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std::formatted_size

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)

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-ids 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.

For basic types and standard string types, the format specification is interpreted as standard format specification.
For chrono types, the format specification is interpreted as chrono format specification.
For std::thread::id, the format specification is interpreted as thread id format specification.
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

#include <format>
#include <iostream>
#include <vector>
#include <string_view>
 
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 { "₀"sv };  // { "\u2080"sv } => { 0xe2, 0x82, 0x80 };
    constexpr auto aprox_equ { "≅"sv }; // { "\u2245"sv } => { 0xe2, 0x89, 0x85 };
    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. Note: 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::string_view{buffer.data(), min_buffer_size} << "\"\n";
 
    // Or we can print the buffer directly by adding the trailing '\0'.
    buffer.push_back('\0');
    std::cout << "Buffer: \"" << 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