Cloning

3.2 Cloning

At run time, we may need to clone an object that is passed by pointer. This is illustrated in listing 2.

Listing 2:

cloneme

1class A
2{
3  public:
4    virtual void m1(void);
5};
6
7class B : public A
8{
9  public:
10    virtual void m1(void);
11};
12
13void test(const A *pA)
14{
15  A *pClone;
16
17  pClone = new A(*pA);
18  // do something that modifies *pClone
19  pA->m1();
20  // get rid of the clone when we are done
21  delete pClone;
22  // leaving *pA intact
23}

However, we have a problem here. The creation of the object pointed to by pClone uses a regular cloning (copy) constructor. Because the type pointed to by pA is class A, we cannot use any other constructors. As a result, the resulting clone is a genuine class A object, but nothing more. Note that pA can point to an object of class B.

This means that when m1 is invoked, the clone invokes A::m1, even though pA->m1() would have invoked B::m1. There are very few, if any, cases that we want a clone to revert to a super close instead of the class of the original object.

Fortunately, there is a way to get around the limitations of the copy constructor method.

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