From cppreference.com
Defined in header
template<class...Args>std::stringformat(std::format_string<Args...>fmt,Args&&...args); (1) (since C++20)template<class...Args>std::wstringformat(std::wformat_string<Args...>fmt,Args&&...args); (2) (since C++20)template<class...Args>std::stringformat(conststd::locale&loc,std::format_string<Args...>fmt,Args&&...args); (3) (since C++20)template<class...Args>std::wstringformat(conststd::locale&loc,std::wformat_string<Args...>fmt,Args&&...args); (4) (since C++20)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 returnstd::vformat(fmt.get(),std::make_format_args(args...));.
2) Equivalent to returnstd::vformat(fmt.get(),std::make_wformat_args(args...));.
3) Equivalent to returnstd::vformat(loc,fmt.get(),std::make_format_args(args...));.
4) Equivalent to returnstd::vformat(loc,fmt.get(),std::make_wformat_args(args...));.
Since
, 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
or
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
std::formatter<T,CharT>::parse() must be constexpr since
(
does not have this requirement)
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-id s 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
specialization for the corresponding argument. Cannot start with }. For basic types and standard string types, the format specification is interpreted as
.
For chrono types, the format specification is interpreted as
.
For range types, the format specification is interpreted as
.
For
and
, the format specification is interpreted as
.
For
and
, see
thread id format specification
and
stacktrace entry format specification
.
For
, no format specifier is allowed.
(since C++23)For other formattable types, the format specification is determined by user-defined formatter specializations.
args... - arguments to be formatted loc -
used for locale-specific formatting Return value
A string object holding the formatted result.
Exceptions
Throws
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
, it is an error if the format string is not a constant expression.
can be used in this case.
std::stringf(std::string_viewruntime_format_string){// return std::format(runtime_format_string, "foo", "bar"); // errorreturnstd::vformat(runtime_format_string,std::make_format_args("foo","bar"));// OK}std::dynamic_format can be used directly on std::format instead of
which requires
as an argument.
std::stringf(std::string_viewruntime_format_string){returnstd::format(std::dynamic_format(runtime_format_string),"foo","bar");}(since C++26)Example
Run this code
#include<format>#include<iostream>#include<string>#include<string_view>template<typename...Args>std::stringdyna_print(std::string_viewrt_fmt_str,Args&&...args){returnstd::vformat(rt_fmt_str,std::make_format_args(args...));}intmain(){std::cout<<std::format("Hello {}!\n","world");std::stringfmt;for(inti{};i!=3;++i){fmt+="{} ";// constructs the formatting stringstd::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
C++20 throws
for invalid format string invalid format string results in compile-time error
C++20 objects that are neither const-usable nor copyable
(such as generator-like objects) are not formattable allow formatting these objects
C++20 there's no user-visible name for this facility the name basic_format_string is exposed See also
(C++20)
writes out formatted representation of its arguments through an output iterator
(function template)
(C++20)
writes out formatted representation of its arguments through an output iterator, not exceeding specified size
(function template)