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C++ Primer Plus

Here s.total_val is the total value for the object passed as an argument, and total_val is the total value for the object to which the message is sent. If s.total_val is greater than total_val, the function returns a reference to s. Otherwise, it returns a reference to the object used to evoke the method. (In OOP talk, that is the object to which the topval message is sent.) Here’s the problem: What do you call that object? If you make the call stock1.topval(stock2), then s is a reference for stock2 (that is, an alias for stock2), but there is no alias for stock1.

The C++ solution to this problem is to use a special pointer called this. The this pointer points to the object used to invoke a member function. (Basically, this is passed as a hidden argument to the method.) Thus, the function call stock1.topval(stock2) sets this to the address of the stock1 object and makes that pointer available to the topval() method. Similarly, the function call stock2.topval(stock1) sets this to the address of the stock2 object. In general, all class methods have a this pointer set to the address of the object that invokes the method. Indeed, total_val in topval() is just shorthand notation for this->total_val. (Recall from Chapter 4, “Compound Types,” that you use the -> operator to access structure members via a pointer. The same is true for class members.) (See Figure 10.4.)

Figure 10.4. this points to the invoking object.

Note

Each member function, including constructors and destructors, has a this pointer. The special property of the this pointer is that it points to the invoking object. If a method needs to refer to the invoking object as a whole, it can use the expression *this. Using the const qualifier after the function argument parentheses qualifies this as being a pointer to const; in that case, you can’t use this to change the object’s value.

What you want to return, however, is not this because this is the address of the object. You want to return the object itself, and that is symbolized by *this. (Recall that applying the dereferencing operator * to a pointer yields the value to which the pointer points.) Now you can complete the method definition by using *this as an alias for the invoking object:

const Stock & Stock::topval(const Stock & s) const

{

if (s.total_val > total_val)

return s; // argument object

else

return *this; // invoking object

}

The fact that the return type is a reference means that the returned object is the invoking object itself rather than a copy passed by the return mechanism. Listing 10.7 shows the new header file.

Listing 10.7. stock20.h

// stock20.h -- augmented version

#ifndef STOCK20_H_

#define STOCK20_H_

#include

class Stock

{

private:

std::string company;

int shares;

double share_val;

double total_val;

void set_tot() { total_val = shares * share_val; }

public:

Stock(); // default constructor

Stock(const std::string & co, long n = 0, double pr = 0.0);

~Stock(); // do-nothing destructor

void buy(long num, double price);

void sell(long num, double price);

void update(double price);

void show()const;

const Stock & topval(const Stock & s) const;

};

#endif

Listing 10.8 presents the revised class methods file. It includes the new topval() method. Also now that you’ve seen how the constructors and destructor work, Listing 10.8 replaces them with silent versions.

Listing 10.8. stock20.cpp

// stock20.cpp -- augmented version

#include

#include "stock20.h"

// constructors

Stock::Stock() // default constructor

{

company = "no name";

shares = 0;

share_val = 0.0;

total_val = 0.0;

}