10.8.2. Using Association

Syntactically, programmers use associated classes similarly to how they use them in an aggregation relationship. Whole-class objects send messages to their aggregated parts with the part name, the arrow operator, and the function name. But whereas aggregation is unidirectional, association is bidirectional, meaning that both associated objects can send messages. Due to its greater flexibility, association can be more complex than aggregation.

A Typical Association Example

Association is a bidirectional relationship between two peers. Program objects can send messages to either peer, and either peer can send messages to its associate along the relationship path. Association relationships can form extended linear or web-like structures, but individual relationships always connect just two classes, and we can often understand their interactions by focusing on class pairs.

Designing With The UML

To illustrate associations, imagine writing a program to implement a two-player board game. The single program runs simultaneously on two computers connected over a local area network. The classes in the following figure are excerpted from an upper-division course project implementing such a game. Each class has a set of distinct responsibilities. When a player moves on one computer, the move is validated, sent to, and displayed on the opposing player's computer. The classes shift program control among themselves by sending messages, allowing each to fulfill its responsibilities.

We can more easily understand how the program objects exchange messages by exploring a few scenarios. Although sequence diagrams are beyond the scope of the textbook, they are a convenient way of exploring the messages objects exchange while a program runs. We only need a few of their basic features, and the brief summary linked here describes them. Programs dynamically alter their execution based on tests made while running. We explore some possible variations with different scenarios that assume differing test outcomes.

Game Source Code

The following member functions focus on the syntax associated objects use to send messages to their associates - that is, on using association.

class Board
{
    private:
        Model* mod;

    public:
        void moveHandler()
        {
            Move m = new Move(...);
            mod->validate(m);
        }

        void update(Move m)
	{
            // update pieces on the board
	}
};
 
 
 
 
 

class Model
{
    private:
        Board* b;
        Game*  g;

    public:
        bool validate(Move m)
        {
            // validate m
	    if (not a legal move)
                return false;
            g->moved(m);
            return true;
        }

        void makeMove(Move m)
        {
            // save the new state
            b->update(m);
        }
};

class Game
{
    private:
        Model*   mod;
        Network* n;

    public:
        void moved(Move m)
        {
            n->sendMove(m);
        }

        void makeMove(Move m)
        {
            mod->makeMove(m);
        }
};
 
 

class Network
{
    private:
        Game* g;

    public:
        void sendMove(Move m)
        {
            writeMove(m);
        }

    private:
        void readMove(Move m)
        {
            g->makeMove(m);
        }
	
	void writeMove(Move M);
};

An Atypical Association Example

Previous examples demonstrated a practice common in hierarchies: one class sharing or delegating some of its responsibilities with other classes. Each example used the display function to demonstrate the concept and syntax. In the case of inheritance, the child's display function displayed its member variables and called the parent's display function to display the inherited members. For aggregation and composition, the whole class's display function displayed the whole's members and called the parts' display functions to complete the operation. This practice is generally safe because these relationships are unidirectional, making an inadvertent cycle less likely.

Alternatively, association is a bidirectional relationship, warranting additional caution. The related class "know about" and can send messages to each other, making it easy to create an endless cycle. The Game and Model classes each have a function named makeMove but don't exhibit this problem because the functions never call each other. The following figure illustrates how easily we can create this error:

class contractor;

class project
{
    private:
	string 		title;
	contractor*	c;

    public:
	void display
	{
            cout << title << endl;
            c->display();
	}
};

class project;

class contractor
{
    private:
	string		name;
	project*	p;

    public:
	void display()
	{
            cout << name << endl;
            p->display();
	}
};

Function calls chained with association must be "complementary" in the same sense previously described for constructors. Programmers must design the functions so that they work together correctly. The following figure uses the display function to illustrate pairs of complementary functions.

class contractor;

class project
{
    private:
        string		title;
        contractor*	c;

    public:
void display()				// Pair a
{
    cout << title << endl;
    c->display();
}
void display()				// Pair b
{
    cout << title << endl;
}
};

class project;

class contractor
{
    private:
        string		name;
        project*	p;

    public:
void display()				// Pair a
{
    cout << name << endl;
}
void display()				// Pair b
{
    cout << name << endl;
    p->display();
}
};