GCC在调试模式下崩溃，在发布模式下运行正常吗？

Question

I have this simple loop, running through an array of base class pointers:

Object * objects[2];

objects[0] = new GreenObject;
objects[1] = new RedObject;
objects[2] = new BlueObject;


for (int i = 0; i < 3; ++i) {
    cout << i << " ";
    objects[i]->info();
}

Under debug mode, the program crashes on the third iteration of the loop, immediately after outputting i , when the info() method is invoked. No such thing happens in release mode, it is running as it should. It is not an issue of the object, since it locks up even if I use other derived classes.

GCC 4.4.0 under Windows 7 64bit

Any ideas?

Answer 1

This is (and for loop) going beyond the end of the array:

objects[2] = new BlueObject;

causing undefined behaviour. The fact it runs in release is just (un)lucky. A subset of undefined behaviour is it behaves as you expect.

Array indexes run from 0 to N - 1 , where N is the number of elements in the array. In the case of objects the valid indexes are 0 and 1 only. Change the declaration of objects to:

Object * objects[3];

Answer 2

Accessing objects[2] is undefined behaviour.

It's a common mistake to expect programming errors to always result in crashes or obvious failures.

Undefined behaviour means anything can happen, including appearing to work in some situations.

If you want predictable, repeatable behaviour then you should write correct code.