Question

// Destructor.  If there is a C object, delete it.
// We don't need to test ptr_ == NULL because C++ does that for us  

    ~scoped_ptr() {
       enum { type_must_be_complete = sizeof(C) };
       delete ptr_;
    }

Note: C is a template parameter

I know we cant delete a null pointer, an exception will be raised.
So in this case, the enum definition must be doing something to prevent that.
In production, sometimes we dont want to end a program simple because we have a null pointer, we may want to look at alternative scenario, when the pointer is null.
And this code is used in production, almost everywhere?

Thanks guys.

Answer 1

it’s effectively a static assertion for the deletion. the implementation wants to know if it is dealing with a type whose declaration is visible before deleting the variable, rather than a forward declaration.

Your compiler will emit an error when you ask it the size of an incomplete type:

struct S;
enum { Size = sizeof(S) };

Update

As your compiler and Matthieu M. will tell you — delete-ing an incomplete type is undefined.