Re: [PATCH,c++] use XALLOCAVEC in the C++ front-end

[Jason Merrill <jason at redhat dot com>]

On 06/08/2010 10:43 AM, Manuel López-Ibáñez wrote:
> On 8 June 2010 16:06, Jason Merrill<jason@redhat.com>  wrote:

> > The error message is unclear, but the issue is that converting from char**
> > to char const** is unsafe, while converting to char const*const* is safe.
>
> What does it mean unsafe in this context? I could produce a patch that
> changes the message but I would like to be sure what to say.
>
> semantics.c:1223:7: error: new ‘const’ qualifier in cast from 'char
> **' to 'const char **' is unsafe [-Werror=cast-qual]

Explanation from the C++ standard (4.4 [conv.qual]):

[ Note: if a program could assign a pointer of type T** to a pointer of 
type const T** (that is, if line #1 below were allowed), a program could 
inadvertently modify a const object (as it is done on line #2). For example,

  int main() {
    const char c = ’c’;
    char* pc;
    const char** pcc = &pc;// #1: not allowed
    *pcc = &c;
    *pc = ’C’; // #2: modifies a const object
  }
--end note ]