Q149324: FIX: Modifying Const Temporary Object Doesn’t Generate Error

Article: Q149324
Product(s): Microsoft C Compiler
Version(s): 
Operating System(s): 
Keyword(s): kbVC400bug kbVC410bug kbVC420bug kbVC500bug kbVC600fix
Last Modified: 30-JUL-2001

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The information in this article applies to:

- The C/C++ Compiler (CL.EXE), used with:
   - Microsoft Visual C++, 32-bit Editions, versions 4.0, 4.1 
   - Microsoft Visual C++, 32-bit Enterprise Edition, version 4.2 
   - Microsoft Visual C++, 32-bit Professional Edition, version 4.2 
   - Microsoft Visual C++, 32-bit Enterprise Edition, version 5.0 
   - Microsoft Visual C++, 32-bit Professional Edition, version 5.0 
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SYMPTOMS
========

The compiler generates the following error when you invoke a non-const member
function on a const object:

  error C2662: 'Set' : cannot convert 'this' pointer from 'const struct A *' to
  'struct A *const '

However, it does not generate this error when you invoke a non-const member
function on a const object that is returned by a function.

RESOLUTION
==========

To work around this problem, make the function return a reference to const.

STATUS
======

This bug was corrected in Microsoft Visual C++, version 6.0.

MORE INFORMATION
================

Sample Code
-----------

  /* Compile options needed: None
  */ 

  truct A {

   int m_i;

   A() { m_i = 0; };

   void Set() {m_i = 5;}
  };

  truct B {

   A m_a;

   // Change the return type to 'const A &' to work around
   const A GetMember() const {return m_a;}
  };

  void TestFunc(const B & b)
  {
   const A a;

   // Next line correctly generates:
   // error C2662: 'Set' : cannot convert 'this' pointer from
   // 'const struct A *' to 'struct A *const '

   // a.Set(); // Uncomment this line to get the error

   // Next line does not generate the error even though
   // B::GetMember returns const A object.
   // Change the return type of B::GetMember to 'const A &'
   // to get the error.

   b.GetMember().Set();

  }

  void main()
  {
   B b;

   TestFunc(b);

  }

Additional query words:

======================================================================
Keywords          : kbVC400bug kbVC410bug kbVC420bug kbVC500bug kbVC600fix 
Technology        : kbVCsearch kbAudDeveloper kbCVCComp
Issue type        : kbbug
Solution Type     : kbfix

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