function_ref

A lightweight non-owning reference to a callable.

Clang + GCC: MSVC:

Use tl::function_ref instead of std::function whenever you don't need to own the callable. The most common case for this is function parameters which aren't stored anywhere:

void foo (function_ref<int(int)> func) {
    std::cout << "Result is " << func(21); //42
}

foo([](int i) { return i*2; });

Full documentation available here.