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    std::formatted_size

    From cppreference.com
     
    C++
     
    Text processing library
     
    Formatting library
    Standard format specification
    Formatting functions
    (C++20)
    (C++20)
    (C++20)
    (C++20)
    (C++20)
    (C++20)
    Format strings
    (C++20)(C++20)(C++20)
    (C++26)
    Formatting concepts
    (C++23)
    Formatter
    (C++20)
    (C++23)
    (C++23)
    (C++23)
    (C++23)
    (C++20)(C++20)(C++20)
    (C++20)(C++20)(C++20)
    (C++23)
    (C++23)
    Formatting arguments
    (C++20)
    (C++20)
    (C++20)(C++20)(C++20)
    (C++20) (deprecated in C++26)
    (C++20)(C++20)
    Format error
    (C++20)
     
    Defined in header <format>
    template< class... Args > std::size_t formatted_size( std::format_string<Args...> fmt, Args&&... args );
    		 (1) (since C++20)
    template< class... Args > std::size_t formatted_size( std::wformat_string<Args...> fmt, Args&&... args );
    		 (2) (since C++20)
    template< class... Args > std::size_t formatted_size( const std::locale& loc, std::format_string<Args...> fmt, Args&&... args );
    		 (3) (since C++20)
    template< class... Args > std::size_t formatted_size( const std::locale& loc, std::wformat_string<Args...> fmt, Args&&... args );
    		 (4) (since C++20)
    See More

    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.

    The behavior is undefined if std::formatter<std::remove_cvref_t<Ti>, CharT> does not meet the BasicFormatter requirements for any Ti in Args.

    Parameters

    fmt - an object that represents the format string. The format string consists of
    • ordinary characters (except { and }), which are copied unchanged to the output,
    • escape sequences {{ and }}, which are replaced with { and } respectively in the output, and
    • replacement fields.

    Each replacement field has the following format:

    { arg-id (optional) } (1)
    { arg-id (optional) : format-spec } (2)
    1) replacement field without a format specification
    2) replacement field with a format specification
    arg-id - specifies the index of the argument in args whose value is to be used for formatting; if it is omitted, the arguments are used in order.

    The arg-id s in a format string must all be present or all be omitted. Mixing manual and automatic indexing is an error.

    format-spec - the format specification defined by the std::formatter specialization for the corresponding argument. Cannot start with }.

    (since C++23)
    (since C++26)
    • For other formattable types, the format specification is determined by user-defined formatter specializations.
    args... - arguments to be formatted
    loc - std::locale used for locale-specific formatting

    Return value

    The total number of characters in the formatted string.

    Exceptions

    Propagates any exception thrown by formatter.

    Example

    Run this code
    #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';
}

    Output: 

    Min buffer size = 37
Buffer: "Hubble's H₀ ≅ *42* miles/sec/mpc."
Buffer: "Hubble's H₀ ≅ *42* miles/sec/mpc."

    Defect reports

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

    DR Applied to Behavior as published Correct behavior
    P2216R3 C++20 throws std::format_error for invalid format string invalid format string results in compile-time error
    P2418R2 C++20 objects that are neither const-usable nor copyable
    (such as generator-like objects) are not formattable     		allow formatting these objects
    P2508R1 C++20 there's no user-visible name for this facility the name basic_format_string is exposed

    See also

    (C++20)
    		 writes out formatted representation of its arguments through an output iterator
    (function template) [edit]
    (C++20)
    		 writes out formatted representation of its arguments through an output iterator, not exceeding specified size
    (function template) [edit]