Q122540: FIX: ‘char *’ Passed to ‘const char * &’ Violates Type Safety

Article: Q122540
Product(s): Microsoft C Compiler
Version(s): 1.0,1.5,1.52,2.0,2.1,4.0,4.1,4.2,5.0
Operating System(s): 
Keyword(s): kbVC150bug kbVC152bug kbVC200bug kbVC210bug kbVC400bug kbVC410bug kbVC420bug kbVC500bug
Last Modified: 11-FEB-2002

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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 1.0, 2.0, 2.1, 4.0, 4.1 
   - Microsoft Visual C++, 32-bit Enterprise Edition, versions 4.2, 5.0 
   - Microsoft Visual C++, 32-bit Professional Edition, versions 4.2, 5.0 
   - Microsoft Visual C++ for Windows, 16-bit edition, versions 1.5, 1.52 
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SYMPTOMS
========

Visual C++ incorrectly allows a pointer to be passed to a function that takes a
non-const reference to a pointer to a const. This breaks C++ type safety and
allows the const data assigned to the reference to be modified by the pointer
after the function has returned.

RESOLUTION
==========

To protect the const data, the reference should be a reference to a const
pointer to a const, that is: const char * const &.

STATUS
======

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

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

A reference may only be initialized with an lvalue expression of the same type
or a class derived from that type. If this is not the case, the reference must
be declared const.

The following sample includes a non-const reference of type const char*, which is
being initialized with an expression of type char*. The compiler should, but
does not, generate an error. The program prints this string:

  const_ptr = Xhis is a 'const char *'

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

  /* Compile options needed: none
  */ 

  #include <iostream.h>

  onst char * const_ptr = "This is a 'const char *'";

  void func ( const char* & reference) {
     reference = const_ptr;
  }

  void main() {
  //  char *non_const_ptr = const_ptr;  // Not allowed.
     char *non_const_ptr;

  //  *const_ptr = 'X';      // Not allowed.

     func( non_const_ptr );
     // 
     //  'const char *' is violated now.
     // 
     *non_const_ptr = 'X';    // Mess up first letter of string

     cout << "const_ptr = " << const_ptr << endl;

  }

Additional query words: 8.0c 8.00c 9.0 9.00 10.00 10.10 10.20

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Keywords          : kbVC150bug kbVC152bug kbVC200bug kbVC210bug kbVC400bug kbVC410bug kbVC420bug kbVC500bug kbVC600fix 
Technology        : kbVCsearch kbAudDeveloper kbCVCComp
Version           : :1.0,1.5,1.52,2.0,2.1,4.0,4.1,4.2,5.0
Issue type        : kbbug
Solution Type     : kbfix

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