Question

#include <iostream> using namespace std;  /*         TA        <-- defines static function      /    \     |      B    <-- subclass TA, inherits it so B::StaticFunc can be used.      \    /        C        <-- want to inherit static func from A, subclass B privately */  template <class T> class TA { public:     // return ptr to new instance of class T     static T * instance()     {         static T inst;         return &inst;     } };   class B : public TA<B> { public:     void Func() { cout << 'B' << endl; } };   /* HERE:  class C : public TA<C> */ class C : public TA<C>, private B { public:     void Func() { cout << 'C' << endl; } };  int main() {     C test();      B::instance()->Func();     C::instance()->Func();      /*      Expected output:        B        C       Actual error:         error: `instance' is not a member of `C'|          If i do not subclass B with C, then it compiles. However, I need         C to provide some other functionality and but a subclassable Singleton     */      return 0; } 

Answer 1

I get a different, more reasonable error (with g++ 4.3.3):

tst.cpp: In function ‘int main()’: tst.cpp:49: error: reference to ‘instance’ is ambiguous tst.cpp:22: error: candidates are: static T* TA<T>::instance() [with T = B] tst.cpp:22: error:                 static T* TA<T>::instance() [with T = C] 

This can be fixed by explicitly specifying which version of instance should be used in C:

class C : public TA<C>, private B { public:     using TA<C>::instance;     void Func() { cout << 'C' << endl; } };