NAME¶

std::make_format_args,std::make_wformat_args - std::make_format_args,std::make_wformat_args

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 );

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<std::remove_const_t<Ti>> does not meet the BasicFormatter
requirements for any Ti in Args.

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

2) Equivalent to return std::make_format_args<std::wformat_context>(args...);.

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().

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.

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

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

Defect reports

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

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

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
(C++20) representation
(function)
vformat_to non-template variant of std::format_to using type-erased argument
(C++20) representation
(function template)