template struct constN { constN() { std::cout << N << '\n'; } };   int main() { std::cout << "The number of lowercase letters in "Hello, world!" is "; constN<count_lower("Hello, world!")>(); // the string literal is implicitly // converted to conststr }

Output:

The number of lowercase letters in "Hello, world!" is 9

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

[edit] See also