tmp/tmpmre6apgj/{from.md → to.md}
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## Queries and queryables <a id="exec.queryable">[[exec.queryable]]</a>
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### General <a id="exec.queryable.general">[[exec.queryable.general]]</a>
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A *queryable object* is a read-only collection of key/value pair where
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each key is a customization point object known as a *query object*. A
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*query* is an invocation of a query object with a queryable object as
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its first argument and a (possibly empty) set of additional arguments. A
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query imposes syntactic and semantic requirements on its invocations.
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Let `q` be a query object, let `args` be a (possibly empty) pack of
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subexpressions, let `env` be a subexpression that refers to a queryable
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object `o` of type `O`, and let `cenv` be a subexpression referring to
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`o` such that `decltype((cenv))` is `const O&`. The expression
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`q(env, args...)` is equal to [[concepts.equality]] the expression
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`q(cenv, args...)`.
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The type of a query expression cannot be `void`.
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The expression `q(env, args...)` is equality-preserving
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[[concepts.equality]] and does not modify the query object or the
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arguments.
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If the expression `env.query(q, args...)` is well-formed, then it is
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expression-equivalent to `q(env, args...)`.
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Unless otherwise specified, the result of a query is valid as long as
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the queryable object is valid.
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### `queryable` concept <a id="exec.queryable.concept">[[exec.queryable.concept]]</a>
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``` cpp
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namespace std {
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template<class T>
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concept queryable = destructible<T>; // exposition only
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}
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```
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The exposition-only `queryable` concept specifies the constraints on the
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types of queryable objects.
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Let `env` be an object of type `Env`. The type `Env` models `queryable`
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if for each callable object `q` and a pack of subexpressions `args`, if
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`requires { q(env, args...) }` is `true` then `q(env, args...)` meets
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any semantic requirements imposed by `q`.
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