POI31-C. Do not treat arrays polymorphically

Because pointer arithmetic does not take account of polymorphism, a major problem with arrays is that they do not interact well with polymorphism (see Stroustrup 06, Meyers 06) as the following example illustrates:

Non-Compliant Code 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[3];
	Derived dat[3];

	walk(dis, 3);
	walk(dat, 3); // 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 1

Instead of having an array of objects, an array of pointers solves the problem of the objects being of different sizes. With the Base and Derived classes as above, we can define the walk and main methods as follows.

void walk(class Base *bar [], int count) {
	for (int i = 0; i < count; i++) {
		(bar[i])->func();
	}
}

int main(void) {
	Base* dis[3] = {new Base, new Base, new Base};
	Base* dat[3] = {new Derived, new Derived, new Derived};

	walk(dis, 3);
	walk(dat, 3);
}

The elements in the arrays are now all the same size (because they are pointers to Base or Derived objects) and there is no problem with the array indexing.

Compliant Solution 2

A better approach would be to use vectors and iterators, instead of arrays, as follows. (Note, however, that we have to have vectors of pointers because containers must be homogeneous.)

void walk(vector<Base*>bar) {
	for_each (bar.begin(), bar.end(), mem_fun(&Base::func));
}

int main(void) {
	vector<Base*> dis(3);
        for (int i=0; i<3; i++) dis[i] = new Base;

	vector<Base*> dat(3);
        for (int i=0; i<3; i++) dat[i] = new Derived;

	walk(dis);
	walk(dat);
}

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?