gwj / asio_examples · Files

//
// query.hpp
// ~~~~~~~~~
//
// Copyright (c) 2003-2020 Christopher M. Kohlhoff (chris at kohlhoff dot com)
//
// Distributed under the Boost Software License, Version 1.0. (See accompanying
// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
//

#ifndef ASIO_QUERY_HPP
#define ASIO_QUERY_HPP

#if defined(_MSC_VER) && (_MSC_VER >= 1200)
# pragma once
#endif // defined(_MSC_VER) && (_MSC_VER >= 1200)

#include "asio/detail/config.hpp"
#include "asio/detail/type_traits.hpp"
#include "asio/is_applicable_property.hpp"
#include "asio/traits/query_member.hpp"
#include "asio/traits/query_free.hpp"
#include "asio/traits/static_query.hpp"

#include "asio/detail/push_options.hpp"

#if defined(GENERATING_DOCUMENTATION)

namespace asio {

/// A customisation point that queries the value of a property.
/**
 * The name <tt>query</tt> denotes a customization point object. The
 * expression <tt>asio::query(E, P)</tt> for some
 * subexpressions <tt>E</tt> and <tt>P</tt> (with types <tt>T =
 * decay_t<decltype(E)></tt> and <tt>Prop = decay_t<decltype(P)></tt>) is
 * expression-equivalent to:
 *
 * @li If <tt>is_applicable_property_v<T, Prop></tt> is not a well-formed
 *   constant expression with value <tt>true</tt>, <tt>asio::query(E,
 *   P)</tt> is ill-formed.
 *
 * @li Otherwise, <tt>Prop::template static_query_v<T></tt> if the expression
 *   <tt>Prop::template static_query_v<T></tt> is a well-formed constant
 *   expression.
 *
 * @li Otherwise, <tt>(E).query(P)</tt> if the expression
 *   <tt>(E).query(P)</tt> is well-formed.
 *
 * @li Otherwise, <tt>query(E, P)</tt> if the expression
 *   <tt>query(E, P)</tt> is a valid expression with overload
 *   resolution performed in a context that does not include the declaration
 *   of the <tt>query</tt> customization point object.
 *
 * @li Otherwise, <tt>asio::query(E, P)</tt> is ill-formed.
 */
inline constexpr unspecified query = unspecified;

/// A type trait that determines whether a @c query expression is well-formed.
/**
 * Class template @c can_query is a trait that is derived from
 * @c true_type if the expression <tt>asio::query(std::declval<T>(),
 * std::declval<Property>())</tt> is well formed; otherwise @c false_type.
 */
template <typename T, typename Property>
struct can_query :
  integral_constant<bool, automatically_determined>
{
};

/// A type trait that determines whether a @c query expression will
/// not throw.
/**
 * Class template @c is_nothrow_query is a trait that is derived from
 * @c true_type if the expression <tt>asio::query(std::declval<T>(),
 * std::declval<Property>())</tt> is @c noexcept; otherwise @c false_type.
 */
template <typename T, typename Property>
struct is_nothrow_query :
  integral_constant<bool, automatically_determined>
{
};

/// A type trait that determines the result type of a @c query expression.
/**
 * Class template @c query_result is a trait that determines the result
 * type of the expression <tt>asio::query(std::declval<T>(),
 * std::declval<Property>())</tt>.
 */
template <typename T, typename Property>
struct query_result
{
  /// The result of the @c query expression.
  typedef automatically_determined type;
};

} // namespace asio

#else // defined(GENERATING_DOCUMENTATION)

namespace asio_query_fn {

using asio::decay;
using asio::declval;
using asio::enable_if;
using asio::is_applicable_property;
using asio::traits::query_free;
using asio::traits::query_member;
using asio::traits::static_query;

void query();

enum overload_type
{
  static_value,
  call_member,
  call_free,
  ill_formed
};

template <typename T, typename Properties, typename = void>
struct call_traits
{
  ASIO_STATIC_CONSTEXPR(overload_type, overload = ill_formed);
  ASIO_STATIC_CONSTEXPR(bool, is_noexcept = false);
  typedef void result_type;
};

template <typename T, typename Property>
struct call_traits<T, void(Property),
  typename enable_if<
    (
      is_applicable_property<
        typename decay<T>::type,
        typename decay<Property>::type
      >::value
      &&
      static_query<T, Property>::is_valid
    )
  >::type> :
  static_query<T, Property>
{
  ASIO_STATIC_CONSTEXPR(overload_type, overload = static_value);
};

