c++ - Singleton and Interface implementation -

i have interface class interface pure virtual methods.

class interface { public:     virtual void dosomething() = 0; } 

also, there class implementation implements interface.

class implementation : public interface {     void dosomething(); } 

in program need have singleton (single instance) of implementation/interface pair. there many pairs of implementation/interface classes in program, few implementation classes 1 interface class.

questions:

1. should invoke interface or implementation class rest of program whenever need use class? how should so?

    //interface needs know implementation  interface * module = interface::instance();   //no enforcement of interface  implementation * module = implementation::instance();   //a programmer needs remember implementation , interface names  interface * module = implementation::instance();   //may there better way 

2. how should method instance() like?

"1) should invoke interface or implementation class rest of program whenever need use class? how should so?"

use interface, less clutter code implementation::instance() calls:

 interface& module = implementation::instance();        // ^  

note reference, assignment , copy won't work.

"2) how should method instance() like?"

the common consensus use scott meyer's approach:

 implementation& instance() {      static implementation theinstance;      return theinstance;  } 

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the better alternative not use singleton @ make code ready operate on 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);  };