Equivalent of C++ std::function

[Yuushi via Digitalmars-d-learn]

I was wondering if D had something akin to std::function in C++.
Say I have some functions in an associative array, for example:

     auto mapping = ['!' : (string a) => toUpper!string(a), '^' : 
(string a) => capitalize!string(a)];

What I want to do is basically declare something like:

    function string(string) transform;
    if(<some condition>) {
         transform = mapping[<lookup>];
    }

In C++, this could be done by declaring:

     std::function<string(string)> transform;

Is there an equivalent D construct for this?