FreeBSD Manual Pages
std::format(3) C++ Standard Libary std::format(3) NAME std::format - std::format Synopsis Defined in header <format> template< class... Args > std::string format( std::format_string<Args...> fmt, Args&&... (1) (since C++20) args ); template< class... Args > std::wstring format( std::wformat_string<Args...> fmt, Args&&... (2) (since C++20) args ); template< class... Args > std::string format( const std::locale& loc, (3) (since C++20) std::format_string<Args...> fmt, Args&&... args ); template< class... Args > std::wstring format( const std::locale& loc, (4) (since C++20) std::wformat_string<Args...> fmt, Args&&... args ); Format args according to the format string fmt, and return the re- sult as a string. If present, loc is used for locale-specific formatting. 1) Equivalent to return std::vformat(fmt.get(), std::make_for- mat_args(args...));. 2) Equivalent to return std::vformat(fmt.get(), std::make_wfor- mat_args(args...));. 3) Equivalent to return std::vformat(loc, fmt.get(), std::make_format_args(args...));. 4) Equivalent to return std::vformat(loc, fmt.get(), std::make_wformat_args(args...));. Since P2216R3, std::format does a compile-time check on the format string (via the helper type std::format_string or std::wformat_string). If it is found to be invalid for the types of the arguments to be formatted, a compilation error will be emitted. If the format string cannot be a compile-time constant, or the com- pile-time check needs to be avoided, use std::vformat or std::runtime_format on fmt (since C++26) instead. The following requirements apply to each type T in Args, where CharT is char for overloads (1,3), wchar_t for overloads (2,4): * std::formatter<T, CharT> must satisfy BasicFormatter * std::formatter<T, CharT>::parse() must be constexpr since P2216R3 (std::vformat does not have this requirement) Parameters 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 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. arg-id - The arg-ids in a format string must all be present or all be omitted. Mixing manual and automatic in- dexing is an error. fmt - the format specification defined by the std::formatter format-spec - specialization for the corresponding argu- ment. Cannot start with }. * 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 range types, the format specification is inter- preted as range format specification. * For std::pair and std::tuple, the format specification is interpreted as tuple format specification. * For std::thread::id and std::stacktrace_entry, see (since C++23) thread id format specification and stacktrace entry format specification. * For std::basic_stacktrace, no format specifier is allowed. * For std::filesystem::path, see path format (since C++26) 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 A string object holding the formatted result. Exceptions Throws std::bad_alloc on allocation failure. Also propagates excep- tion thrown by any formatter. Notes It is not an error to provide more arguments than the format string requires: std::format("{} {}!", "Hello", "world", "something"); // OK, produces "Hello world!" As of P2216R3, it is an error if the format string is not a constant expression. std::vformat can be used in this case. std::string f(std::string_view runtime_format_string) { // return std::format(runtime_format_string, "foo", "bar"); // er- ror return std::vformat(runtime_format_string, std::make_for- mat_args("foo", "bar")); // OK } std::runtime_format can be used directly on std::format instead of std::vformat which requires std::basic_format_args as an argument. (since std::string f(std::string_view runtime_format_string) C++26) { return std::format(std::runtime_format(runtime_format_string), "foo", "bar"); } Example // Run this code #include <format> #include <iostream> #include <string> #include <string_view> template<typename... Args> std::string dyna_print(std::string_view rt_fmt_str, Args&&... args) { return std::vformat(rt_fmt_str, std::make_format_args(args...)); } int main() { std::cout << std::format("Hello {}!\n", "world"); std::string fmt; for (int i{}; i != 3; ++i) { fmt += "{} "; // constructs the formatting string std::cout << fmt << " : "; std::cout << dyna_print(fmt, "alpha", 'Z', 3.14, "unused"); std::cout << '\n'; } } Output: Hello world! {} : alpha {} {} : alpha Z {} {} {} : alpha Z 3.14 Defect reports The following behavior-changing defect reports were applied retroac- tively to previously published C++ standards. DR Applied to Behavior as published Correct behavior P2216R3 C++20 throws std::format_error for invalid format string results in invalid format string compile-time er- ror objects that are neither P2418R2 C++20 const-usable nor copyable allow formatting these objects (such as generator-like objects) are not formattable P2508R1 C++20 there's no user-visible name for the name ba- sic_format_string is this facility exposed 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 2024.06.10 std::format(3)
NAME | Synopsis | Parameters | Return value | Exceptions | Notes | Example | Output: | See also
Want to link to this manual page? Use this URL:
<https://man.freebsd.org/cgi/man.cgi?query=std::format&sektion=3&manpath=FreeBSD+Ports+15.1.quarterly>