std::make_format_args, std::make_wformat_args

Since C++20

Header: <format>

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

# Declarations

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

(since C++20)

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

(since C++20)

# Parameters

args...: values to be used as 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{0uz}; 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...));
}
 
template<typename T>
const T& unmove(T&& x)
{
    return x;
}
 
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(unmove(1), unmove(2.0), unmove('3'), "4"));
}

# Defect reports

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
P2905R2	C++20	make_format_args accepted rvalue arguments by forwarding references	only takes lvalue references
P2909R4	C++20	char or wchar_t might be formatted asout-of-range unsigned integer values	code units are converted to the correspondingunsigned type before such formatting
LWG 3631	C++20	cv-qualified arguments were incorrectly handled after P2418R2	handling corrected