C++ Primer Plus

Any function with a definition in the class declaration automatically becomes an inline function. Thus, Stock::set_tot() is an inline function. Class declarations often use inline functions for short member functions, and set_tot() qualifies on that account.

You can, if you like, define a member function outside the class declaration and still make it inline. To do so, you just use the inline qualifier when you define the function in the class implementation section:

class Stock

{

private:

    ...

    void set_tot();  // definition kept separate

public:

    ...

};

inline void Stock::set_tot()  // use inline in definition

{

    total_val = shares * share_val;

}

The special rules for inline functions require that they be defined in each file in which they are used. The easiest way to make sure that inline definitions are available to all files in a multifile program is to include the inline definition in the same header file in which the corresponding class is defined. (Some development systems may have smart linkers that allow the inline definitions to go into a separate implementation file.)

Incidentally, according to the rewrite rule, defining a method within a class declaration is equivalent to replacing the method definition with a prototype and then rewriting the definition as an inline function immediately after the class declaration. That is, the original inline definition of set_tot() in Listing 10.1 is equivalent to the one just shown, with the definition following the class declaration.

Which Object Does a Method Use?

Now we come to one of the most important aspects of using objects: how you apply a class method to an object. Code such as this uses the shares member of an object:

shares += num;

But which object? That’s an excellent question! To answer it, first consider how you create an object. The simplest way is to declare class variables:

Stock kate, joe;

This creates two objects of the Stock class, one named kate and one named joe.

Next, consider how to use a member function with one of these objects. The answer, as with structures and structure members, is to use the membership operator:

kate.show();    // the kate object calls the member function

joe.show();     // the joe object calls the member function

The first call here invokes show() as a member of the kate object. This means the method interprets shares as kate.shares and share_val as kate.share_val. Similarly, the call joe.show() makes the show() method interpret shares and share_val as joe.shares and joe.share_val, respectively.

Note

When you call a member function, it uses the data members of the particular object used to invoke the member function.

Similarly, the function call kate.sell() invokes the set_tot() function as if it were kate.set_tot(), causing that function to get its data from the kate object.

Each new object you create contains storage for its own internal variables, the class members. But all objects of the same class share the same set of class methods, with just one copy of each method. Suppose, for example, that kate and joe are Stock objects. In that case, kate.shares occupies one chunk of memory, and joe.shares occupies a second chunk of memory. But kate.show() and joe.show() both invoke the same method—that is, both execute the same block of code. They just apply the code to different data. Calling a member function is what some OOP languages term sending a message. Thus, sending the same message to two different objects invokes the same method but applies it to two different objects (see Figure 10.2).

Figure 10.2. Objects, data, and member functions.

Using Classes