Explanation of this new() statement

Question

I am reviewing a section of C++ code an I came accross this statement block:

static void Vector3DefaultConstructor(Vector3 *self)
{    
    new(self) Vector3();    
}

I have not come accross the new operator being used in this way before. Can someone explain why new is being called in this way?

Answer 1

This is called "placement new". By default, it does not allocate memory but instead constructs the object at the given location (here, self). It can, however, be overloaded for a class.

See the FAQ for more info.

The correct way to destroy an object constructed using placement new is by calling the destructor directly:

obj->~Vector3();

Answer 2

Can someone explain why new is being called in this way?

I can tell you what it does. It effectively calls the constructor on an arbitrary piece of memory, thus constructing the object in that piece of memory. Regular new both allocates memory and constructs an object in that memory; placement new only does the latter part.

As to why anyone would write that code? No clue. It would make more sense if the object took a void* rather than a Vector3*. But unless Vector3::Vector3() is private, there's no reason to hide the placement new usage inside a static function like this.