operator = on vectors of different dimensions

Question

consider the following code:

class vector{
   // ...
   vector(int size){ /*...*/ };
   vector& operator= (const vector& other){
   // ...
   }
};

int main(){
   vector v1(5), v2(10);
   v1 = v2;
}

What is my operator = supposed to do here? v1 does not have enough capacity to store the elements of v2. From my point of view, it can either reinitialize itself to a capacity of 10 and copy the other vector's elements or throw an exception. I usually choose the former approach but increasingly often see the latter one. Which one is the correct one?

Answer 1

Typically a user would expect that after an assignment x = y, the equality x == y should be true: assignment confers semantic equivalence. Doing anything else would be highly unusual and surprising. That probably includes not throwing an exception in response to the instruction "make x like y".

Answer 2

It all depends on what a vector is.

If it's an automatic resizing vector, you would expect a resize. std::vector does just that.

If it's a mathematical vector for matrix operations, where you don't allow the vector size to change, then it should throw an exception.