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std::formatted_size(3) C++ Standard Libary std::formatted_size(3) NAME std::formatted_size - std::formatted_size Synopsis Defined in header <format> template< class... Args > std::size_t formatted_size( /*format_string<Args...>*/ fmt, (1) (since C++20) Args&&... args ); template< class... Args > std::size_t formatted_size( /*wformat_string<Args...>*/ fmt, (2) (since C++20) Args&&... args ); template< class... Args > std::size_t formatted_size( const std::locale& loc, (3) (since C++20) /*format_string<Args...>*/ fmt, Args&&... args ); template< class... Args > std::size_t formatted_size( const std::locale& loc, (4) (since C++20) /*wformat_string<Args...>*/ fmt, Args&&... args ); 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 lo- cale-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 parameter of unspecified type, whose initialization is valid only if the argument is convertible to std::string_view (for (1,3)) or std::wstring_view (for (2,4)), and the result of conver- sion is a constant expression and a valid format string for Args. 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) fmt - 1) replacement field without a format specification 2) replacement field with a format specification specifies the index of the argument in args whose value is to be used for formatting; if it is omitted, the arguments arg-id - are used in order. The arg-ids in a format string must all be present or all be omitted. Mixing manual and automatic index- ing is an error. format-spec - the format specification defined by the std::formatter specialization for the corresponding argu- ment. * For basic types and standard string types, the format specification is interpreted as standard format specification. * For chrono types, the format specification is inter- preted as chrono 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 // Run this code #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 { ""sv }; // { "\u2080"sv } => { 0xe2, 0x82, 0x80 }; constexpr auto aprox_equ { ""sv }; // { "\u2245"sv } => { 0xe2, 0x89, 0x85 }; constexpr int Ho { 42 }; // H 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 in- clude 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 H *42* miles/sec/mpc." Buffer: "Hubble's H *42* miles/sec/mpc." Defect reports The following behavior-changing defect reports were applied retroac- tively to previously published C++ standards. DR Applied to Behavior as published Cor- rect behavior throws std::format_error for invalid invalid format string P2216R3 C++20 format string results in compile-time error objects that are neither const-usable nor P2418R2 C++20 copyable allow formatting these (such as generator-like objects) are not objects formattable See also format_to writes out formatted representation of its arguments through an output (C++20) iterator (function template) format_to_n writes out formatted representation of its arguments through an output (C++20) iterator, not exceeding specified size (function template) http://cppreference.com 2022.07.31 std::formatted_size(3)
NAME | Synopsis | Parameters | Return value | Exceptions | Example | Output: | See also
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