Re: Geeting hash_map values back

James Kanze <>
Wed, 23 Jan 2008 00:31:24 -0800 (PST)
On Jan 22, 2:48 pm, "C C++ C++" <> wrote:

On Jan 22, 6:35 pm, James Kanze <> 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

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)
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