PRB: const char* not same as const CPTR where #typedef char* C (123461)

SYMPTOMS

The following program compiles, but one of the following warnings appears:

   C4090: 'initializing' : different 'const' qualifiers:

-or-

   C2440: 'initializing' : cannot convert from 'const char *' to 'char *'
                           Conversion loses qualifiers

   const char *ReturnConstantPtr()
   {
      const char *p = "can't touch this";
      return p;
   }

   void main()
   {
       char *p = ReturnConstantPtr();
   }

However, the above program will compile without exception if a type definition is used for char* as follows:

   typedef char* CPTR;

   const CPTR ReturnConstantPtr()
   {
       const CPTR p = "can't touch this";
       return p;
   }

   void main()
   {
       CPTR p = ReturnConstantPtr();
   }

CAUSE

This occurs for two reasons. First, typedefs are not macros, so modifiers like const or volatile apply to the whole construct. When the following are used

   #typedef  char*  CPTR
   const  CPTR  cPtr;

they are equivalent to "char* const cPtr;" not "const char* cPtr;."

Second, since "char* const cPtr;" declares cPtr as a constant pointer to character data and "const char* cPtr;" declares cPtr as a pointer to constant character data, the two declarations are quite different.

Thus, a "different const qualifier" warning is generated for

   char *p = ReturnConstantPtr();

because it is trying to set p equal to a pointer that points to a constant character whose value *p could try to change. Alternatively, no warning is generated for

   CPTR p = ReturnConstantPtr();

because it is setting p equal to a constant pointer that points to a character whose value *p may freely change.

REFERENCES

For more information on the const keyword, search the online documentation provided with Visual C++, 32-bit edition, for "const" or "constant values." For more information on the typedef keyword, search the online documentation for "#typedef" or "typedef specifier."