Why am I getting the 'MyMessageBox does not name a type' error in C ?

""MyMessageBox does not name a type" Error in C

This error arises when a class attempts to use another class that has not been defined beforehand. In the given code, the User class declares a member variable named dataMsgBox, which is an instance of the MyMessageBox class. However, at the time of compilation, MyMessageBox has not been defined yet, resulting in the error.

To resolve this, we employ forward declaration. We declare the User class without defining it first, which allows MyMessageBox to reference it even without its complete definition. The sequence of definitions should then be reversed, with MyMessageBox defined before User.

class User; // Forward declaration of User

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

class User
{
public:
    MyMessageBox dataMsgBox;
};

Alternatively, instead of passing pointers, consider passing references to prevent null values:

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

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