Linking issue while using C++ Templates

[Ian Lance Taylor]

Rajat <myself_rajat@yahoo.com> writes:

> //A.h -- sample code.
> template<typename T> class A
> {
> public:
>     static A * getInstance ()
>     {
>         if (!_instance)
>             _instance = new A ();
>         return _instance;
>     }
>
>  T * getApp () const { return _pApp; }
>
> private:
>     T * _pApp;
>     static A * _instance;
> };

...

> File x.cpp compiles perfactly, but while linking it throws error saying,
> x.o(.gnu.linkonce.t._ZN9AI11BE11getInstanceEv+0x9): In function `A<B>::getInstance()':
> A.h:: undefined reference to `A<B>::_instance'
> collect2: ld returned 1 exit status
>
> Any clue ?

You have to define your static variables, not just declare them.  This
is standard C++--it doesn't have anything to do with gcc as such.

template<typename T> A<T>* A<T>::_instance;

Ian