c++qtsizeofqscopedpointer

The use of sizeof in the implementation of QScopedPointer


To understand how Qt prevent incomplete type I went through the header file of qscopedpointer.h.The related part is as follows:

template <typename T>
struct QScopedPointerDeleter
{
    static inline void cleanup(T *pointer)
    {
        // Enforce a complete type.
        // If you get a compile error here, read the section on forward declared
        // classes in the QScopedPointer documentation.
        typedef char IsIncompleteType[ sizeof(T) ? 1 : -1 ];
        (void) sizeof(IsIncompleteType);

        delete pointer;
    }
};

I know when using sizeof on incomplete type the compilation will fail.But what do the array and second sizeof do? Is sizeof alone not enough?


Solution

  • An array is used so the negative size of it will give a compile time error. The second line makes sure the compiler can't skip the evaluation of sizeof(T) by ensuring the actual size of IsIncompleteType is used.