std::runtime_format
| 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
| 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 > 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) | |
str with s.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 3.14See also
| (C++20) |
stores formatted representation of the arguments in a new string (function template) |
| (C++20) |
non-template variant of std::format using type-erased argument representation (function) |
| (C++20)(C++20)(C++20) |
class template that performs compile-time format string checks at construction time (class template) |