Implicitly and Explicitly Using the this Pointer to Access an Object’s Data Members

Figure 9.23 demonstrates the implicit and explicit use of the this pointer to enable a member function of class Test to print the private data x of a Test object. In the next example and in Chapter 10, we show some substantial and subtle examples of using this.

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 1   // Fig. 9.23: fig09_23.cpp
 2   // Using the this pointer to refer to object members.
 3   #include <iostream>
 4   using namespace std;
 5
 6   class Test
 7   {
 8   public:
 9      explicit Test( int = 0 ); // default constructor
10      void print() const;
11   private:
12      int x;
13   }; // end class Test
14
15   // constructor
16   Test::Test( int value )
17      : x( value ) // initialize x to value
18   {
19      // empty body
20   } // end constructor Test
21
22   // print x using implicit and explicit this pointers;
23   // the parentheses around *this are required
24   void Test::print() const
25   {
26      // implicitly use the this pointer to access the member x
27      cout << "        x = " << x;                             
28
29      // explicitly use the this pointer and the arrow operator
30      // to access the member x                                
31      cout << " this->x = " << this->x;                      
32
33      // explicitly use the dereferenced this pointer and
34      // the dot operator to access the member x         
35      cout << " (*this).x = " << ( *this ).x << endl;   
36   } // end function print
37
38   int main()
39   {
40      Test testObject( 12 ); // instantiate and initialize testObject
41
42      testObject.print();
43   } // end main

        x = 12
  this->x = 12
(*this).x = 12

Fig. 9.23. using the this pointer to refer to object members.

For illustration purposes, member function print (lines 24–36) first prints x by using the this pointer implicitly (line 27)—only the name of the data member is specified. Then print uses two different notations to access x through the this pointer—the arrow operator (->) off the this pointer (line 31) and the dot operator (.) off the dereferenced this pointer (line 35). Note the parentheses around *this (line 35) when used with the dot member selection operator (.). The parentheses are required because the dot operator has higher precedence than the * operator. Without the parentheses, the expression *this.x would be evaluated as if it were parenthesized as *(this.x), which is a compilation error, because the dot operator cannot be used with a pointer.

One interesting use of the this pointer is to prevent an object from being assigned to itself. As we’ll see in Chapter 10, self-assignment can cause serious errors when the object contains pointers to dynamically allocated storage.

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