Unit testing a function that returns a pointer (C++)

Question

I have the following function:

unsigned* b_row_to_array(b_row r) {
    unsigned a[] = {(r >> 8) & 3,
        (r >> 6) & 3,
        (r >> 4) & 3,
        (r >> 2) & 3,
        r & 3};
    return a;
}

That I wrote the following test function for:

TEST_METHOD(BRowToArrayTest) {
    unsigned expected[] = { 0, 0, 0, 0, 0 };
    unsigned* computed = b_row_to_array(0);
    for (int i = 0; i < 1; i++) {
        Assert::AreEqual(expected[i], computed[i]);
    }
}

But the test fails, saying "Expected:<0>, Actual:<42348989>", where the value for Actual is different with every test, so it must be reading in the wrong address. Am I using pointers wrong or does it read out-of-bounds because the test function is not in the same project? How can I solve this?

Ah, I take it you're hinting at that a[] is no longer at the address that the pointer points to? If so, how do I resolve this, seeing as C++ doesn't allow for returning an array directly. — Arthelais, Apr 03 '15 at 15:04
Why can't you use a standard container, like `std::vector`? If you have to stay with your array, maybe do it the C way and pass a pointer to `a` to your function and then fill it in the function. — Unapiedra, Apr 03 '15 at 15:13

score 0 · Accepted Answer · answered Apr 03 '15 at 15:04

That's because your function is return a pointer to a local variable that is destroyed immediately. It's not different than had you written:

T* b_row_to_array(b_row r) {
    T a = { ... };
    return &a;
    // <== a destroyed, so we have dangling pointer
}

If you want that to work, you will need a to last. That means either allocating it:

unsigned* b_row_to_array(b_row r) {
    unsigned* a = new unsigned[5];
    ...
    return a;
}

Or returning it by value:

std::array<unsigned, 5> b_row_to_array(b_row r);

I used to allocation method and it resolved my problem, thank you very much. — Arthelais, Apr 03 '15 at 15:09

Unit testing a function that returns a pointer (C++)

1 Answers1