Namespaces
Variants
Views
Actions

std::make_format_args, std::make_wformat_args

From cppreference.com
< cpp‎ | utility‎ | format
 
 
Utilities library
Language support
Type support (basic types, RTTI)
Library feature-test macros (C++20)
Dynamic memory management
Program utilities
Coroutine support (C++20)
Variadic functions
Debugging support
(C++26)
Three-way comparison
(C++20)
(C++20)(C++20)(C++20)
(C++20)(C++20)(C++20)
General utilities
Date and time
Function objects
Formatting library (C++20)
(C++11)
Relational operators (deprecated in C++20)
Integer comparison functions
(C++20)(C++20)(C++20)   
(C++20)
Swap and type operations
(C++14)
(C++11)
(C++11)
(C++11)
(C++17)
Common vocabulary types
(C++11)
(C++17)
(C++17)
(C++17)
(C++11)
(C++17)
(C++23)
Elementary string conversions
(C++17)
(C++17)

 
Formatting library
Standard format specification
Formatting functions
(C++20)
(C++20)
(C++20)
(C++20)
Format strings
Formatting concepts
Formatter
(C++20)
Formatting arguments
(C++20) (deprecated in C++26)
make_format_argsmake_wformat_args
(C++20)(C++20)
Format error
 
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)

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...);.

Contents

[edit] Parameters

args... - values to be used as formatting arguments

[edit] 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:

[edit] 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)
(DR)
Formatting of code units as unsigned integers

[edit] 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"));
}

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

[edit] 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
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 as
out-of-range unsigned integer values
code units are converted to the corresponding
unsigned type before such formatting
LWG 3631 C++20 cv-qualified arguments were incorrectly handled after P2418R2 handling corrected

[edit] See also

class that provides access to all formatting arguments
(class template) [edit]
(C++20)
non-template variant of std::format using type-erased argument representation
(function) [edit]
non-template variant of std::format_to using type-erased argument representation
(function template) [edit]