NAME¶

std::runtime_format - std::runtime_format

Synopsis¶

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

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¶

fmt - a string view

Return value¶

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

Class template runtime-format-string <CharT>

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

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*/& ) = (3)
delete;

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¶

// Run this code

#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

See also¶

format stores formatted representation of the arguments in a new string
(C++20) (function template)
vformat non-template variant of std::format using type-erased argument
(C++20) representation
(function)
basic_format_string
format_string class template that performs compile-time format string checks
wformat_string at construction time
(C++20) (class template)
(C++20)
(C++20)