Using placement new for a polymorphic unique_ptr
c++, c++11, new-operator, smart-pointers
Solution
This isn't going to work. A `Foo` object is simply too big to fit in the memory allocated for a `Bar` object. If you want `bp` to point to a `Foo` object, you're going to have to allocate enough space for one.
Placement new constructs an object in memory you already own. It's on you to make sure that memory is big enough to hold the object you're constructing.
Problem
I read this post about using placement new to reset a `boost::shared_ptr` whilst avoiding additional memory allocations, and assume that the same, if not similar, can be done for a `std::unique_ptr`? My question is when the `std::unique_ptr` is of type `Base*` and so can point to any `Derived*`, will the `placement new` work as intended if the `Derived` classes vary in size? Something like this maybe: ``` class Base { public: Base() {} virtual ~Base(){} }; class Foo : public Base { public: Foo() : Base() {} virtual ~Foo(){} int a; int b; }; class Bar : public Base { public: Bar() : Base() {} virtual ~Bar() {} int a; }; int main() { std::unique_ptr<Base> bp(new Bar()); bp->~Base(); //edit: call destructor void* rawP = dynamic_cast<void*>(bp.release());//edit: cast to void* bp.reset(new(rawP) Foo()); return 0; } ```