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.
An object holding the runtime format string of the exposition-only type:
Class templateruntime-format-string <CharT>
template< class CharT >
struct /*runtime-format-string*/;
The returned object contains an exposition-only non-static data member str of type std::basic_string_view<CharT>.
/*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] NotesSince 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 <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[edit] See also
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