std::runtime_format - cppreference.com

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Defined in header `<format>`
`/runtime-format-string/<char> runtime_format( std::string_view fmt ) noexcept;`	(1)	(since C++26)
`/runtime-format-string/<wchar_t> runtime_format( std::wstring_view fmt ) noexcept;`	(2)	(since C++26)

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.

Parameters

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<CharT>.

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.

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

Feature-test macro	Value	Std	Feature
`__cpp_lib_format`	`202311L`	(C++26)	Runtime format strings

Example

#include <format>
#include <print>
#include <string>
#include <string_view>

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

	stores formatted representation of the arguments in a new string (function template) [edit]
	non-template variant of std::format using type-erased argument representation (function) [edit]
	class template that performs compile-time format string checks at construction time (class template) [edit]