FIX: 'char ' Passed to 'const char &' Violates Type Safety

ID: Q122540

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, 4.2, 5.0
- Microsoft Visual C++ for Windows, 16-bit edition, versions 1.5, 1.52

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