digitalmars.D.learn - const void* when wrapping C++
- Andrej Mitrovic <andrej.mitrovich gmail.com> Sep 15 2012
C/C++ sometimes uses a declaration such as 'cost void*', which is a pointer to const void. D doesn't have an equivalent to this, since using const(void)* is illegal. The problem is if there are two overloads in a C++ class such as this: foo( const void* ) // equivalent to const(void)*, illegal in D foo( void * const ) // equivalent to const(void*), ok in D then this becomes hard to recreate on the D side. It's not easy to simulate it with a template either, since 'void' can't be passed to a template as a type: foo( Const!(void)* ) // clever try but won't work, can't pass 'void' to a template Some other template tricks might be used but it could really complicate the user code. SWIG for example by default ignores one of the methods. The alternative is to rename one of the methods on the D side, but this hardly fixes anything since now the user has to figure out which method to call as the two methods end up having the same type signature (void*). So the question is why is const(void)* illegal in D? It could have some helpful semantics, e.g. the compiler could limit type casts: const(void)* ptr; const(int)* intPtr = cast(const(int)*)ptr; // ok int* intPtr = cast(int*)ptr; // not ok And it would make wrapping C/C++ easier.
Sep 15 2012