Difference between revisions of "cpp/utility/format/make format args"

Latest revision as of 13:24, 26 February 2024

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>.

[edit] Parameters

[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:

• 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().

[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.

[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"));
}

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.

[edit] See also