Memory leak in C++ (Valgrind)

Question

I implement the stack with a minimum. In this program, I get an error from valgrind. Something is wrong with the push() and main() functions. When I add delete st; to the push() function, I get even more errors. I check it through valgrind./a.out. Sorry for the long code. I also wrote the rest of the functions for stack. But there is no error in them, I left those in the code where there may be an error.

#include <cstring>
#include <iostream>

struct Stack {
  int data;
  int min;
  Stack* next;
};

void Push(Stack** top, int n) {
  Stack* st = new Stack();
  st->data = n;
  if (*top == NULL) {
    *top = st;
    (**top).min = n;
  } else {
    st->min = ((n <= (**top).min) ? n : (**top).min);
    st->next = *top;
    *top = st;
  }
  std::cout << "ok" << std::endl;
}

void Pop(Stack** top) {
  if (*top != NULL) {
    std::cout << (**top).data << std::endl;
    *top = (*top)->next;
  } else {
    std::cout << "error" << std::endl;
  }
}

int main() {
  Stack* top = nullptr;
  int m;
  std::cin >> m;
  std::string str;
  for (int i = 0; i < m; ++i) {
    std::cin >> str;
    if (str == "push") {
      int value;
      std::cin >> value;
      Push(&top, value);
    }
    if (str == "pop") {
      Pop(&top);
    }
  }
  delete top;
}

Answer 1

When you just delete top , you destruct it (in your case it's nothing, but you can distract yourself for reading about destructors if interested) and free the dynamic memory allocated for top . However, you actually want to also delete top->next , top->next->next (if present) etc. A hotfix:

while (top) { // same as "while (top != nullptr) {"
    Stack* next = top->next; // we can't use `top` after we `delete` it, save `next` beforehand
    delete top;
    top = next;
}

Now, about more general things. The course teaches you some really old C++ (almost just plain C; even C here is bad though). At the very least, your whole Push() can be replaced (thanks to lvalue references ( Type& ), std::min and aggregate initialization ) with:

void push(Stack*& top, int n) {
    top = new Stack{n, std::min(n, top ? top->min : n), top};
    std::cout << "ok\n";
}

I'm new to C++ programming. I used to write in Python

Good job. Sadly, such teaching shows C++ as something too old and horrifying.

Edit

here's a new in Push, so there should most likely be a delete in Pop

That's right (thanks to @molbdnilo). You should delete pop ped elements instead of just leaking them.