Because pointer arithmetic does not take account of polymorphism, a major problem with arrays is that they do not interact well with polymorphism Stroustrup 06, Meyers 06 as the following example illustrates:
Non-compliant Example
class Base {
public:
virtual void func(void) {
cout << "Base" << endl;
}
};
class Derived : public Base {
public:
int i;
Derived() { i = 0; }
void func(void) {
cout << "Derived " << ++i << endl;
}
};
void walk(class Base *bar, int count) {
for (int i = 0; i < count; i++) {
bar[i].func();
}
}
int main(void) {
Base dis[5];
Derived dat[5];
walk(dis, 5);
walk(dat, 5); // crashes
}
In the last call to walk(), dat[] is treated as a Base[] and the subscripting no longer works correctly when sizeof(Derived) != sizeof(Base). This is because walk() incorrectly believes that the size of each element in bar[] is sizeof(Base). To locate the second element in the array (located at bar[1]), walk() adds the sizeof(Base) to the address bar. Assuming the derived object is larger (which is often the case), the resulting pointer refers to a point within the first element and not to the start of the second element located at bar + sizeof(Derived).
Compliant Solution
The function walk should use a vector and an iterator. (This is left as an exercise for the reader.)
Consequences
Using arrays polymorphically can result in memory corruption which could lead to an attacker being able to execute arbitrary code.
References
- Sutter 04 Item 100: Don't treat arrays polymorphically.
- Meyers 06 Item 3: Never treat arrays polymorphically.
- Stroustrup 06 What's wrong with arrays?