How do I fix the error C2440: 'return' : cannot convert from 'int' to 'elem *'?

Question

I was trying to do build my program and it kept giving me the following error:

Error C2440: 'return': cannot convert from 'int' to 'elem *'

Problem is that the only way I've found to fix it was to change it to return=0 but that would make the whole thing pointless given that I want it to return 1 when I have successfully removed an element from the queue. Do you have any idea how to fix it?

    struct elem { int key;
        elem *next; 
        } *first=NULL, *last=NULL;   

elem *push(int n, elem *&first, elem *&last)  
    { elem *p=last; 
    last=new elem; 
    last->key=n; 
    last->next=NULL; 
    if(p!=NULL) 
        p->next=last; 
    else 
        first=last; 
    } 

elem *pop(int &n, elem *&first, elem *&las) 
           { elem *p=NULL;
           if (first)  
            {       n=first->key; 
                    p=first;  
                    first=first->next;;

                        if (first==NULL) 
                            last=first;  
                         delete p;
                         return 1;       //this here gives me the error
                }      
                else     
                    return 0;
            }

Answer 1

c++ won't let you convert an integral type to a pointer without reinterpret_cast or c-style cast however your function is misleading and you are doing things that should never be done.

char *ptr = 1; // error
char *ptr = reinterpret_cast<char*>(0x164651); // compiles but you must make sure this address will be valid !

You declare your function to return an elem * and makes this an indication to success or failure, In this case you should return nullptr on failure and a pointer to the pushed or popped element on success.

elem *pop(int &n, elem *&first, elem *&las) 
           { elem *p=NULL;
           if (first)  
            {       n=first->key; 
                    p=first;  
                    first=first->next;;

                        if (first==NULL) 
                            last=first;  
                         return p;       // return the popped element to the caller
                }      


else     
                return nullptr; // failed to pop any element
        }

If you insist on indicating success and failure by 1 and 0 values so you should use an integral type as the return type no a pointer.

int pop(int &n, elem *&first, elem *&las) // returns 1 on success otherwise 0

and in c++ it's more better to use exceptions to indicate failure. This allows to separate failure path from return path and save a lot of error checking which you will find yourself doing most of the time. So Always use c++ exceptions and RAII if possible, it will save you much.