Storing a string into a Queue, using seperate class definitions, C++ - c++

Question

I am trying to store a string input (a math equation) into a Queue linked list. To add a character into the Queue, i need to access the Queue function "Enqueue( )" from inside a separate class: the "Calculate" class.
I keep getting error messages ("Queue': undeclared identifier") and ("QueObj": undeclared identifier).
My code mostly looks like this (removed most unrelated code):
#include <iostream>
#include <stdlib.h>
#include <string>
class Calculate // Convert from string to node
{
public:
double Calc(string text)
{
input = text; // math notation, user input sent from main()
/* error message here -> */ Queue queObj; // Queue object created to access Queue functions
/* error message here -> */ queObj.Enqueue(text); // Enqueues the notation into a Queue
};
private:
string input; // The input string for the math equation
};
class Queue
{
public:
void Enqueue(string queVal) // Enqueue definitions
{
// Enqueue instructions
};
void Dequeue() // Dequeue definitions
{
// Dequeue instructions
};
bool IsEmpty() // Queue size check
{
// Queue size check
};
private:
Node *head = NULL; // Pointer for start of queue (set to NULL)
Node *tail = NULL; // Pointer for end of queue (set to NULL)
friend class Calculate; // Friend class allows Calculate to access Queue private data
};
int main()
{
string Eq1 = "1 + 2"; // math equation
Calculate calcObj; // object to access calculate class functions and variables
calcObj.Calc(Eq1); // stores the equation into calculate class
return 0;
}

Answer 1

What is simply happening is that the class Queue is not visible to the class Calculate since the compiler (roughly speaking) reads and compiles code in top to bottom fashion. So the compiler is unaware that there exists a class named Queue while it is parsing the contents of the class Calculate.
There are two ways to make this work:
Put the class declarations at the top of all the class definitions. A class declaration looks as follows:
class Queue
{
public:
void Enqueue(string queVal); // Enqueue definitions
void Dequeue(); // Dequeue definitions
bool IsEmpty(); // Queue size check
private:
Node *head = NULL; // Pointer for start of queue (set to NULL)
Node *tail = NULL; // Pointer for end of queue (set to NULL)
friend class Calculate; // Friend class allows Calculate to access Queue private data
};
And then in the bottom, you can define all the functions as follows:
void Queue::Enqueue(string queval)
{
// Enqueue instructions
}
// rest of the functions
Create a header file with all such class declarations in it, and then include that header file. And then create a .cpp file with all the function definitions in it. While compiling the program, link both the object files (recommended option, since it is less cluttered and easily scalable).
Also, unrelated to the problem here, but you should ideally explicitly mention the return type of main as follows
int main()
This code might even not compile under newer C++ standards, because of not explicitly mentioning the return type of main.
Also, you don't need to use a semi-colon (;) after function definitions.
Lastly, format you code properly (specifically, don't use inconsistent indentations).