std::runtime_format - cppreference.com (original) (raw)

Returns an object that stores a runtime format string directly usable in user-oriented formatting functions and can be implicitly converted to std::basic_format_string.

Contents

• 1 Parameters
• 2 Return value
• 3 Class template runtime-format-string
  • 3.1 Member objects
  • 3.2 Constructors and assignments
  • 3.3 Notes
  • 3.4 Example
  • 3.5 See also

[edit] Parameters

[edit] Return value

An object holding the runtime format string of the exposition-only type:

Class template _runtime-format-string_ <CharT>

| template< class CharT > struct /*runtime-format-string*/; | | (exposition only*) | | ------------------------------------------------------------ | | ------------------- |

Member objects

The returned object contains an exposition-only non-static data member _str_ of type std::basic_string_view.

Constructors and assignments

| /*runtime-format-string*/( std::basic_string_view<CharT> s ) noexcept; | (1) | | | ---------------------------------------------------------------------------------------------------------------------- | --- | | | /*runtime-format-string*/( const /*runtime-format-string*/& ) = delete; | (2) | | | /*runtime-format-string*/& operator=( const /*runtime-format-string*/& ) = delete; | (3) | |

1. Initializes _str_ with s.

2. Copy constructor is explicitly deleted. The type is neither copyable nor movable.

3. The assignment is explicitly deleted.

[edit] Notes

Since the return type of runtime_format is neither copyable nor movable, an attempt of passing runtime_fmt as glvalue inhibits the construction of std::basic_format_string which results in program ill-formed. To construct std::basic_format_string with runtime_format, the returned value of runtime_format is passed directly on std::basic_format_string as prvalue where copy elision is guaranteed.

auto runtime_fmt = std::runtime_format("{}");   auto s0 = std::format(runtime_fmt, 1); // error auto s1 = std::format(std::move(runtime_fmt), 1); // still error auto s2 = std::format(std::runtime_format("{}"), 1); // ok

[edit] Example

#include #include #include #include   int main() { std::print("Hello {}!\n", "world");   std::string fmt; for (int i{}; i != 3; ++i) { fmt += "{} "; // constructs the formatting string std::print("{} : ", fmt); std::println(std::runtime_format(fmt), "alpha", 'Z', 3.14, "unused"); } }

Output:

Hello world! {}  : alpha {} {}  : alpha Z {} {} {}  : alpha Z 3.14

[edit] See also