Using "assert" with pointers in C++

Question

When do we need to use "assert" for pointers in C++, and when they are used, how are they most commonly implemented?

Answer 1

Generally you would use an assert to check a condition that, if false, would indicate a bug in your application. So if a NULL pointer shouldn't ever be encountered at some point in the application, unless there's a bug, then assert it. If it might be encountered due to some invalid input then you need to do proper error handling.

Answer 2

You don't need to use assert on pointers at all. The idea is to ensure you don't crash when dereferencing your pointers when they're null.

You can do this with assert but it's not a very professional way to handle errors like this since it invariably terminates the program - not a good idea if the user hasn't, for example, saved their last three hours worth of data entry.

What you should do with pointers is to check them for null-ness and fail gracefully. In other words, have your function return an error of some sort or do nothing (not everyone will agree with this approach but it's perfectly acceptable if it's documented).

The assert stuff is meant, in my opinion, for catching problems during development which is why you'll find assert does nothing in release builds under some compilers. It is not a substitute for defensive programming.

As to how to do it:

#include <assert.h>
void doSomethingWithPointer (int *p) {
    assert (p != 0);
    cout << *p << endl;
}

but this would be better done as:

void doSomethingWithPointer (int *p) {
    if (p != 0)
        cout << *p << endl;
}

In other words, even if your "contract" (API) states that you're not allowed to receive null pointers, you should still handle them gracefully. An old quote: be conservative in what you give, liberal in what you accept (paraphrased).