Re: Geeting hash_map values back
On Jan 22, 2:48 pm, "C C++ C++" <m.azm...@gmail.com> wrote:
On Jan 22, 6:35 pm, James Kanze <james.ka...@gmail.com> wrote:
[...]
Use static_cast. With hash_map, there should be no problem. In
general, however, do be careful that you go through the right
types---something like:
extern "C" void*
threadFunction( void* p )
{
Base* args = static_cast< Base* >( p ) ;
// ...
}
// ...
Derived args ;
pthread_create( ..., &args ) ;
has undefined behavior. (The call to pthread_create should be:
pthread_create( ..., static_cast< Base* >( &args ) ) ;
fourth agument should be void *, how can you pass above one ?
A pointer to object type converts implicitly to a void*; you
don't need an explicit conversion.
The important thing about void*, here, is that the *only* thing
you can really do with it, legally, is convert it back to the
original type. If you convert a Derived* (here, &args) to
void*, and then convert the void* to a Base* in threadFunction,
you have undefined behavior. In the simplest cases, it will
probably work, but throw in a bit of multiple inheritance or
virtual inheritance, and it's almost certainly not going to
work.
Whence the static_cast to Base*, before the implicit conversion
to void*. (The implicit conversion has the semantics of a
static_cast. And no, static_cast is not transitive:
static_cast< void* >( &aDerived ) is *not* the same as
static_cast< void* >( static_cast< Base* >( &aDerived ) ).)
--
James Kanze (GABI Software) email:james.kanze@gmail.com
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