template <typename T, typename Property>
struct call_traits<T, void(Property),
  typename enable_if<
    (
      is_applicable_property<
        typename decay<T>::type,
        typename decay<Property>::type
      >::value
      &&
      !static_query<T, Property>::is_valid
      &&
      query_member<T, Property>::is_valid
    )
  >::type> :
  query_member<T, Property>
{
  ASIO_STATIC_CONSTEXPR(overload_type, overload = call_member);
};

template <typename T, typename Property>
struct call_traits<T, void(Property),
  typename enable_if<
    (
      is_applicable_property<
        typename decay<T>::type,
        typename decay<Property>::type
      >::value
      &&
      !static_query<T, Property>::is_valid
      &&
      !query_member<T, Property>::is_valid
      &&
      query_free<T, Property>::is_valid
    )
  >::type> :
  query_free<T, Property>
{
  ASIO_STATIC_CONSTEXPR(overload_type, overload = call_free);
};

struct impl
{
  template <typename T, typename Property>
  ASIO_NODISCARD ASIO_CONSTEXPR typename enable_if<
    call_traits<T, void(Property)>::overload == static_value,
    typename call_traits<T, void(Property)>::result_type
  >::type
  operator()(
      ASIO_MOVE_ARG(T),
      ASIO_MOVE_ARG(Property)) const
    ASIO_NOEXCEPT_IF((
      call_traits<T, void(Property)>::is_noexcept))
  {
    return static_query<
      typename decay<T>::type,
      typename decay<Property>::type
    >::value();
  }

  template <typename T, typename Property>
  ASIO_NODISCARD ASIO_CONSTEXPR typename enable_if<
    call_traits<T, void(Property)>::overload == call_member,
    typename call_traits<T, void(Property)>::result_type
  >::type
  operator()(
      ASIO_MOVE_ARG(T) t,
      ASIO_MOVE_ARG(Property) p) const
    ASIO_NOEXCEPT_IF((
      call_traits<T, void(Property)>::is_noexcept))
  {
    return ASIO_MOVE_CAST(T)(t).query(ASIO_MOVE_CAST(Property)(p));
  }

  template <typename T, typename Property>
  ASIO_NODISCARD ASIO_CONSTEXPR typename enable_if<
    call_traits<T, void(Property)>::overload == call_free,
    typename call_traits<T, void(Property)>::result_type
  >::type
  operator()(
      ASIO_MOVE_ARG(T) t,
      ASIO_MOVE_ARG(Property) p) const
    ASIO_NOEXCEPT_IF((
      call_traits<T, void(Property)>::is_noexcept))
  {
    return query(ASIO_MOVE_CAST(T)(t), ASIO_MOVE_CAST(Property)(p));
  }
};

template <typename T = impl>
struct static_instance
{
  static const T instance;
};

template <typename T>
const T static_instance<T>::instance = {};

} // namespace asio_query_fn
namespace asio {
namespace {

static ASIO_CONSTEXPR const asio_query_fn::impl&
  query = asio_query_fn::static_instance<>::instance;

} // namespace

template <typename T, typename Property>
struct can_query :
  integral_constant<bool,
    asio_query_fn::call_traits<T, void(Property)>::overload !=
      asio_query_fn::ill_formed>
{
};

#if defined(ASIO_HAS_VARIABLE_TEMPLATES)

template <typename T, typename Property>
constexpr bool can_query_v
  = can_query<T, Property>::value;

#endif // defined(ASIO_HAS_VARIABLE_TEMPLATES)

template <typename T, typename Property>
struct is_nothrow_query :
  integral_constant<bool,
    asio_query_fn::call_traits<T, void(Property)>::is_noexcept>
{
};

#if defined(ASIO_HAS_VARIABLE_TEMPLATES)

template <typename T, typename Property>
constexpr bool is_nothrow_query_v
  = is_nothrow_query<T, Property>::value;

#endif // defined(ASIO_HAS_VARIABLE_TEMPLATES)

template <typename T, typename Property>
struct query_result
{
  typedef typename asio_query_fn::call_traits<
      T, void(Property)>::result_type type;
};

} // namespace asio

#endif // defined(GENERATING_DOCUMENTATION)

#include "asio/detail/pop_options.hpp"

#endif // ASIO_QUERY_HPP