34 Execution control library [exec]

A queryable object is a read-only collection of key/value pair where each key is a customization point object known as a query object.

A query is an invocation of a query object with a queryable object as its first argument and a (possibly empty) set of additional arguments.

A query imposes syntactic and semantic requirements on its invocations.

Let q be a query object, let args be a (possibly empty) pack of subexpressions, let env be a subexpression that refers to a queryable object o of type O, and let cenv be a subexpression referring to o such that decltype((cenv)) is const O&.

The expression q(env, args...) is equal to ([concepts.equality]) the expression q(cenv, args...).

The type of a query expression cannot be void.

The expression q(env, args...) is equality-preserving ([concepts.equality]) and does not modify the query object or the arguments.

If the expression env.query(q, args...) is well-formed, then it is expression-equivalent to q(env, args...).

Unless otherwise specified, the result of a query is valid as long as the queryable object is valid.