INFO: New C++ Type Conversions May Generate C2446 or C2664 (87020)

SUMMARY

The Microsoft compilers listed above, in accordance with restrictions defined by the Annotated Reference Manual (ARM) and later X3J16 working papers, will prevent type conversions that would potentially allow the modification of data qualified as type "const". An example of this is as follows:

int             I =    8;
int          * pI =   &I;

const int      cI =   11;
const int  *  pcI =  &cI;
const int ** ppcI = &pcI;

ppcI = &pI; // This should not be allowed because later attempts
            // to perform the assignment "*pI = 0;" would
            // modify the value "**ppcI" that was declared as
            // being constant.

If the code above is compiled using C/C++ version 7.0, the following two errors will be generated:

error C2446: '=' : no conversion between 'const int __near * __near * ' and 'int __near *__near * '

error C2664: 'f_ppci' : cannot convert parameter 1 from 'int __near *__near * ' to 'const int __near *__near * '

If this same code is compiled using Visual C++ version 4.0, the following error will be generated:

error C2446: '=' : no conversion from 'int ** ' to 'const int ** ' (new behavior; please see help)

If this same code is compiled using Visual C++ version 5.0, the following error will be generated:

error C2440: '=' : cannot convert from 'int ** ' to 'const int ** ' Conversion loses qualifiers

The Microsoft C/C++ compiler is correctly generating the errors. According to the ANSI committee for C++, the compiler should produce an error.

MORE INFORMATION

There are eight legal conversions allowed by C++ (see ARM section 13.2 page 3), two of which are applicable to this example (if T represents a type):

   T  --> T const (or const T)
   T* --> T const * (or const T *)

Using the T symbol, the example given above (ppcI = &pI) could be expressed as follows:

   T** --> const T**

To demonstrate why this conversion is not legal, substitute for T in the rules given earlier and show that the conversion "T** --> const T**" cannot be derived from those two rules:

Let "int**" be T. By the first rule, then, the following is possible:

   int** --> int** const

You might immediately assume that because "T const" is the same as "const T", then "int** const" is the same as "const int**", but this is not true. The "const" keyword modifies whatever falls directly to the right of it. For example:

const int* D;  // "int" is to the right of "const," so D is a
               // pointer to a constant integer. The compiler
               // will protect the value of *D

int* const D;  // D is to the right of "const" and D is a
               // pointer, so D would be called a "constant pointer
               // to an integer." The compiler will allow you to
               // modify *D, but not D itself.

const int D;   // These two are identical.
int const D;

Hence, the first legal conversion (T --> T const) does not apply to this example.

Try the second rule, and let 'int*' be T, which gives you:

   'int*'* --> 'int*' const *

The type "int* const *" can be translated as "a constant pointer to a pointer that points to an integer." The target is "a pointer to a pointer that points to a constant integer." So the second legal conversion is also not applicable to this example. With this, it is apparent that there is no legal combination of rules that will allow the conversion "T** --> const T**" to be made.