C++ free all memory used by struct

Question

Quick question; I've googled around and found some answers already, but I'm a bit paranoid so I want to be sure.

Consider this situation:

struct CoordLocation
{
    float X;
    float Y;
    float Z;
};

int main()
{
    CoordLocation *coord = new CoordLocation();
    delete coord;
    return 0;
}

Will calling delete also clear the memory used by the fields X, Y, Z? Some answers I found mentioned that I'd just delete the POINTER, not the actually referenced object this way. What if...

struct CoordLocation
{
    float *X;
    float *Y;
    float *Z;
};

int main()
{
    CoordLocation *coord = new CoordLocation();
    delete coord;
    return 0;
}

And what if I manually free the memory for each object inside the struct's constructor/destructor?

struct CoordLocation
{
    CoordLocation()
    {
         *X = new float;
         *Y = new float;
         *Z = new float;
    }
    ~CoordLocation()
    {
         delete X; delete Y; delete Z;
    }
    float *X;
    float *Y;
    float *Z;
};

int main()
{
    CoordLocation *coord = new CoordLocation();
    delete coord;
    return 0;
}

I noticed that for a simple situation such as:

   float *a = new float;
   *a = 5.0f;
   printf("%f", *a);
   delete a;
   printf("%f", &a);

printf would print 5.0, so the variable pointed to by a is not exactly destroyed.

So my question is: How can I reliably free (as in no memory leaks) ALL the memory used by the struct in this case?

struct CoordLocation
{
    float X;
    float Y;
    float Z;
};

int main()
{
    CoordLocation *coord = new CoordLocation();
    delete coord;
    return 0;
}

Thanks!

Solution

You only need to delete memory you allocate with new.

printf would print 5.0, so the variable pointed to by a is not exactly destroyed.

You're actually running into undefined behavior. Although the value is still there, the memory was released and can be reused.

So the following:

struct CoordLocation
{
    float X;
    float Y;
    float Z;
};

can't create a memory leak if you omit the destructor.

Your next snippet:

struct CoordLocation
{
    float *X;
    float *Y;
    float *Z;
};

int main()
{
    CoordLocation *coord = new CoordLocation();
    delete coord;
    return 0;
}

can potentially create a memory leak, but not as it is. The following will:

int main()
{
    CoordLocation *coord = new CoordLocation();
    coord->X = new float();
    delete coord;
    return 0;
}

Your third example

struct CoordLocation
{
    CoordLocation()
    {
         *X = new float;
         *Y = new float;
         *Z = new float;
    }
    ~CoordLocation()
    {
         delete X; delete Y; delete Z;
    }
    float *X;
    float *Y;
    float *Z;
};

int main()
{
    CoordLocation *coord = new CoordLocation();
    delete coord;
    return 0;
}

won't create a memory leak because you free all the memory that you allocate. If you were to omit the destructor or forget to call delete coord;, they you'd have a memory leak.

A good rule of thumb: call a delete for every new and a delete[] for every new[] and you're safe.

Comments

[mjfgates]

Perhaps, "WILL be reused." Said in a threatening tone of voice, while holding a lit flashlight under your chin.

[Bojan Komazec]

Luchian, what happens in third example if second or third new throws exception? There are no exception handlers in main so terminate and abort will be executed and program will be terminated but prior to that, isn't there a memory leak as delete coord; (and so delete X;) is never executed?

[Luchian Grigore]

@BojanKomazec AH! It depends on what you mean by a memory leak - stackoverflow.com/questions/9921590/…

[Mark B]

The third example will create a double delete if you ever copy construct one.

[Bojan Komazec]

@LuchianGrigore Yeah, that's one of those border cases. It might or might not be regarded as a leak; am still reading answers. You raised very interesting question there.

[Piotr Dabkowski]

Amazing answer! You should write tutorials ;)