nullptr as a template parameter

Question

I have a template like:

template <class A, class B>
void func(A* a, B* b){
  ...
}

In some cases it happens that parameter B* b is not needed and therefore, I try to use a nullptr:

MyA a;
func(&a, nullptr);

The compiler doesn't like that since nullptr somehow is not a type.

How can I deal with that situation? The only idea is to just use a dummy type for that case.

Answer 1

The problem is that nullptr is not actually a pointer, but an object of type nullptr_t. So it cannot match either A* or B*. One option would be to provide an overload to specifically handle nullptr_t.

template<class A>
void func(A* a, nullptr_t)
{
    func(a, (int*)nullptr);
}

If you also want to allow the first argument to be nullptr, you can provide 2 more overloads. One to handle just the first argument, and one to handle both.

template<class B>
void func(nullptr_t, B* b)
{
    func((int*)nullptr, b);
}

void func(nullptr_t, nullptr_t)
{
    func((int*)nullptr, (int*)nullptr);
}

For any more arguments, this approach becomes unfeasible without code generation, as the number of overloads required is an exponential function of the number of arguments. In that case, I would recommend jrok's approach.