I am learning C++ and I have problem to understand why this works. I understand that a reference is just synonym for a certain object. But I do not understand why this
std::vector<int> v{1,2,3}; for (auto &i : v) //using reference i *= i; outputs 1,4,9 and this
std::vector<int> v{1,2,3}; for (auto i : v) //without using reference i *= i; outputs 1,2,3?
Thank you in advance
It's the same reason why:
int a=2; int &b=a; b=3; why this ends up setting a to 3, but:
int a=2; int b=a; b=3; a is still 2, and only b is 3.
The vector itself is irrelevant, this has to do with the basic concept of what a reference is. A reference creates an alias for the referenced object. A "non-reference" creates a new object. auto &i creates a reference. auto i creates a new object and initializes it by copying its value from another object, in this case each value in the vector.
The reference is actually a reference to a certain object in memory.
So when you do auto &i you create a reference to an object (in that case it is an element from the vector). Any operations are performed on the referenced object - i.e. the element of the vector. i *= i changes the value of the element, because i is the reference to this element.
And when you do auto i you create a new variable (object) which is being stored on a stack. That object only lives in the scope of the for loop. So the changes made to it do not affect any other data.
Hope this helps.
