bslmf_decay.h

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/// @file bslmf_decay.h
///
/// The content of this file has been pre-processed for Doxygen.
///
 
 
// bslmf_decay.h                                                      -*-C++-*-
#ifndef INCLUDED_BSLMF_DECAY
#define INCLUDED_BSLMF_DECAY
 
#include <bsls_ident.h>
BSLS_IDENT("$Id: $")
 
/// @defgroup bslmf_decay bslmf_decay
/// @brief  Convert a type to the type used for pass-by-value.
/// @addtogroup bsl
/// @{
/// @addtogroup bslmf
/// @{
/// @addtogroup bslmf_decay
/// @{
///
/// <h1> Outline </h1>
/// * <a href="#bslmf_decay-purpose"> Purpose</a>
/// * <a href="#bslmf_decay-classes"> Classes </a>
/// * <a href="#bslmf_decay-description"> Description </a>
///   * <a href="#bslmf_decay-usage"> Usage </a>
///     * <a href="#bslmf_decay-usage-example-1">  Usage Example 1 </a>
///
/// # Purpose {#bslmf_decay-purpose}
/// Convert a type to the type used for pass-by-value.
///
/// # Classes {#bslmf_decay-classes}
///
/// -  bsl::decay: type trait computing return type for type parameter
/// -  bsl::decay_t: alias to the return type of the `bsl::decay` meta-function
///
/// @see  bslmf_removeextent
///
/// # Description {#bslmf_decay-description}
/// This component provides a metafunction, `bsl::decay`, that
/// applies array-to-pointer and function-to-pointer conversion and
/// cv-qualification removal to a type, thus modeling the decay of an argument
/// type when passed by-value into a function.  `bsl::decay` provides identical
/// functionality to the C++11 standard metafunction `std::decay`.  From the
/// C++14, standard description of `std::decay`:
///
/// Let `U` be `remove_reference_t<T>`.  If `is_array<U>::value` is `true`, the
/// member typedef `type` shall equal `remove_extent_t<U>*`.  If
/// `is_function<U>::value` is `true`, the member typedef `type` shall equal
/// `add_pointer_t<U>`.  Otherwise the member typedef `type` equals
/// `remove_cv_t<U>`.  [ *Note*: This behavior is similar to the
/// lvalue-to-rvalue (4.1), array-to-pointer (4.2), and function-to-pointer
/// (4.3) conversions applied when an lvalue expression is used as an rvalue,
/// but also strips cv-qualifiers from class types in order to more closely
/// model by-value argument passing.  - *end note* ]
///
/// ## Usage {#bslmf_decay-usage}
///
///
///
/// ###  Usage Example 1 {#bslmf_decay-usage-example-1}
///
///
/// A class template needs to cache a value of type `T`. There is nothing in the
/// definition of the class that would prevent it from working for `T` of
/// function type or array-of-unknown bound, but one cannot simply declare a
/// member of either of those types.  Instead, we use `bsl::decay<T>::type`,
/// which can be stored, copied, and compared as needed:
/// @code
/// #ifndef INCLUDED_BSLMF_DECAY
/// #include <bslmf_decay.h>
/// #endif
///
/// template <class t_TYPE>
/// class Thing {
///   public:
///
/// #ifdef BSLS_COMPILERFEATURES_SUPPORT_ALIAS_TEMPLATES
/// @endcode
/// Note that if the current compiler supports alias templates C++11 feature, we
/// can use `bsl::decay_t` alias to the "result" type of the `bsl::decay`
/// meta-function, that avoids the `::type` suffix and `typename` prefix in the
/// declaration of the function return type:
/// @code
///     using CacheType = bsl::decay_t<t_TYPE>;
/// #else
///     typedef typename bsl::decay<t_TYPE>::type CacheType;
/// #endif
///
/// private:
///     CacheType d_cache;
///     // ...
///
/// public:
///     CacheType cache() const { return d_cache; }
/// };
/// @endcode
/// Now verify that for function and array types, `cache()` will return a simple
/// pointer:
/// @code
/// int main()
/// {
///     typedef const int A1[];
///     typedef double A2[3][4];
///     typedef void F1(int);
///     typedef int (&F2)();
///
///     assert((bsl::is_same<const int*,    Thing<A1>::CacheType>::value));
///     assert((bsl::is_same<double(*)[4],  Thing<A2>::CacheType>::value));
///     assert((bsl::is_same<void (*)(int), Thing<F1>::CacheType>::value));
///     assert((bsl::is_same<int (*)(),     Thing<F2>::CacheType>::value));
///
///     return 0;
/// }
/// @endcode
/// @}
/** @} */
/** @} */
 
/** @addtogroup bsl
 * @{
 */
/** @addtogroup bslmf
 * @{
 */
/** @addtogroup bslmf_decay
 * @{
 */
 
#include <bslscm_version.h>
 
#include <bslmf_addpointer.h>
#include <bslmf_assert.h>
#include <bslmf_isarray.h>
#include <bslmf_isfunction.h>
#include <bslmf_removecv.h>
#include <bslmf_removeextent.h>
#include <bslmf_removereference.h>
 
#include <bsls_compilerfeatures.h>
 
namespace bsl {
 
// Forward declaration
template <class t_TYPE, bool t_IS_ARRAY, bool t_IS_FUNC> struct decay_imp;
 
                        // ====================
                        // class template decay
                        // ====================
 
/// Metafunction to return the variant of the specified parameter `t_TYPE`
/// used for pass-by-reference, e.g., by applying array-to-pointer and
/// function-to-pointer conversion.
///
/// See @ref bslmf_decay 
template <class t_TYPE>
class decay {
 
    typedef typename bsl::remove_reference<t_TYPE>::type U;
    enum {
        k_ISARRAY = is_array<U>::value,
        k_ISFUNC  = is_function<U>::value
    };
 
  public:
    typedef typename decay_imp<U, k_ISARRAY, k_ISFUNC>::type type;
};
 
#ifdef BSLS_COMPILERFEATURES_SUPPORT_ALIAS_TEMPLATES
 
// ALIASES
 
/// `decay_t` is an alias to the return type of the `bsl::decay`
/// meta-function.  Note, that the @ref enable_if_t  avoids the `::type` suffix
/// and `typename` prefix when we want to use the result of the
/// meta-function in templates.
template <class t_TYPE>
using decay_t = typename decay<t_TYPE>::type;
 
#endif  // BSLS_COMPILERFEATURES_SUPPORT_ALIAS_TEMPLATES
 
// ============================================================================
//                      CLASS TEMPLATE IMPLEMENTATION
// ============================================================================
 
template <class t_TYPE, bool t_IS_ARRAY, bool t_IS_FUNC>
struct decay_imp : remove_cv<t_TYPE> {
    BSLMF_ASSERT(!(t_IS_ARRAY || t_IS_FUNC));
};
 
template <class t_TYPE>
struct decay_imp<t_TYPE, true /* t_IS_ARRAY */, false /* t_IS_FUNC */> {
    typedef typename remove_extent<t_TYPE>::type *type;
};
 
template <class t_TYPE>
struct decay_imp<t_TYPE, false /* t_IS_ARRAY */, true /* t_IS_FUNC */> {
    typedef typename add_pointer<t_TYPE>::type type;
};
 
}  // close namespace bsl
 
#endif // ! defined(INCLUDED_BSLMF_DECAY)
 
// ----------------------------------------------------------------------------
// Copyright 2016 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 ----------------------------------
 
/** @} */
/** @} */
/** @} */