Question

I have a problem understanding why a certain implicit conversion is not working as I expect it. I have the following class

ref class ManagedWStringArrayWrapper
{
    wchar_t** m_pointer;

public:

    operator wchar_t**()
    {
        return m_pointer;
    }
};

and I thought this would implicitly convert to const wchar_t ** as well – but it doesn’t. Can someone tell me why?

Answer 1

Conversion from T** to T const** is not as intuitive as you might expect — in fact, it’s given as an example in the standard itself as const-incorrect code.

The example given is:

#include <cassert>  

int main() {  
  char* p = 0;  

  //char const** a = &p; // not allowed, but let's pretend it is  
  char const** a = (char const**)&p; // instead force the cast to compile  

  char const* orig = "original";  
  *a = orig; // type of *a is char const*, which is the type of orig, this is allowed  

  assert(p == orig); // oops! char* points to a char const*  
}

Study the above scenario for a while and it will become clear that this is not the same as a conversion T* → T const*. Not at all!

I asked the same question in a blog post, and the FAQ has an entry on it too.