Question

I’m starting to learn C++ (coming from Java), so bear with me.

I can’t seem to get my method declaration to accept a class I’ve made.

‘Context’ has not been declared

I think I’m not understanding a fundamental concept, but I don’t know what.

Expression.h

#include "Context.h"  
class Expression {  
public:  
    void interpret(Context *);  // This line has the error
    Expression();  
    virtual ~Expression();  
};  

Context.h

#include <stack>  
#include <vector>  
#include "Expression.h"  

class Context {  
private:  
    std::stack<Expression*,std::vector<Expression*> > theStack;  
public:  
    Context();  
    virtual ~Context();  
};

Answer 1

You have to forward declare Expression in Context or vice versa (or both), otherwise you have a cyclic dependency. For example,

Expression.h:

class Context; // no include, we only have Context*.

class Expression {  
public:  
    void interpret(Context *);  // This line has the error
    Expression();  
    virtual ~Expression();  
};

Context.h:

#include <stack>  
#include <vector>  

class Expression; // No include, we only have Expression*

class Context {  
private:  
    std::stack<Expression*,std::vector<Expression*> > theStack;  
public:  
    Context();  
    virtual ~Context();  
};

You can perform the forward declarations because the full definition of the classes isn’t needed, since you are only referring to pointers to the other class in each case. It is likely that you will need the includes in the implementation files (that is, #include "Context.h" in Expression.cpp and #include Expression.h in Context.cpp).

Finally, remember to put include guards in your header files.