< cpp‎ | experimental‎ | ranges
Technical specifications
Filesystem library (filesystem TS)
Library fundamentals (library fundamentals TS)
Library fundamentals 2 (library fundamentals 2 TS)
Extensions for parallelism (parallelism TS)
Extensions for parallelism 2 (parallelism TS v2)
Extensions for concurrency (concurrency TS)
Concepts (concepts TS)
Ranges (ranges TS)
Special mathematical functions (special math TR)
General utilities library
Utility components
Function objects
Metaprogramming and type traits
Tagged pairs and tuples
struct identity;
(ranges TS)

ranges::identity is a function object type whose operator() returns its argument unchanged. It is used as the default projection for all Ranges TS algorithms.

Member types

Member type Definition
is_transparent /* unspecified */

Member functions

returns the argument unchanged
(public member function)


template< class T>
constexpr T&& operator()( T&& t ) const noexcept;

Returns std::forward<T>(t).


t - argument to return

Return value



The member type is_transparent indicates to the caller that this function object is a transparent function object: it accepts arguments of arbitrary types and uses perfect forwarding, which avoids unnecessary copying and conversion when the function object is used in heterogeneous context, or with rvalue arguments. In particular, template functions such as std::set::find and std::set::lower_bound make use of this member type on their Compare types.