What is the point of function pointers?

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Most examples boil down to callbacks: You call a function f() passing the address of another function g(), and f() calls g() for some specific task. If you pass f() the address of h() instead, then f() will call back h() instead.

Basically, this is a way to parametrize a function: Some part of its behavior is not hard-coded into f(), but into the callback function. Callers can make f() behave differently by passing different callback functions. A classic is qsort() from the C standard library that takes its sorting criterion as a pointer to a comparison function.

In C++, this is often done using function objects (also called functors). These are objects that overload the function call operator, so you can call them as if they were a function. Example: 

class functor {
  public:
     void operator()(int i) {std::cout << "the answer is: " << i << '\n';}
};

functor f;
f(42);

The idea behind this is that, unlike a function pointer, a function object can carry not only an algorithm, but also data: 

class functor {
  public:
     functor(const std::string& prompt) : prompt_(prompt) {}
     void operator()(int i) {std::cout << prompt_ << i << '\n';}
  private:
     std::string prompt_;
};

functor f("the answer is: ");
f(42);

Another advantage is that it is sometimes easier to inline calls to function objects than calls through function pointers. This is a reason why sorting in C++ is sometimes faster than sorting in C.

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