does not name a type error in C++

Question

Hi,

I have two classes declared as below:

class User
{
public:
 MyMessageBox dataMsgBox;
};

class MyMessageBox
{
public:
 void sendMessage(Message *msg, User *recvr);
 Message receiveMessage();
 vector<Message> *dataMessageList;
};

When I try to compile it using gcc, it gives the following error: 'MyMessageBox does not name a type'. Please help me in this regard.

Thanks, Rakesh.

Answer 1

When the compiler compiles the class User, and gets to the MyMessageBox line, MyMessageBox has not yet been defined. It has no idea it exists.

You need to make sure MyMessageBox is defined before you use it as a member. This is solved by reversing the definition order.

Now, you have a cyclic dependency. What you can do is forward declare User so message box can still take a pointer or reference to it, without knowing the full definition:

class User; // let the compiler know such a class will exist

class MyMessageBox
{
public:
 void sendMessage(Message *msg, User *recvr); // that's ok
 Message receiveMessage();
 vector<Message> *dataMessageList;
};

class User
{
public:
 MyMessageBox dataMsgBox; // also ok, since it's defined
};

You cannot do this the other way around. The reason is for the compiler to know how much memory User takes up, it needs to know the size of it's members. If you were to say:

class MyMessageBox;

class User
{
public:
 MyMessageBox dataMsgBox; // size not available! it's an incomplete type
};

It wouldn't work, since it doesn't know the size yet. (Because it has no full definition.)

---

On a side note, this function:

 void sendMessage(Message *msg, User *recvr);

Probably shouldn't take either of those by pointer. You can't send a message without a message, nor can you send a message without a user to send it to. And both of those situations come true if you pass null to the function.

Rather, use a reference (possibly const):

 void sendMessage(const Message& msg, User& recvr);

So it's both required to give both parameters, and easier to use.

Answer 2

You must declare the prototype it before using it:

class User;

class MyMessageBox
{
public:
 void sendMessage(Message *msg, User *recvr);
 Message receiveMessage();
 vector<Message> *dataMessageList;
};

class User
{
public:
 MyMessageBox dataMsgBox;
};

edit: Swapped the types

Answer 3

C++ compilers process their input once. Each class you use must have been defined first. You use MyMessageBox before you define it. In this case, you can simply swap the two class definitions.

Answer 4

1. Forward declare User
2. Put the declaration of MyMessageBox before User

Answer 5

You need to define MyMessageBox before User -- because User include object of MyMessageBox by value (and so compiler should know its size).

Also you'll need to forward declare User befor MyMessageBox -- because MyMessageBox include member of User* type.