Function passed as template argument

I’m looking for the rules involving passing C++ templates functions as arguments.

This is supported by C++ as shown by an example here:

#include <iostream>

void add1(int &v)
{
  v+=1;
}

void add2(int &v)
{
  v+=2;
}

template <void (*T)(int &)>
void doOperation()
{
  int temp=0;
  T(temp);
  std::cout << "Result is " << temp << std::endl;
}

int main()
{
  doOperation<add1>();
  doOperation<add2>();
}

Learning about this technique is difficult, however. Googling for “function as a template argument” doesn’t lead to much. And the classic C++ Templates The Complete Guide surprisingly also doesn’t discuss it (at least not from my search).

The questions I have are whether this is valid C++ (or just some widely supported extension).

Also, is there a way to allow a functor with the same signature to be used interchangeably with explicit functions during this kind of template invocation?

The following does not work in the above program, at least in Visual C++, because the syntax is obviously wrong. It’d be nice to be able to switch out a function for a functor and vice versa, similar to the way you can pass a function pointer or functor to the std::sort algorithm if you want to define a custom comparison operation.

   struct add3 {
      void operator() (int &v) {v+=3;}
   };
...

    doOperation<add3>();

Pointers to a web link or two, or a page in the C++ Templates book would be appreciated!