Question

Following results are very interesting and I am having difficulty understanding them. Basically I have a class which has an int:

class TestClass{
public:
    int test;
    TestClass() { test = 0; };
    TestClass(int _test) { test = _test; };
    ~TestClass() { /*do nothing*/ };
};

A test function which accepts a pointer of TestClass

void testFunction1(TestClass *ref){
    delete ref;
    TestClass *locTest = new TestClass();
    ref = locTest;
    ref->test = 2;
    cout << "test in testFunction1: " << ref->test << endl;
}

This is what I am doing in main:

int main(int argc, _TCHAR* argv[])
{
    TestClass *testObj = new TestClass(1);
    cout << "test before: " << testObj->test << endl;
    testFunction1(testObj);
    cout << "test after: " << testObj->test << endl;
    return 0;
}

I was expecting output to be:

test before: 1
test in testFunction1: 2
test after: 1

But I get the following output:

test before: 1
test in testFunction1: 2
test after: 2

Can someone explain this. Interesting thing is that changing testFunction1 to:

void testFunction1(TestClass *ref){
    //delete ref;
    TestClass *locTest = new TestClass();
    ref = locTest;
    ref->test = 2;
    cout << "test in testFunction1: " << ref->test << endl;
}

i.e. I do not delete ref before pointing it to new location, I get the following output:

test before: 1
test in testFunction1: 2
test after: 1

I would really appreciate if someone can explain me this strange behavior. Thanks.

Answer 1

When you access the object after deleting it, the behaviour is undefined.

The behaviour that you see follows from the memory allocation algorithm in your system.

That is after deleting your first testclass object, you allocate memory for a new object. The runtime simply reuses the memory.

To check what is going on, print the values of the pointers:

void testFunction1(TestClass *ref){
    cout << ref << endl;
    delete ref;
    TestClass *locTest = new TestClass();
    cout << locTest << endl;
    ref = locTest;
    ref->test = 2;
    cout << "test in testFunction1: " << ref->test << endl;
}