bslmf_unwraprefdecay.h

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/// @file bslmf_unwraprefdecay.h
///
/// The content of this file has been pre-processed for Doxygen.
///
 
 
// bslmf_unwraprefdecay.h                                             -*-C++-*-
#ifndef INCLUDED_BSLMF_UNWRAPREFDECAY
#define INCLUDED_BSLMF_UNWRAPREFDECAY
 
#include <bsls_ident.h>
BSLS_IDENT("$Id: $")
 
/// @defgroup bslmf_unwraprefdecay bslmf_unwraprefdecay
/// @brief  Provide a meta-function to decay and unwrap reference wrappers.
/// @addtogroup bsl
/// @{
/// @addtogroup bslmf
/// @{
/// @addtogroup bslmf_unwraprefdecay
/// @{
///
/// <h1> Outline </h1>
/// * <a href="#bslmf_unwraprefdecay-purpose"> Purpose</a>
/// * <a href="#bslmf_unwraprefdecay-classes"> Classes </a>
/// * <a href="#bslmf_unwraprefdecay-canonical-header"> Canonical Header </a>
/// * <a href="#bslmf_unwraprefdecay-description"> Description </a>
///   * <a href="#bslmf_unwraprefdecay-usage"> Usage </a>
///     * <a href="#bslmf_unwraprefdecay-example-1-unwrap-reference-wrapped-argument-types"> Example 1: Unwrap Reference Wrapped Argument Types </a>
///
/// # Purpose {#bslmf_unwraprefdecay-purpose}
/// Provide a meta-function to decay and unwrap reference wrappers.
///
/// # Classes {#bslmf_unwraprefdecay-classes}
///
/// -  bsl::unwrap_ref_decay: standard meta-function to unwrap decayed ref wrapper
/// -  bsl::unwrap_ref_decay_t: alias to the return type of the meta-function
///
/// # Canonical Header {#bslmf_unwraprefdecay-canonical-header}
/// bsl_functional.h, bsl_type_traits.h
///
/// @see  bslmf_referencewrapper, bslmf_unwrap_reference
///
/// # Description {#bslmf_unwraprefdecay-description}
/// This component defines a meta-function `bsl::unwrap_ref_decay`
/// that may be used to unwrap a possibly cv-qualified `bsl::reference_wrapper`
/// (which is an alias of `std::reference_wrapper` if that exists)
/// specialization or reference thereto of some type `U`, resulting in `U&`.  In
/// case the specified type template argument is not a specialization of
/// `bsl::reference_wrapper` the result is the type, but decayed (`bsl::decay`).
///
/// `bsl::unwrap_ref_decay` meets the requirements of the @ref unwrap_ref_decay 
/// template defined in the C++20 standard [meta.trans.other].
///
/// ## Usage {#bslmf_unwraprefdecay-usage}
///
///
/// In this section we show intended use of this component.
///
/// ### Example 1: Unwrap Reference Wrapped Argument Types {#bslmf_unwraprefdecay-example-1-unwrap-reference-wrapped-argument-types}
///
///
/// Suppose that we work in a programming environment where function argument
/// types may be presented as is, or wrapped in a `bsl::reference_wrapper` and
/// we would like to create a member or local variable while obeying the request
/// for the reference type that the reference-wrapper represents.  As parameters
/// may be cv-qualified and may be references we need to first decay them (see
/// `bsl::decay`).  When the argument type decays into a reference-wrapper we
/// want to use a reference to the wrapped type, and simply use the decayed
/// variations of types that are not wrapped.  This is a use case for
/// `bsl::unwrap_ref_decay`.
///
/// First, we create types that may be the bases for reference-wrapped, and
/// normal type parameters:
/// @code
/// typedef bsl::reference_wrapper<int *>  WrappedType;
/// typedef int                           *NotWrappedType;
/// @endcode
/// Next, we create types that represent possible function argument types:
/// @code
/// typedef WrappedType                     WrappedTypeArray[5];
/// typedef WrappedType                    *WrappedTypePointer;
/// typedef const WrappedType               ConstWrappedType;
/// typedef volatile WrappedType            VolatileWrappedType;
/// typedef const volatile WrappedType      CvWrappedType;
/// typedef WrappedType&                    WrappedTypeRef;
/// typedef const WrappedType&              ConstWrappedTypeRef;
/// typedef volatile WrappedType&           VolatileWrappedTypeRef;
/// typedef const volatile WrappedType&     CvWrappedTypeRef;
///
/// typedef NotWrappedType                  NotWrappedTypeArray[5];
/// typedef NotWrappedType                 *NotWrappedTypePointer;
/// typedef const NotWrappedType            ConstNotWrappedType;
/// typedef volatile NotWrappedType         VolatileNotWrappedType;
/// typedef const volatile NotWrappedType   CvNotWrappedType;
/// typedef NotWrappedType&                 NotWrappedTypeRef;
/// typedef const NotWrappedType&           ConstNotWrappedTypeRef;
/// typedef volatile NotWrappedType&        VolatileNotWrappedTypeRef;
/// typedef const volatile NotWrappedType&  CvNotWrappedTypeRef;
/// @endcode
/// Finally we can verify and demonstrate how all these types turn into a
/// decayed type, and only those that decay into a reference-wrapper type will
/// become themselves references.  Notice that an array of reference wrappers
/// decays into a pointer to a wrapper, so it will not turn into a reference.
/// @code
/// assert((true == bsl::is_same<bsl::unwrap_ref_decay<WrappedTypeArray>::type,
///                              WrappedType *>::value));
/// assert((true == bsl::is_same<
///                     bsl::unwrap_ref_decay<WrappedTypePointer>::type,
///                     WrappedTypePointer>::value));
///
/// assert((true == bsl::is_same<bsl::unwrap_ref_decay<WrappedType>::type,
///                              int *&>::value));
/// assert((true == bsl::is_same<
///                     bsl::unwrap_ref_decay<ConstWrappedType>::type,
///                     int *&>::value));
/// assert((true == bsl::is_same<
///                     bsl::unwrap_ref_decay<VolatileWrappedType>::type,
///                     int *&>::value));
/// assert((true == bsl::is_same<bsl::unwrap_ref_decay<CvWrappedType>::type,
///                              int *&>::value));
/// assert((true == bsl::is_same<bsl::unwrap_ref_decay<WrappedTypeRef>::type,
///                              int *&>::value));
/// assert((true == bsl::is_same<
///                     bsl::unwrap_ref_decay<ConstWrappedTypeRef>::type,
///                     int *&>::value));
/// assert((true == bsl::is_same<
///                     bsl::unwrap_ref_decay<VolatileWrappedTypeRef>::type,
///                     int *&>::value));
/// assert((true == bsl::is_same<bsl::unwrap_ref_decay<CvWrappedTypeRef>::type,
///                              int *&>::value));
///
/// // Not wrapped types decay
/// assert((true == bsl::is_same<
///                     bsl::unwrap_ref_decay<NotWrappedTypeArray>::type,
///                     NotWrappedType *>::value));
/// assert((true == bsl::is_same<
///                     bsl::unwrap_ref_decay<NotWrappedTypePointer>::type,
///                     NotWrappedTypePointer>::value));
///
/// assert((true == bsl::is_same<bsl::unwrap_ref_decay<NotWrappedType>::type,
///                              NotWrappedType>::value));
/// assert((true == bsl::is_same<
///                     bsl::unwrap_ref_decay<ConstNotWrappedType>::type,
///                     NotWrappedType>::value));
/// assert((true == bsl::is_same<
///                     bsl::unwrap_ref_decay<VolatileNotWrappedType>::type,
///                     NotWrappedType>::value));
/// assert((true == bsl::is_same<bsl::unwrap_ref_decay<CvNotWrappedType>::type,
///                              NotWrappedType>::value));
/// assert((true == bsl::is_same<
///                     bsl::unwrap_ref_decay<NotWrappedTypeRef>::type,
///                     NotWrappedType>::value));
/// assert((true == bsl::is_same<
///                     bsl::unwrap_ref_decay<ConstNotWrappedTypeRef>::type,
///                     NotWrappedType>::value));
/// assert((true == bsl::is_same<
///                     bsl::unwrap_ref_decay<VolatileNotWrappedTypeRef>::type,
///                     NotWrappedType>::value));
/// assert((true == bsl::is_same<
///                     bsl::unwrap_ref_decay<CvNotWrappedTypeRef>::type,
///                     NotWrappedType>::value));
/// @endcode
/// Note, that (when available) the `bsl::unwrap_ref_decay_t` avoids the
/// `::type` suffix and `typename` prefix when we want to use the result of the
/// `bsl::unwrap_ref_decay` meta-function in templates.
/// @}
/** @} */
/** @} */
 
