std:: format

Format args according to the format string fmt , and return the result as a string. If present, loc is used for locale-specific formatting.

Since P2216R3 , std::format does a compile-time check on the format string (via the helper type std::format_string or std::wformat_string ). If it is found to be invalid for the types of the arguments to be formatted, a compilation error will be emitted. If the format string cannot be a compile-time constant, or the compile-time check needs to be avoided, use std::vformat or std::runtime_format on fmt (since C++26) instead.

The following requirements apply to each type T in Args , where CharT is char for overloads (1,3) , wchar_t for overloads (2,4) :

• std:: formatter < T, CharT > must satisfy BasicFormatter
• std:: formatter < T, CharT > :: parse ( ) must be constexpr since P2216R3 ( std::vformat does not have this requirement)

Parameters

Return value

A string object holding the formatted result.

Exceptions

Throws std::bad_alloc on allocation failure. Also propagates exception thrown by any formatter.

Notes

It is not an error to provide more arguments than the format string requires:

std::format("{} {}!", "Hello", "world", "something"); // OK, produces "Hello world!"

As of P2216R3 , it is an error if the format string is not a constant expression. std::vformat can be used in this case.

std::string f(std::string_view runtime_format_string)
{
    // return std::format(runtime_format_string, "foo", "bar"); // error
    return std::vformat(runtime_format_string, std::make_format_args("foo", "bar")); // OK
}

std::string f(std::string_view runtime_format_string)
{
    return std::format(std::runtime_format(runtime_format_string), "foo", "bar");
}

Example

#include <format>
#include <iostream>
#include <string>
#include <string_view>
 
template<typename... Args>
std::string dyna_print(std::string_view rt_fmt_str, Args&&... args)
{
    return std::vformat(rt_fmt_str, std::make_format_args(args...));
}
 
int main()
{
    std::cout << std::format("Hello {}!\n", "world");
 
    std::string fmt;
    for (int i{}; i != 3; ++i)
    {
        fmt += "{} "; // constructs the formatting string
        std::cout << fmt << " : ";
        std::cout << dyna_print(fmt, "alpha", 'Z', 3.14, "unused");
        std::cout << '\n';
    }
}

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

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

See also