将传递引用变量分配给struct成员时会发生什么

Question

I defined this struct :

template<class T>
struct Node {
   T value;
   Node(const T& v) : value(v) {}
};

I want to know what exactly happens when I do

int i = 3;
Node<int>* my_node = new Node<int>(i);

By printing out the address of i and the address my_node->value I realized that they don't refer to the same address. I did expect this behavior because it doesn't make sense to force my new pointer to be placed at a specific address, to be able to wrap my passed-by-reference argument!!!

But I'm wondering what's the role of & ? Is it just to make sure argument is not copied when passed to the method? And if yes, is that just to avoid heavy copying? Or is there another purpose for using & ?

And value(v) or value = v simply copy the whole datatype into a new memory space, no matter if v is passed by reference or not, am I right?

Edit: I'm sorry I tried to shorten my code and ended up making it ambiguous. Just fixed it.

Answer 1

This statement is badly formed: Node* my_node = new Node(i);

But I'm wondering what's the role of &? Is it just to make sure argument is not copied when passed to the method? And if yes, is that just to avoid heavy copying (in case of a big datatype other than int in this example)? Or is there another purpose for using &?

The role of & in this case is exactly as you have said. The general rule of thumb that I use, is to pass built-ins (eg int, double) by value and pass everything else by const reference to avoid unnecessary copying.

And value(v) or value = v simply copy the whole datatype into a new memory space, no matter if v is passed by reference or not, am I right?

That is correct also. But for non-built-ins, these two statements differ when used in a constructor. The first is initialization and is done in the initialization list (which you have done in your example), the latter is assignment and is done in the constructor body. For non-built-ins, it is generally more efficient to use the initialization list. Otherwise, the object is default initialized before you do the assignment.