Why is there a memory leak in the following C++ code?

Question

Assume a class called Vec exists with a vector called arr as it's only member . The following code does NOT leak memory. (Which means my destructor for Vec is working as expected.)

int main() {
    Vec *obj = new Vec(5); // initializes a vector of size 5
    delete obj;
}

However, in the following code, there is a memory leak. But why? I seem to have a delete for each use of new. What am I missing?

int main() {
    Vec* obj;
    obj = new Vec(5);

    if (true) {
        delete obj;
        Vec* obj = new Vec(6);
    }

    delete obj;
}

PS: I checked for memory leak using valgrind.

Answer 1

Scope.

Vec* obj = new Vec(6); makes a new variable named obj that only exists within the if's body and hides the obj at the outer scope. The inner obj goes out of scope and vanishes at the end of the if's body, taking the last pointer to that new allocation with it. The code then re-deletes the obj at the outer scope (not a good thing to do).

Solution:

int main() {
    Vec* obj;
    obj = new Vec(5);

    if (true) {
        delete obj;
        obj = new Vec(6); // re-uses the original obj
    }

    delete obj;
}