Question

Which is the difference between these two codes? Is there a memory leak in the first case?

no destructor defined

 class Library
{
private:
    Book books[50];
    int index;
public:
    Library()
    {

        index=0;
    }
};

or with destructor defined

class Library
{
private:
    Book *books;
    int index;
public:
    Library()
    {
        books=new Book[50];
        index=0;
    }
    ~Library()
    {
        delete books;
    }
};

Answer 1

delete books;

should be

delete[] books;

In the second one: books is allocated as an array, so must be deleted as an array. The first version does not have a memory leak.

The difference is that in the first case the memory for books is is contained within the allocation for each instance of Library, wheras in the second case it is allocated separately, using the heap.