std::format(3)

NAME
std::format - std::format

Synopsis
   Defined in header <format>
   template< class... Args >
   std::string format( /*format_string<Args...>*/ fmt, Args&&... args (1) (since C++20)
   );
   template< class... Args >
   std::wstring format( /*wformat_string<Args...>*/ fmt, Args&&... (2) (since C++20)
   args );
   template< class... Args >
   std::string format( const std::locale& loc, (3) (since C++20)
   /*format_string<Args...>*/ fmt, Args&&... args );
   template< class... Args >
   std::wstring format( const std::locale& loc, (4) (since C++20)
   /*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.str, std::make_format_args(args...));
   2) equivalent to return std::vformat(fmt.str, std::make_wformat_args(args...));
   3) equivalent to return std::vformat(loc, fmt.str, std::make_format_args(args...));
   4) equivalent to return std::vformat(loc, fmt.str, std::make_wformat_args(args...));
   The behavior is undefined if std::formatter<Ti, CharT> does not meet the
   BasicFormatter requirements for any Ti in Args (as required by std::make_format_args
   and std::make_wformat_args), where CharT is char for overloads (1,3), wchar_t for
   overloads (2,4).

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 conversion 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 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 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
 }

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
                      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)