4 ms·
I found it's much easier to understand the concepts behind move operations if you write an object that implemented move semantics by itself. class Object {
by gpu_explorer 7y ago
I found it's much easier to understand the concepts behind move operations if you write an object that implemented move semantics by itself.
class Object
{
int* data = new int[32];
void move_into( Object& object )
{
object.data = data;
data = nullptr;
}
~Object()
{
delete[] data;
}
}
Object a;
Object b;
a.move_into( b );
I think much of the confusion arises from wondering where is the allocation for int* data located? It's somewhere on the heap. What if the data member were int data[32] instead? What would the class look like? What would 'a' look like afterwards?
- earenndil 7y agoDon't you have to destroy the target object, before you move into it?
- gpu_explorer 7y agoIn this example no because of pointer. Where is the allocation for int* data located? It's not inside the object. You can think easily of the state of 'a' after calling move_into. But maybe with int data[32] as a data member this is harder to reason about. This is a way to illustrate move semantics and confusion behind what happens to the 'moved from' object.
- AnimalMuppet 7y agoYou made the moved-to pointer point to the moved-from allocated heap data. Now nothing points to the heap data that was initially allocated by the moved-to object. So if you don't delete[] the moved-to data pointer, then you leaked 32 bytes of heap.
- gpu_explorer 7y agoWhat happens when the program exits?
- AnimalMuppet 7y agoThen it doesn't matter. It matters if you run out of heap before the program exits, though.
- mark-r 7y agoNo, you don't have to destroy the target object. You may need to destroy some of its members depending on the object implementation.
- marksmith2996 7y agoYes, this is a memory leak.
- stinos 7y agoWhy would you alter data (set it to nullptr) in a destructor?
- gpu_explorer 7y agoFrom my copy paste example. I fixed thank you.
- slavik81 7y agoI appreciate it's just an example, but it should be mentioned that this is a really bad class. If you write a custom destructor, you must also specify the move/copy constructors and move/assignment operators. Otherwise, the same data pointer will be deleted twice if the object is ever copied.
- gpu_explorer 7y agoYes I know this of course but it seemed that leaving things out is faster and people can know it's just a toy.