Lesson 3 A safe array Objective Grouping arrays in a class

# Grouping C++ Class Arrays

Examine creating "has-a" relationship by making the safe array class a member of another class. Now that we have created a safe array, let's use our `class vect` as a member of the `class pair_vect`. Using a `class` as a member of another `class` is known as the hasa relationship. Complicated objects can be designed from simpler ones by incorporating them with the hasa relationship.
```#include "vect.h" //the safe array class
//from the previous lesson

class pair_vect {
public:
pair_vect(int i) : a(i), b(i), size(i){ }
int& first_element(int i);
int& second_element(int i);
int ub()const {return size -1;}
private:
vect a, b;
int size;
};

int& pair_vect::first_element(int i){
return a.element(i);
}

int& pair_vect::second_element(int i)
{ return b.element(i);}
```

Notice how the `pair_vect` constructor is a series of initializers. The initializers of the `vect` members `a` and `b` invoke `vect::vect(int)`. Let's use the `pair_vect class` to build a table of age and weight relationships.
```int main()
{
int     i;
pair_vect age_weight(5);  //age and weight

cout << "table of age, weight\n";
for (i = 0; i <= age_weight.ub(); ++i) {
age_weight.first_element(i) = 21 + i;
age_weight.second_element(i) = 135 + i;
cout << age_weight.first_element(i) << ","
<< age_weight.second_element(i)<< endl;
}
}
```