Please help me to make a function stop/return an error without returning any value

[Bubiche]

I'm having an assignment to implement a stack class using singly linked list and here is my Pop function:

#include <iostream>
using namespace std;

struct Node
{
	int val;
	Node* Next;
}

class stack
{
private:
	Node* top;
public:
	int Pop()
	{
	        if(top==NULL)
		      return ???; //what to do here???
	         Node* tmp=top;
	         int a=top->val;
	         top = top->Next;
	         delete tmp;
	         return a;
	}
}

As you can see, I don't know what the Pop function will do if the stack is empty, please help.
Thank you for your time and patience.

[Zhuge]

This looks like a job for exceptions:
http://www.cplusplus.com/doc/tutorial/exceptions/

[Cody0023]

You can return 1 or throw an exception

[Bubiche]

Thank you guys, I've spent some time to read the tutorial about exceptions (never heard of it before). Here's what I came up with, is it alright? I still feel that there's an error in the return part

#include <iostream>
#include <conio.h>
using namespace std;

struct Node
{
	int val;
	Node* Next;
}

class stack
{
private:
	Node* top;
public:
	int Pop()
	{
		try
		{
			Node* tmp=top;
			int a=top->val;
			top = top->Next;
			delete tmp;
			if(top==NULL)
				throw 'n';
		}
		catch(char c)
		{
			cout<<"ERROR";
		}
		return a;
	}
}

[ne555]

It shouldn't be an error to empty an stack

you should let the client to catch the exception.

[JLBorges]

You have four typical options; choose one, and document it.

Option one:

// invariant: !empty()
// calling Pop() on empty stack will result in undefined behaviour
int Pop() ;

Option two:

// invariant: !empty()
// invariant is asserted: assert( !empty() )
// NDEBUG: calling Pop() on empty stack will result in undefined behaviour
int Pop() ;

Option three:

// invariant: !empty()
// calling Pop() on empty stack throws an excdeption (std::out_of_range)
int Pop() ;

Option four:

// invariant: !empty()
// invariant is asserted: assert( !empty() )
// NDEBUG: calling Pop() throws an exception (std::out_of_range)
int Pop() ; 

My personal preference is for option two (provide assistance for debugging, but do not penalize a programmer who writes correct code because there could be others who may write incorrect code).

int Pop()
{
    assert( top != nullptr ) ; // #include <cassert>

    Node* tmp = top ;
    const int a = top->val ; // UB if top is null
    
    top = top->Next ;
    delete tmp ;
    
    return a ;
}