Programming Tutorials

const Pointers in C++

By: Lakshmi in C++ Tutorials on 2007-09-14  

You can use the keyword const for pointers before the type, after the type, or in both places. For example, all of the following are legal declarations:

const int * pOne;
int * const pTwo;
const int * const pThree;

pOne is a pointer to a constant integer. The value that is pointed to can't be changed.

pTwo is a constant pointer to an integer. The integer can be changed, but pTwo can't point to anything else.

pThree is a constant pointer to a constant integer. The value that is pointed to can't be changed, and pThree can't be changed to point to anything else.

The trick to keeping this straight is to look to the right of the keyword const to find out what is being declared constant. If the type is to the right of the keyword, it is the value that is constant. If the variable is to the right of the keyword const, it is the pointer variable itself that is constant.

const int * p1;  // the int pointed to is constant
int * const p2;  // p2 is constant, it can't point to anything else

const Pointers and const Member Functions

When a function is declared const, the compiler flags as an error any attempt to change data in the object from within that function.

If you declare a pointer to a const object, the only methods that you can call with that pointer are const methods. Program below illustrates this.

Using pointers to const objects.

1:      // 
2:      // Using pointers with const methods
3:
4:      #include <iostream.h>
5:
6:      class Rectangle
7:      {
8:      public:
9:           Rectangle();
10:           ~Rectangle();
11:           void SetLength(int length) { itsLength = length; }
12:           int GetLength() const { return itsLength; }
13:
14:           void SetWidth(int width) { itsWidth = width; }
15:           int GetWidth() const { return itsWidth; }
16:
17:      private:
18:           int itsLength;
19:           int itsWidth;
20:      };
21:
22:      Rectangle::Rectangle():
23:      itsWidth(5),
24:      itsLength(10)
25:      {}
26:
27:      Rectangle::~Rectangle()
28:      {}
29:
30:      int main()
31:      {
32:           Rectangle* pRect =  new Rectangle;
33:           const Rectangle * pConstRect = new Rectangle;
34:           Rectangle * const pConstPtr = new Rectangle;
35:
36:           cout << "pRect width: " << pRect->GetWidth() << " feet\n";
37:           cout << "pConstRect width: " << pConstRect->GetWidth() << " feet\n";
38:           cout << "pConstPtr width: " << pConstPtr->GetWidth() << " feet\n";
39:
40:           pRect->SetWidth(10);
41:           // pConstRect->SetWidth(10);
42:           pConstPtr->SetWidth(10);
43:
44:           cout << "pRect width: " << pRect->GetWidth() << " feet\n";
45:           cout << "pConstRect width: " << pConstRect->GetWidth() << " feet\n";
46:           cout << "pConstPtr width: " << pConstPtr->GetWidth() << " feet\n";
47:      return 0;
48: }

Output: pRect width: 5 feet
pConstRect width: 5 feet
pConstPtr width: 5 feet
pRect width: 10 feet
pConstRect width: 5 feet
pConstPtr width: 10 feet

Analysis: Lines 6-20 declare Rectangle. Line 15 declares the GetWidth() member method const. Line 32 declares a pointer to Rectangle. Line 33 declares pConstRect, which is a pointer to a constant Rectangle. Line 34 declares pConstPtr, which is a constant pointer to Rectangle.
Lines 36-38 print their values.

In line 40, pRect is used to set the width of the rectangle to 10. In line 41, pConstRect would be used, but it was declared to point to a constant Rectangle. Therefore, it cannot legally call a non-const member function; it is commented out. In line 38, pConstPtr calls SetAge(). pConstPtr is declared to be a constant pointer to a rectangle. In other words, the pointer is constant and cannot point to anything else, but the rectangle is not constant.