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std::make_format_args, std::make_wformat_args

Defined in header <format>

Defined in header `<format>`
template< class Context = std::format_context, class... Args > /format-arg-store/<Context, Args...> make_format_args( Args&&... args );	(1)	(since C++20)
template< class... Args > /format-arg-store/<std::wformat_context, Args...> make_wformat_args( Args&&... args );	(2)	(since C++20)

template< class Context = std::format_context, class... Args >
/*format-arg-store*/<Context, Args...>
    make_format_args( Args&&... args );

(1)

(since C++20)

template< class... Args >
/*format-arg-store*/<std::wformat_context, Args...>
    make_wformat_args( Args&&... args );

(2)

(since C++20)

Returns an object that stores an array of formatting arguments and can be implicitly converted to std::basic_format_args<Context>.

The behavior is undefined if typename Context::template formatter_type<Ti> does not meet the BasicFormatter requirements for any Ti in Args.

Parameters

args...

values to be used as formatting arguments

Returns

An object that holds the formatting arguments.

Notes

A formatting argument has reference semantics for user-defined types and does not extend the lifetime of args. It is the programmer's responsibility to ensure that args outlive the return value. Usually, the result is only used as argument to formatting function.

Example

#include <array>
#include <format>
#include <iostream>
#include <string_view>
 
void raw_write_to_log(std::string_view users_fmt, std::format_args&& args) {
    static int n{};
    std::clog << std::format("{:04} : ", n++) << std::vformat(users_fmt, args) << '\n';
}
 
template <typename... Args>
constexpr void log(Args&&... args) {
 
    // Generate formatting string "{} "...
    std::array<char, sizeof...(Args) * 3 + 1> braces{};
    constexpr const char c[4] = "{} ";
    for (auto i{0u}; i != braces.size() - 1; ++i) {
        braces[i] = c[i % 3];
    }
    braces.back() = '\0';
 
    raw_write_to_log(std::string_view{braces.data()}, std::make_format_args(args...));
}
 
int main()
{
    log("Number", "of", "arguments", "is", "arbitrary.");
    log("Any type that meets the `BasicFormatter` requirements", "can be printed.");
    log("For example:", 1, 2.0, '3', "*42*");
 
    raw_write_to_log("{:02} │ {} │ {} │ {}", std::make_format_args(1, 2.0, '3', "4"));
}

Output:

0000 : Number of arguments is arbitrary. 
0001 : Any type that meets the `BasicFormatter` requirements can be printed. 
0002 : For example: 1 2.0 3 *42* 
0003 : 01 │ 2.0 │ 3 │ 4

Defect reports

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

DR	Applied to	Behavior as published	Correct behavior
P2418R2	C++20	objects that are neither const-usable nor copyable (such as generator-like objects) are not formattable	allow formatting these objects