std::make_f...format_args(3)

🌸 NAME 🌸

okay so this is the C++ Standard Libary manual page for the duo std::make_format_args,std::make_wformat_args — and yes, the NAME repeats itself exactly like that: std::make_format_args,std::make_wformat_args. periodt. 💅

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✨ Synopsis ✨

Defined in header <format>
template< class Context = std::format_context, class... Args >

/*format-arg-store*/<Context, Args...>                         (1)
(since C++20)

    make_format_args( Args&... args );
template< class... Args >

/*format-arg-store*/<std::wformat_context, Args...>            (2)
(since C++20)

    make_wformat_args( Args&... args );

So what does it do? It returns an object that stores an array of formatting arguments and can be implicitly converted to std::basic_format_args<Context>. <sparkle>iconic behavior, honestly</sparkle>. ✨

⚠️ The behavior is undefined if typename Context::template formatter_type<std::remove_const_t<Ti>> does not meet the BasicFormatter requirements for any Ti in Args. No cap, that's undefined behavior territory. 💀

⚠️ The program is ill-formed if for any type Ti in Args, Ti does not satisfy __formattable_with<Context>. Not cute, not allowed. 😭

(2) is equivalent to return std::make_format_args<std::wformat_context>(args...);. 🫶

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💖 Parameters 💖

• args... — values to be used as formatting arguments 🎀

✨ Returns ✨

An object that holds the formatting arguments. 🦋

For each argument t of type T, let TD be std::remove_const_t<std::remove_reference_t<T>>. The corresponding std::basic_format_arg in the result is determined as below:

• if TD is bool or Context::char_type, the std::basic_format_arg stores t;
• otherwise, if TD is char and Context::char_type is wchar_t, the std::basic_format_arg stores static_cast<wchar_t>(static_cast<unsigned char>(t));
• otherwise, if TD is a signed integer type whose size is not greater than int, the std::basic_format_arg stores static_cast<int>(t);
• otherwise, if TD is a unsigned integer type whose size is not greater than unsigned int, the std::basic_format_arg stores static_cast<unsigned int>(t);
• otherwise, if TD is a signed integer type whose size is not greater than long long, the std::basic_format_arg stores static_cast<long long>(t);
• otherwise, if TD is a unsigned integer type whose size is not greater than unsigned long long, the std::basic_format_arg stores static_cast<unsigned long long>(t);
• otherwise, if TD is float, double, or long double, the std::basic_format_arg stores t;
• otherwise, if TD is a std::basic_string_view or std::basic_string specialization and TD::char_type is Context::char_type, the std::basic_format_arg stores std::basic_string_view<Context::char_type>(t.data(), t.size());
• otherwise, if std::decay_t<TD> is Context::char_type* or const Context::char_type*, the std::basic_format_arg stores static_cast<const Context::char_type*>(t);
• otherwise, if std::is_void_v<std::remove_pointer_t<TD>> is true or std::is_null_pointer_v<TD> is true, the std::basic_format_arg stores static_cast<const void*>(t);
• otherwise, the std::basic_format_arg stores a std::basic_format_arg<Context>::handle to t, along with extra data needed for handle::format().

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🧚 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. bestie, please keep those args alive — this is not a drill. 😭🫶

• Feature-test macro: __cpp_lib_format_uchar — Value: 202311L — Std: (C++20) — Feature: Formatting of code units as unsigned integers (DR)

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🌈 Example 🌈

// Run this code

#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"));
}

🌸 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

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💀 Defect reports 💀

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

• P2418R2 — Applied to: C++20
  • Behavior as published: objects that are neither const-usable nor copyable (such as generator-like objects) are not formattable
  • Correct behavior: allow formatting these objects
• P2905R2 — Applied to: C++20
  • Behavior as published: make_format_args accepted rvalue arguments by forwarding references
  • Correct behavior: only takes lvalue references
• P2909R4 — Applied to: C++20
  • Behavior as published: char or wchar_t might be formatted as out-of-range unsigned integer values
  • Correct behavior: code units are converted to the corresponding unsigned type before such formatting
• LWG 3631 — Applied to: C++20
  • Behavior as published: cv-qualified arguments were incorrectly handled after P2418R2
  • Correct behavior: handling corrected

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👑 See also 👑

• basic_format_args, format_args, wformat_args — class that provides access to all formatting arguments (C++20) (class template) (C++20) (C++20)
• vformat — non-template variant of std::format using type-erased argument representation (C++20) (function)
• vformat_to — non-template variant of std::format_to using type-erased argument representation (C++20) (function template)

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💖 that's the whole page, bestie. source/version/date sign-off: http://cppreference.com · 2024.06.10 · page title std::make_f...format_args(3). periodt, go format something cute. 🎀