The pair<T1,T2> is a simple and handy way to carry around a pair of objects. One is of type T1, and another of type T2; they may be the same type, but you don't get anything extra if they are. The two members can be accessed directly, as .first and .second.

Construction is simple. The default ctor initializes each member with its respective default ctor. The other simple ctor,

    pair (const T1& x, const T2& y);
   

does what you think it does, first getting x and second getting y.

There is a constructor template for copying pairs of other types:

    template <class U, class V> pair (const pair<U,V>& p);
   

The compiler will convert as necessary from U to T1 and from V to T2 in order to perform the respective initializations.

The comparison operators are done for you. Equality of two pair<T1,T2>s is defined as both first members comparing equal and both second members comparing equal; this simply delegates responsibility to the respective operator== functions (for types like MyClass) or builtin comparisons (for types like int, char, etc).

The less-than operator is a bit odd the first time you see it. It is defined as evaluating to:

    x.first  <  y.first  ||
	( !(y.first  <  x.first)  &&  x.second  <  y.second )
   

The other operators are not defined using the rel_ops functions above, but their semantics are the same.

Finally, there is a template function called make_pair that takes two references-to-const objects and returns an instance of a pair instantiated on their respective types:

    pair<int,MyClass> p = make_pair(4,myobject);