'Object' has not been declared error

Question

I'm new to c++ coming from Java so I'm not sure what I'm doing wrong here the following line in person.h is giving me the error Transaction has not been declared.

void pay(Transaction transaction);

I have a Transaction object, do I have to declare / include it in the person.h file somewhere?

here is my person.h source

#ifndef PERSON_H_
#define PERSON_H_

#include "Transaction.h"

using std::string;

class Person {
public:

    class Transaction;

    Person();
    virtual ~Person();
    void pay(Transaction* transaction);
};

#endif /* PERSON_H_ */

Answer 1

In C++, seperating the declarations from the definitions is a big thing. Declarations tell how to use the code, and are in header (.h) files. Definitions are the code itself, and are in source (.cpp) files.

If a file only refers to Transaction by pointer or reference (as is common in a header), then you only need to predeclare the transaction class, with class Transaction;. If the file needs an actual Transaction value, then you need to #include "Transaction.h".

If it still says Transaction is an undefined symbol, that means you have a circular dependency: A series of headers that include each other in a loop. In that case, you need to alter one of the header files to only use pointers and references, and predeclare the classes in the other headers.

Answer 2

When you declare a class inside another class, you're declaring an internal class.

So

class Person {
public:

    class Transaction;
};

Declares a class whose fully qualified name is Person::Transaction. Even if there is another class named Transaction at global scope, references to Transaction will be assumed to refer to Person::Transaction. Since you haven't defined that class, you get your error.

What you want is ::Transaction, either from including the header or from forward declaring outside your class declaration. Do either

class Transaction;
class Person {
};

OR

#include "Transaction.h"
class Person
{
};

The former only allows passing by reference. Passing by value or calling a function requires the full class definition provided by include.