Default function that just returns the passed value?

Question

As a lazy developer, I like to use this trick to specify a default function:

template 

        

Answer 1

There is no standard functor that does this, but it is easy enough to write (though the exact form is up for some dispute):

struct identity {
    template
    constexpr auto operator()(U&& v) const noexcept
        -> decltype(std::forward(v))
    {
        return std::forward(v);
    }
};

This can be used as follows:

template 
void index(std::array &x, Function&& f = Function())
{
    for (unsigned int i = 0; i < Size; ++i) {
        x[i] = f(i);
    }
}