Singleton and Interface implementation

Question

I have an interface class Interface with pure virtual methods.

class Interface
{
public:
    virtual void DoSomething() = 0;
}

Answer 1

"1) Should I invoke Interface or Implementation class from the rest of my program whenever I need to use the class? How exactly should I do so?"

Use the interface, that will less clutter your code with Implementation::Instance() calls:

 Interface& module = Implementation::Instance();
       // ^ 

Note the reference, assignment and copy won't work.

"2) How should the method Instance() look like?"

The common consensus is to use Scott Meyer's approach:

 Implementation& Instance() {
     static Implementation theInstance;
     return theInstance;
 }

---

The better alternative is not to use a singleton at all but make your code ready to operate on the Interface exclusively:

 class Interface {
      // ...
 };

 class Impl : public Interface {
      // ...
 };

 class Client {
     Interface& if_;
 public:
     Client(Interface& if__) : if_(if__) {}
      // ...
 }

 int main() {
     Impl impl;
     Client client(impl);
 };