Why does G++ tell me "Stack" is not declared in this scope?

c++

Solution

Your `Stack` is declared in `Stack.cpp`, as you said. You are trying to use it in `StackTest.cpp`. Your `Stack` is not declared in `StackTest.cpp`. You can't use it there. This is what the compiler is telling you.

You have to define classes in all translation units (.cpp files), in which you are planning to be using them. Moreover, you have to define them identically in all of these translation units. In order to satisfy that requirement, class definitions are usually separated into header files (.h files) and included (using `#include`) into each .cpp file where they are needed.

In your case you need to create header file `Stack.h`, containing the definition of `Stack` class (and constant definitions in this case) and nothing else

const int MaxStack = 10000; 
const char EmptyFlag = '\0'; 

class Stack { 

    char items[MaxStack]; 
    int top; 
public: 
    enum { FullStack = MaxStack, EmptyStack = -1 }; 
    enum { False = 0, True = 1}; 
    // methods 
    void init(); 
    void push(char); 
    char pop(); 
    int empty(); 
    int full(); 
    void dump_stack(); 
}; 

(Header files also benefint from use of so called include guards, but it will work as shown above for now).

This class definition should be moved from `Stack.cpp` to `Stack.h`. Instead you will include this .h file into `Stack.cpp`. Your `Stack.cpp` will begin as follows

#include<iostream>     

#include "Stack.h"

using namespace std;    

void Stack::init()      
{      
    top = EmptyStack;      
}      

// and so on...

The rest of your former `Stack.cpp`, i.e. member definitions, should remain as is.

The `Stack.h` should also be included into `StackTest.cpp`, in the same way, so your `StackTest.cpp` should begin as

#include <iostream>        

#include "Stack.h"

using namespace std; 

// and so on...

That's, basically, it. (And instead of providing an `init` method a better idea would be to create a constructor for the `Stack` class. But that's a different story).

Problem

Original source