Boost C++ Libraries

The header <boost/functional/identity.hpp> provides the function object boost::identity whose operator() returns its argument. It is an implementation of C++20's std::identity that supports C++03 and above.

It is commonly used as the default projection in constrained algorithms.

template<class Range, class Projection = boost::identity>
void print(Range&& range, Projection projection = {});

namespace boost {

struct identity {
    using is_transparent = unspecified;

    template<class T>
    T&& operator()(T&& value) const noexcept;
};

} /* boost */