std::formatted_size

Since C++20

Header: <format>

Determine the total number of characters in the formatted string by formatting args according to the format string fmt. If present, loc is used for locale-specific formatting.

# Declarations

template< class... Args >
std::size_t formatted_size( std::format_string<Args...> fmt, Args&&... args );

(since C++20)

template< class... Args >
std::size_t formatted_size( std::wformat_string<Args...> fmt, Args&&... args );

(since C++20)

template< class... Args >
std::size_t formatted_size( const std::locale& loc,
std::format_string<Args...> fmt, Args&&... args );

(since C++20)

template< class... Args >
std::size_t formatted_size( const std::locale& loc,
std::wformat_string<Args...> fmt, Args&&... args );

(since C++20)

# Parameters

# Return value

The total number of characters in the formatted string.

# Example

#include <format>
#include <iomanip>
#include <iostream>
#include <string_view>
#include <vector>
 
int main()
{
    using namespace std::literals::string_view_literals;
 
    constexpr auto fmt_str{"Hubble's H{0} {1} {2:*^4} miles/sec/mpc."sv};
    constexpr auto sub_zero{"\N{SUBSCRIPT ZERO}"sv}; // "₀" or {0342, 130, 128}
    constexpr auto aprox_equ{"\N{APPROXIMATELY EQUAL TO}"sv}; // "≅" or {0342, 137, 133}
    constexpr int Ho{42}; // H₀
 
    const auto min_buffer_size{std::formatted_size(fmt_str, sub_zero, aprox_equ, Ho)};
    std::cout << "Min buffer size = " << min_buffer_size << '\n';
 
    // Use std::vector as dynamic buffer. The buffer does not include the trailing '\0'.
    std::vector<char> buffer(min_buffer_size);
 
    std::format_to_n(buffer.data(), buffer.size(), fmt_str, sub_zero, aprox_equ, Ho);
    std::cout << "Buffer: "
              << std::quoted(std::string_view{buffer.data(), min_buffer_size})
              << '\n';
 
    // Print the buffer directly after adding the trailing '\0'.
    buffer.push_back('\0');
    std::cout << "Buffer: " << std::quoted(buffer.data()) << '\n';
}

# Defect reports

DRApplied toBehavior as publishedCorrect behavior
P2216R3C++20throws std::format_error for invalid format stringinvalid format string results in compile-time error
P2418R2C++20objects that are neither const-usable nor copyable(such as generator-like objects) are not formattableallow formatting these objects
P2508R1C++20there’s no user-visible name for this facilitythe name basic_format_string is exposed

# See also