Again with the type erasure: std::function

We observed that for std::any, the salient operations were as follows:

• Constructing a copy of the contained object
• Constructing a copy of the contained object "by move"
• Getting the typeid of the contained object

Suppose we were to add one to this set of salient operations? Let's say our set is:

• Constructing a copy of the contained object
• Constructing a copy of the contained object "by move"
• Getting the typeid of the contained object
• Calling the contained object with a particular fixed sequence of argument types A..., and converting the result to some particular fixed type R

The type-erasure of this set of operations corresponds to the standard library type std::function<R(A...)>!

    int my_abs(int x) { return x < 0 ? -x : x; }
    long unusual(long x, int y = 3) { return x + y; }

    void test()
  ...