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When dealing with generic code in C++, I would find a std::identity functor (like std::negate) very useful. Is there a particular reason why this is not present in the standard library?

Vincent
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    It was present in pre-C++11 drafts. IIRC, it was used to prevent template argument deduction in [`std::forward`](http://en.cppreference.com/w/cpp/utility/forward). Somewhere along the way, someone realized `remove_reference` was needed for `forward`, which also took care of the non-deducible context part, and maybe `identity` was no longer used by anything else, so it got dropped. – Praetorian Mar 10 '16 at 04:56
  • Interestingly, some C++ implementations seem to use a `std::_Identity` template internally. – JAB Sep 07 '16 at 14:12

2 Answers2

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Soon after std::identity was introduced, issues began to appear, starting with conflicts to pre-cpp98 definitions of std::identity appearing as extensions: https://groups.google.com/a/isocpp.org/forum/#!topic/std-proposals/vrrtKvA7cqo This site might give a little more history for it.

  • Identity can be used to prevent template argument deduction. Boost::clamp uses this trick. See also https://stackoverflow.com/questions/41767240/what-is-stdidentity-and-how-it-is-used. – gast128 Dec 11 '17 at 16:13
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Since C++20, there is an std::identity functor type with an operator() template member function. This function call operator returns its argument.

For example, if you have such a function template:

template<typename T, typename Operation>
void print_collection(const T& coll, Operation op) {
    std::ostream_iterator<typename T::value_type> out(std::cout, " ");
    std::transform(std::begin(coll), std::end(coll), out, op);
    std::cout << '\n';
}

and wanted to print the elements of vec:

std::vector vec = {1, 2, 3};

you would do something like:

print_collection(vec, [](auto val) { return val; });

With std::identity, you could do:

print_collection(vec, std::identity());

The line above seems to specify this intention more clearly.

JFMR
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