Returning a reference to *this, from an anonymous object on stack

Suppose the class Coord3D has a method

Point2D& Coord3D::get2DPart(){
    return *(Point2D*)this;
}

(Coord3D is just int x; int y; int z;, and Point2D is the 2D version of it with exactly int x; and int y;)

Would the following be safe:

Point2D x = Coord3D(1,2,3).get2DPart();

Or even something like

Coord3D x = Coord3D(1,2,3).getSelf();

where getSelf() is a function which does return *this;

Seems like it'll probably be safe, but people I asked a开发者_开发知识库ren't sure about it. Is this OK?

Edit: this is what coord3D and Point2D are:

struct Coord3D{
   float x; float y; float z;
};
struct Point2D{
   float x; float y;
};

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Point2D x = Coord3D(1,2,3).get2DPart();

is equivalent to:

Point2D x( Coord3D(1,2,3).get2DPart() );

The temporary Coord3D you create here will last as long as Point2D's constructor is being executed. Therefore, using Coord3D'a this in that scope is safe.

Note that *(Point2D*)this smells. If Point3D inherits from Point2D, you can just return *this. If they are unrelated, it's dangerous.

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Edit, following the question's update regarding the types:

Given Point2D and Point3D are not related, casting one into the other might work, but it's highly unadvised. A better approach would be to have Point3D inherit from Point2D. Another option would be to add, say, a GetPoint2D() to Point3D, and have it create a new 2D point out of the 3D. However, you might have then a real issue with returning references to local variables. Lastly, if you do take the risk and cast as is, at least use reinterpret_cast and not the C-style cast.

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Assuming the cast is valid in the first place (common subsequence of a standard layout type or whatever applies in your case), then yes this is valid as *this will live until ;.