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 result as a string.
   If present, loc is used for locale-specific formatting.
   1) Equivalent to return std::vformat(fmt.get(), std::make_format_args(args...));.
   2) Equivalent to return std::vformat(fmt.get(), std::make_wformat_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 compile-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-id s in a format string must all be present or all
                        be omitted. Mixing manual and automatic indexing is an
                        error.
   fmt - the format specification defined by the std::formatter
             format-spec - specialization for the corresponding argument. 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 interpreted as chrono
                 format specification.
               * For range types, the format specification is interpreted
                 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 exception 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"); // error
     return std::vformat(runtime_format_string, std::make_format_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 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 results in
                      invalid format string compile-time error
                      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 basic_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)