In c++, how to create a conversion from T to T*?

Question

In preparing to a OOP exam, I enjoyed seeing g++ compile the following code (without an instantiation) even though it appeared to make no sense:

template<class T> void f() {
    T t = "a";
    t += 5.6;
    t->b();
    T* p = t;
    p = p*(t/"string");
}

I then set out on a challenge to make this instantiate and compile.

I created the following class:

class A {
    public:
    A(const char* s) {}
    void operator+=(double d) {}
    A operator/(char* str) {return A("");}
    A* operator->() {return this;}
    A* operator=(A& a) {return &a;}
    void b() {}
};
A* operator*(A* a, A b) {return new A("");}

which allowed almost all of the template to work, except the line

T* p = t;

My question is, what operator or constructor will make this line work? Currently it gives me "error: cannot convert ‘A’ to ‘A*’ in initialization"

Answer 1

My C/C++ is pretty rusty but I really want to say that it's just

T* p = &t;

Answer 2

Use the type conversion operator?

class A {
   //...
public:
    template< class T > 
    operator T*() { return this; }
};

But clearly, it's bad practice.

Answer 3

That is a completely meaningless line.

But to make it compilable, you can provide the conversion operator:

operator A* () { return 0; }

Hope you realize how evil this is.

Answer 4

class A {
  // ...
  template <typename T>
  operator T*() const
  {
    // ...
  }
  // ...
};

Answer 5

While Klaim told you how, implicit conversions to pointers of T may lead to subtile bugs - so please don't do this in production code.
Explicitly taking the address via the & operator avoid these bugs and says more clearly what your intent was.

Also note that your assignment operator looks, lets say unusual. See the C++ FAQ lite entry for more details.