C 帮助(ISO C 禁止指针和整数之间的比较.）

我正在尝试制作一个带有堆栈的非常简单的程序，但是当我尝试运行它时，我似乎会遇到错误。该错误显示" ISO C 禁止指针和整数之间的比较。"。任何帮助都非常感谢。

我的代码：

#include <iostream>
#include <string>
using namespace std;
const int maxstack = 5;
struct stacktype{
    string name[maxstack];
    int top;
};
void createstack(stacktype &stack);
void destroystack(stacktype &stack);
bool fullstack(stacktype stack);
void push(stacktype &stack, string &newelement);
bool emptystack(stacktype stack);
void pop(stacktype &stack, string &poppedelement);
int main(){
    stacktype stack;
    string newelement, poppedelement;
    char quest;
    createstack(stack);
    cout<<"Do you want to enter data? (y/n)";
    cin>>quest;
    while((quest == "y" || quest == "Y") && !(fullstack(stack))){ //I get the error on this line
        cout<<"Please enter name";
        cin>>newelement;
        push(stack, newelement);
        cout<<"Do you want to enter data? (y/n)";
        cin>>quest;
    }
    cout<<endl<<endl;
    while(!emptystack(stack)){
        pop(stack, poppedelement);
        cout<<poppedelement<<endl;
    }
    destroystack(stack);
    system("PAUSE");
    return 0;
}
void createstack(stacktype &stack){
    stack.top = -1;
}
void destroystack(stacktype &stack){
    stack.top = -1;;
}
bool fullstack(stacktype stack){
    if(stack.top == maxstack - 1){
        return true;
    }else{
        return false;
    }
}
void push(stacktype &stack, string &newelement){
    stack.top++;
    stack.name[stack.top] = newelement;
}
bool emptystack(stacktype stack){
    if(stack.top == -1){    
        return true;
    }else{
        return false;
    }
}
void pop(stacktype &stack, string &poppedelement){
    poppedelement = stack.name[stack.top];
    stack.top--;
}