An associator defines a relationship between different types and objects where, given:
a source object s of type S,
type requirements R, and
a candidate object c of type C meeting the type requirements R,
an associated type A meeting the type requirements R may be computed, and an associated object a of type A may be obtained.
An associator shall be a class template that takes two template type arguments. The first template argument is the source type S. The second template argument is the candidate type C. The second template argument shall be defaulted to some default candidate type D that satisfies the type requirements R.
An associator shall additionally satisfy the requirements in Table [tab:async.reqmts.associator.requirements]. In this table, X is a class template that meets the associator requirements, S is the source type, s is a value of type S or const S, C is the candidate type, c is a (possibly const) value of type C, D is the default candidate type, and d is a (possibly const) value of type D that is the default candidate object.
| expression | return type | assertion/note pre/post-conditions |
| X<S>::type | X<S, D>::type | |
| X<S, C>::type | The associated type. | |
| X<S>::get(s) | X<S>::type | Returns X<S>::get(S, d). |
| X<S, C>::get(s, c) | X<S, C>::type | Returns the associated object. |
The associator's primary template shall be defined. A program may partially specialize the associator class template for some user-defined type S.
Finally, the associator shall provide the following type alias and function template in the enclosing namespace:
template<class S, class C = D> using X_t = typename X<S, C>::type;
template<class S, class C = D>
typename X<S, C>::type get_X(const S& s, const C& c = d){
return X<S, C>::get(s, c);
}
where X is replaced with the name of the associator class template. [ Note: This function template is provided as a convenience, to automatically deduce the source and candidate types. — end note ]