/** @addtogroup bsl
 * @{
 */
/** @addtogroup bslmf
 * @{
 */
/** @addtogroup bslmf_unwraprefdecay
 * @{
 */
 
#include <bslscm_version.h>
 
#include <bslmf_decay.h>
#include <bslmf_referencewrapper.h>
#include <bslmf_unwrapreference.h>
 
#include <bsls_compilerfeatures.h>
#include <bsls_libraryfeatures.h>
 
#if BSLS_LIBRARYFEATURES_HAS_CPP20_BASELINE_LIBRARY
  #include <type_traits>  // 'std::unwrap_reference', 'std::unwrap_reference_t'
#endif
 
                        // =======================
                        // struct unwrap_ref_decay
                        // =======================
 
namespace bsl {
 
#ifdef BSLS_LIBRARYFEATURES_HAS_CPP20_BASELINE_LIBRARY
using std::unwrap_ref_decay;
using std::unwrap_ref_decay_t;
#else
 
/// This `struct` template implements the @ref unwrap_ref_decay  meta-function
/// defined in the C++20 standard [meta.trans.other], providing an alias,
/// `type`, that returns the result.  `type` is `bsl::decay<t_TYPE>::type`
/// unless `bsl::decay<t_TYPE>::type` is a specialization of
/// `bsl::reference_wrapper` (which is an alias to `std::reference_wrapper<U>`
/// when that exists) for a type `U`, in which case `type` shall be `U&`.
///
/// See @ref bslmf_unwraprefdecay 
template <class t_TYPE>
struct unwrap_ref_decay {
    // TYPES
 
    /// This `typedef` is an alias to the reference-unwrapped type of the
    /// decayed `t_TYPE` template parameter.
    typedef typename bsl::unwrap_reference<
                                 typename bsl::decay<t_TYPE>::type>::type type;
};
 
#ifdef BSLS_COMPILERFEATURES_SUPPORT_ALIAS_TEMPLATES
/// @ref unwrap_ref_decay_t  is an alias to the return type of the
/// 'bsl::unwrap_ref_decay' meta-function.
template <class t_TYPE>
using unwrap_ref_decay_t = typename unwrap_ref_decay<t_TYPE>::type;
#endif  // BSLS_COMPILERFEATURES_SUPPORT_ALIAS_TEMPLATES
 
#endif  // else of BSLS_LIBRARYFEATURES_HAS_CPP20_BASELINE_LIBRARY
 
}  // close namespace bsl
 
#endif
 
// ----------------------------------------------------------------------------
// Copyright 2024 Bloomberg Finance L.P.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
// ----------------------------- END-OF-FILE ----------------------------------
 
/** @} */
/** @} */
/** @} */