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Re: -Wcast-qual and casting away


On Thu, May 21, 2009 at 12:10 AM, Ian Lance Taylor <iant@google.com> wrote:
> Consider this C/C++ program:
>
> extern void **f1();
> void f2(const char *p) { *(const void **)f1() = p; }
>
> If I compile this program with g++ -Wcast-qual, I get this:
>
> foo2.cc:2: warning: cast from type ‘void**’ to type ‘const void**’ casts away qualifiers
>
> If I compile this program with gcc -Wcast-qual, I do not get any
> warning.
>
> Let's overlook the fact that the text of the g++ warning does not make
> any sense--I am certainly not casting anything away. ?The warning is
> conceptually plausible for the same reason that you can't assign a
> char** variable to a const char** variable without a cast. ?At least, I
> think one could make a argument that that is so. ?But it's not a *very*
> strong argument, as -Wcast-qual is documented to warn about cases where
> a type qualifier is removed, and that is manifestly not happening here.
> -Wcast-qual is useful to catch certain programming errors; I don't think
> anybody adding a const qualifier is actually making a mistake.

In fact, the contrary is true.  The warning speaks the truth.
The implicit conversion is unsafe and the cast is explicitly
by-passing that const-safety built into the type system.

-- Gaby


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