Home Is there a way to make the compiler include functions from outer scopes when picking a candidate?

Questions

Is there a way to make the compiler include functions from outer scopes when picking a candidate?

January 1, 2022

Consider this class and variadic member function:

class foo
{
public:
    template<class C, class... Cs>
    void add(C&& c, Cs&&... cs)
    {
        ...
        
        add(cs...);
    }
private:
    void add(){ }
};

Is there a way to place the empty add overload that terminates the recursion inside another scope? Please ignore whether or not this is generally a good idea, I am strictly asking for how one could do it, if at all. I would like to place it inside an impl namespace like so:

namespace impl
{
    void add() {}
}

class foo
{
public:
    template<class C, class... Cs>
    void add(C&& c, Cs&&... cs)
    {
        ...
        
        using namespace impl;
        add(cs...);
    }
};

But the above code does not work when instatiated. The compiler complains that it can’t find a matching function call for add when the parameter pack is empty.

Is there a way to achieve this using some scope-hackery?

>Solution :

How about using if constexpr:

namespace impl { void add() {} }

class foo {
 public:
  template<class C, class... Cs>
  void add(C&& c, Cs&&... cs)
  {
    if constexpr (sizeof...(Cs) == 0)
      impl::add();
    else
      add(cs...);
  }
};