Contains - by value.

class X
{
private Y itsY;
};

!! indicates that X and Y have identical lifetimes.
When X is destroyed, Y will be destroyed too

Uses.

class X
{
public: void F(const Y&);
};

Y is used within the source code of class X.
Have different lifeline. Y may outlive X.

Contains - by reference.

class X
{
private:
Y* itsY;
};

This is used to indicate that X and Y have dissimilar lifetimes.
Y may outlive X.

Inheritance.

Class Y:X

{}

This indicates that Y is a public
base class of X. have same lifetime.

Invoking a member function.

void X::F(Y& theY)
{
P p;
V v = theY.M(p);
}

Using a container class.

class X
{
private:
Set<Y*> itsYs;
};

Contains array or so.

Can have different lifetimes