CODEWITHSUNDEEP
c++stackqueue
Jaden
Jaden

Reputation: 375

Trying to create queue using stacks. Why am I getting a void to int conversion error?

This is my first attempt at creating a queue using two stacks. I am not sure if I am on the right track because I have been unable to check my work due to an '=': cannot convert from 'void' to 'int' error. The error lies on this line: x = enQ.pop();. enQ is not a function, so how could it be void?


My code is not yet finished of course.

stack<int> enQ;
stack<int> deQ;

void enQueue(int x) {
    enQ.push(x);
    cout << x << " has been added to the queue." << endl;
}

void deQueue() {
    while (enQ.size() != 0) {
        int x;
        x = enQ.pop();
        enQ.pop();
        deQ.push(x);
        cout << x << " had been pushed to DEQUEUE" << endl;
    }
}

Upvotes: 2

Views: 239

Answers (2)

Chen OT
Chen OT

Reputation: 3614

You can check this post to see why STL separate the T kind_of_pop_and_get() into pop and top. Former only removes the top element on stack, and latter simply copying the top element without touching stack.

Why doesn't std::queue::pop return value.?

In short, it is for exception safety.

The kind_of_pop_and_get() will do

  • Remove element from stack.
  • Then return the element to copy assignment/constructor of receiver.

Exception may throw during object construction and start the stack unwinding. Then the stack state could be affected even the kind_of_pop_and_get() call is not completed.

Upvotes: 1

songyuanyao
songyuanyao

Reputation: 172994

Because the function std::stack::pop returns nothing (i.e. its return type is void).

You could change

x = enQ.pop();

to

x = enQ.top();
enQ.pop();

Upvotes: 1

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