INFO: Initializing const Member Data

• Microsoft C/C++ for MS-DOS, version 7.0,
• Microsoft Visual C++ for Windows, versions 1.0, 1.5, 1.51
• Microsoft Visual C++ 32-bit Edition, versions 1.0, 2.0, 2.1, 4.0, 5.0

The sample source code below demonstrates how to initialize a const member of a class. Note that the code uses a member initialization list because a const member may not appear on the left side of the assignment operator. The value must be set when the object is constructed because the value of a const member can be set only at initialization.

If an application attempts to initialize a const member with the assignment operator, the compiler generates the following error messages:

   error C2758: 'a' : must be initialized in constructor base/member
      initializer list
   error C2166: lvalue specifies const object

   error C2166: l-value specifies const object

Sample Code

   /*
    * Compile options needed: none
    */

   #include <iostream.h>

   class demo
   {
      const int a;

   public:
      demo(int val = 0) : a(val) {}
      int get_a(void) { return a; }
   };

   void main()
   {
      demo first;
      demo second(5);

      cout << "First = " << first.get_a() << endl;

      cout << "Second = " << second.get_a() << endl;
   }

Output

First = 0 Second = 5

Keywords          : CPPLngIss
Version           : MS-DOS:7.0;WINDOWS:1.0,1.5,1.51;WINDOWS NT:1.0,2.0,2.1,4.0,5.0
Platform          : MS-DOS NT WINDOWS
Issue type        : kbinfo