[C++] Optional parameter for reference to pointer?

Question

Hi,

how do I the second parameter become optional?

template <typename T>
inline void Delete (T *&MemoryToFree,
 T *&MemoryToFree2 = ){
 delete MemoryToFree;
 MemoryToFree = NULL;

 delete MemoryToFree2;
 MemoryToFree2 = NULL;

}

I tried several things after the = operator, like NULL, (T*)NULL etc can this be done?

the only way the compiler let me do it was this: using overload...

    template <typename T, typename T2>
inline void Delete (T *&MemoryToFree, T2 *&MemoryToFree2){
 delete MemoryToFree;
 MemoryToFree = NULL;

 delete MemoryToFree2;
 MemoryToFree2 = NULL;
}

just to learn...

Answer 1

You could just overload the function

template <typename T>
inline void Delete (T *&MemoryToFree){
        delete MemoryToFree;
        MemoryToFree = NULL;
}

template <typename T, typename T2>
inline void Delete (T *&MemoryToFree, T2 *&MemoryToFree2){
        delete MemoryToFree;
        MemoryToFree = NULL;

        delete MemoryToFree2;
        MemoryToFree2 = NULL;
}

Answer 2

No this is not possible. You cannot use default paramters with references.

Answer 3

The following code complies fine for me using gcc 4.3.2.

void fn( int*const & i = 0 )
{
    delete i;
}

so maybe you could adapt that. You need the *const to permit the default argument. Default arguments are permitted for references-to-const.

Edit

Just realised that marking it *const is no use to you since you want to be able to null off the pointer as well as delete it.

Answer 4

You could always write a simple "on-demand static lvalue generator" and use it as the default value for your parameter

template <typename T> inline T& get_lvalue() {
  static T t;
  return t;
}

In your code

template <typename T> 
inline void Delete(T *&MemoryToFree, T *&MemoryToFree2 = get_lvalue<T*>())