Why does Visual C++ warn on implicit cast from const void ** to void * in C, but not in C++?

Question

Summary

The C/C++ compiler in Microsoft Visual Studio gives warning C4090 when a C program tries to convert a pointer to pointer to const data (like <

Answer 1

Apparently this is simply a bug in VC++.

If you declare const char **x; the result is a pointer to a "read-only" pointer to chars, and it's not itself a "read-only" pointer (I use the term "read-only" because const-ness term pushes the wrong concept that the character being pointed to is constant while this is false in general... const with references and pointers is a property of the reference or of the pointer and tells nothing about constness of the pointed-to or referenced data).

Any read/write pointer can be converted to a void * and VC++ has no real reason to emit a warning when compiling that code, neither in C nor in C++ mode.

Note that this is not formally a problem because the standard doesn't mandate which warnings should or should not be issued and therefore a compiler is free to emit warnings for perfectly valid code still remaining compliant. VC++ actually emits a plethora of those warnings for valid C++ code...