Shared void pointers. Why does this work?
c++, c++11, shared-ptr, smart-pointers, visual-c++
Solution
The `shared_ptr` constructor that you use is actually a constructor template that looks like:
template <typename U>
shared_ptr(U* p) { }
It knows inside of the constructor what the actual type of the pointer is (`X`) and uses this information to create a functor that can correctly `delete` the pointer and ensure the correct destructor is called. This functor (called the `shared_ptr`'s "deleter") is usually stored alongside the reference counts used to maintain shared ownership of the object.
Note that this only works if you pass a pointer of the correct type to the `shared_ptr` constructor. If you had instead said:
SharedVoidPointer sp1(static_cast<void*>(x));
then this would not have worked because in the constructor template, `U` would be `void`, not `X`. The behavior would then have been undefined, because you aren't allowed to call `delete` with a void pointer.
In general, you are safe if you always call `new` in the construction of a `shared_ptr` and don't separate the creation of the object (the `new`) from the taking of ownership of the object (the creation of the `shared_ptr`).
Problem
To solve a very peculiar problem in my application I need a shared-pointer to allocated data, but to the outside world, the underlying data type should remain hidden. I could solve this by making some kind of Root class of which all my other classes inherit, and use a shared_ptr on this Root class, like this: ``` std::shared_ptr<Root> ``` However: - I don't want all my classes to inherit from this Root class just to be able to have this shared pointer - Sometimes I want to return a shared pointer to std::vector, or std::list, or std::set, ... which obviously don't inherit from my Root class Strange enough, it seems that you can create a shared_ptr on void and this seems to work correctly, like shown in this example: ``` class X { public: X() {std::cout << "X::ctor" << std::endl;} virtual ~X() {std::cout << "X::dtor" << std::endl;} }; typedef std::shared_ptr<void> SharedVoidPointer; int main() { X *x = new X(); SharedVoidPointer sp1(x); } ``` x is correctly deleted and in a larger experiment I could verify that the shared pointer does indeed what it needs to do (delete x afer the last shared_ptr turns out the light). Of course this solves my problem, since I can now return data with a SharedVoidPointer data member and be sure that it's correctly cleaned up where it should be. But is this guaranteed to work in all cases? It clearly works in Visual Studio 2010, but does this also work correctly on other compilers? On other platforms?