How can I create a polymorphic object on the stack?

Question

How do I allocate a polymorphic object on the stack? I\'m trying to do something similar to (trying to avoid heap allocation with new)?:

A* a = NULL;

switch         


        

Answer 1

You can do it with placement new. This will place the items on the stack, in the memory contained in the buffer. However, these variables are not automatic. The downside is that your destructors won't run automatically, you would need to properly destruct them just as you've created them when they go out of scope.

A reasonable alternative to manually calling the destructor is to wrap your type in a smart pointer, as shown below:

class A
{
public:
   virtual ~A() {}
};

class B : public A {};
class C : public B {};

template
class JustDestruct
{
public:
   void operator()(const T* a)
   {
      a->T::~T();
   }
};

void create(int x)
{
    char buff[1024] // ensure that this is large enough to hold your "biggest" object
    std::unique_ptr> t(buff);

    switch(x)
    {
    case 0:
       ptr = new (buff) A();
       break;

    case 1:
       ptr = new (buff) B();
       break;

    case 2:
       ptr = new (buff) C();
       break;
    }

    // do polymorphic stuff
}