Is it ok to static_cast a void* pointer

Question

For example, propose map<int,void*>hold where void* is always storing pointers from classA is it safe to cast it back later via static_cast?

classA* ptr = static_cast<classA*>( holditerator->second );

the reason void* is used is because hold is member of a class defined on a header used by some cpp files which don't know what classA is. I would have to include the header of classA definitions on these cpp files which can't be done by many reasons.

Answer 1

Yes, static_cast is OK in that case and the right thing to use.

I have to ask why you don't store classA* pointers in the first place though. If you want to put derived class pointers into it, then beware, you need to upcast/upconvert (implicitly or explicitly) the derived class pointers to classA* before you put them into the map.

But even if you put also derived class pointers into the map, a base class pointer would suffice because a derived class pointer is implicitly convertible to a base class pointer.

The reason void* is used is because hold is member of a class define on a header used by some cpp files which don't know what classA is.

That can be a valid reason to prevent layering violations.

I would have to include the header of classA definitions on these cpp files which can't be done by many reasons.

That's most probably not necessary in your case. A forward declaration suffices. If the header knows what is put into the map, but just wants to avoid including additional headers, this is the way to go.

Answer 2

As Johannes explained, the static_cast is ok. Another technique to prevent the dependencies to ClassA in the cpp files is to use the pimpl idiom.

// in header file
class classB {
public:
    classB();
    ~classB();
private:
    class impl;
    unique_ptr<impl> pimpl;
};



// in implementation file
#include "classA.hpp"

class classB::impl 
{
    std::map<int, classA> hold;  // hidden in implementation file
};

classB::classB() : pimpl{ new impl{ /*...*/ } } { }
classB::~classB() { }