Thread: Potential memory leak and design considerations

Consider the following example:

Code:

#include <iostream>
#include <stack>
 
struct Foo{ 
 Foo( int n ):num(n){}
 int num;
};
 
class Bar{
 
 public:
 
  Bar(){
   for( int i = 0; i < 10; i++ )
    stackOFoos.push( Foo(i) );
  }
 
  const Foo& takeAFoo(){
 Foo* afoo = &stackOFoos.top();
 stackOFoos.pop();
 return *afoo;
}
 
  int howManyFoos(){ return stackOFoos.size(); }
 
 private:
  std::stack<Foo> stackOFoos;
};
 
int main(){
 
 Bar bar;
 std::cout << bar.howManyFoos() << "\n";
 
 Foo afoo = bar.takeAFoo();
 std::cout << bar.howManyFoos() << "\n";
 
 return 0;
}

1. Does the takeAFoo() function create a memory leak? Or is the foo& it returns bound to the scope of "afoo" in main, and since afoo isn't a pointer in main, no memory is leaked?

2. Also, takeAFoo won't work if the function is declared const like this:

Code:

const Foo& takeAFoo() const{
   Foo* afoo = &stackOFoos.top();
   stackOFoos.pop();
   return *afoo;
  }

Why is that?

3. As a design consideration, I do want to be able to take a foo out of the bar class and do whatever I want with it. Is there a better way to go about achieving that kind of behavior?

1) I would suppose that

Code:

stackOFoos.pop();

destructs the item you just took the pointer of. So you'll be returning an object that was just destructed. I think you will need to return a copy of top.

2) That's because you are calling a non-const method on a member (pop). If you have a feeling that the operation should be constant and the popping of the stack shouldn't logically mean that you changing the Bar, you can make the stack mutable.

3) Return the top by value I guess.

You can't do that.
You're first taking the address of an item inside the stack, then you're deleting that item that the pointer was pointing to, then you're dereferencing that pointer to potentially garbage.
If you want to pop an item off the stack you must receive it by value. You can't leak because it's all done by value. Dont worry, NRVO etc will probably kick in an eliminate the copy since library writers tend to be smart enough to do that.
It wont compile if the function is declared const because it modifies its member variable through pop.

Many thanks guys!

2) That's because you are calling a non-const method on a member (pop). If you have a feeling that the operation should be constant and the popping of the stack shouldn't logically mean that you changing the Bar, you can make the stack mutable.

So that means if I want to access inner elements of stacks, queues, etc, (and potentially make copies of those elements) I can do it if the stack is mutable, and I won't actually be modifying the original stack?

If a member is mutable then the const-ness restriction of methods doesn't apply to this member.

Don't get too excited over mutable though. You shouldn't use it just so you can change any non-const method into a const one.