C++ std::pair code understanding

Question

So I am looking through this handout that describes the code for std::pair . Below is the code:

template <class U, class V>
struct pair {
    U first;
    V second;
    pair(const U& first = U(), const V& second = V()) :
        first(first), second(second) {}
};
template <class U, class Y>
pair<U, V> make_pair(const U& first, const V& second);

I am trying to understand this code but I am having problems, specifically at the line pair in the struct. I understand that we store two create two variables first and second according to the respective classes.

In the argument of the pair function, I see that we create a new class U and V and assign them respectively to first and second , but I don't clearly understand how the const U& works because of the ampersand sign. What's more confusing is the use of a colon after the function declaration which I have never seen before used in c++.

I also don't understand the line below declaring first(first) and second(second) especially with the brackets. Isn't first a type, so how are we able to call a function from first ?

Answer 1

We'll address this by dividing it into parts.

U& means that we are passing a variable of type U that will be used by reference - the variable used by the constructor is the same one (same memory address and value) that is given as an argument. By saying const U& first = U() we are saying that we promise not to change the first passed into the constructor ( const ... ), we want first to be taken by reference ( ...U&... ), and if we don't provide first in the constructor we should use a U provided by U 's default constructor ( ... = U() ). For more information on references, this page should help.

first(first) is part of a "constructor initialization list" - the preferred method for initializing class member variables in a constructor. We tell the constructor that we are initializing the member variable of pair called first ( first(...) ) with the argument first provided by the constructor (the U& we discussed earlier). For more information on constructor initialization lists, this page should help.