C++：将空对象分配给现有对象

Question

EDIT: fixed up code to clear up some confusion

As the title says I am wondering what happens to an object when I assign an empty object to it.

Here's a specific example with just the operator overloads and class data members:

class triangle
{
  vector3d p[3];
  vector2d *uv = nullptr;
 int nChannels = 0;
 public:
void operator=(const triangle &obj)
{
  delete[] uv;
  nChannels= obj.nChannels;
  memcpy(p, obj.p, sizeof(vec3d) * 3);
  uv = new vec2d[3 * obj.nChannels];
  memcpy(uv, obj.uv, 3 * obj.nChannels * sizeof(vec2d));

}

}

int main()
{
 triangle p;
 d = p; // where d is some initialized or uninitialized instance of triangle
 d = p;
}

In short, what happens when I assign an object which has one member that has not been initialized, to another object and what happens when I call new triangle[0], I would expect when I call new triangle[0] to be given a nullptr but I am not so sure anymore.

Answer 1

what happens when I assign an object who has one member that has not been initialized

The assignment operator will be called. In this case, you've provided a user declared assignment operator.

The provided operator (like an implicitly generated one) reads the state of the right hand operand, which in this case is default initialised. p lacks an initialiser, so the implicit default constructor leaves it with an indeterminate value. The behaviour of reading an indeterminate value is undefined.

That is, if the program could be compiled in the first place. It uses undeclared identifiers, and is ill-formed.

and what happens when I call new triangle[0]

An array of length 0 would be created.

would expect when I call new triangle[0] to be given a nullptr

Your expectation is misguided. The resulting pointer would be non-null (unless you used std::nothrow and the allocation failed). Indirecting through that pointer would have undefined behaviour. The program would leak memory unless the allocation is released with delete[] .