Q92614: INFO: Initializing const Member Data

Article: Q92614
Product(s): Microsoft C Compiler
Version(s): 1.0,1.5,1.51,2.0,2.1,4.0,5.0
Operating System(s): 
Keyword(s): kbLangCPP kbVC100 kbVC150 kbVC151 kbVC200 kbVC210 kbVC400 kbVC500
Last Modified: 18-DEC-2001

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The information in this article applies to:

- Microsoft Visual C++, versions 1.0, 1.5, 1.51, 2.0, 2.1, 4.0 
- Microsoft Visual C++, 32-bit Enterprise Edition, version 5.0 
- Microsoft Visual C++, 32-bit Professional Edition, version 5.0 
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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

Using Visual C++ 32-bit Edition, version 4.0, this error is reported as:

  error C2166: l-value specifies const object

The member initialization list is the preferred method to initialize the
members of a class. For example, when initializing an object of a user-
defined type, it is more efficient to use a member initialization list to
initialize a data member than to perform an assignment in the constructor.
The latter approach requires two calls: one to construct the data member
and another to the operator=() function to assign a value to the member.

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

SUMMARY
=======



Additional query words:

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Keywords          : kbLangCPP kbVC100 kbVC150 kbVC151 kbVC200 kbVC210 kbVC400 kbVC500 
Technology        : kbVCsearch kbVC400 kbAudDeveloper kbvc150 kbvc100 kbVC500 kbVC151 kbVC200 kbVC210 kbVC32bitSearch kbVC500Search
Version           : :1.0,1.5,1.51,2.0,2.1,4.0,5.0
Issue type        : kbinfo

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