// bslmf_decay.h -*-C++-*-
#ifndef INCLUDED_BSLMF_DECAY
#define INCLUDED_BSLMF_DECAY
#include <bsls_ident.h>
BSLS_IDENT("$Id: $")
//@PURPOSE: Convert a type to the type used for pass-by-value.
//
//@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_ALSO: bslmf_removeextent
//
//@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
///-----
//
/// 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:
//..
// #ifndef INCLUDED_BSLMF_DECAY
// #include <bslmf_decay.h>
// #endif
//
// template <class t_TYPE>
// class Thing {
// public:
//
//#ifdef BSLS_COMPILERFEATURES_SUPPORT_ALIAS_TEMPLATES
//..
// 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:
//..
// 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; }
// };
//..
// Now verify that for function and array types, 'cache()' will return a simple
// pointer:
//..
// 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;
// }
//..
#include <bslscm_version.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
// ====================
template <class t_TYPE>
class 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.
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
template <class t_TYPE>
using decay_t = typename decay<t_TYPE>::type;
// 'decay_t' is an alias to the return type of the 'bsl::decay'
// meta-function. Note, that the 'enable_if_t' avoids the '::type' suffix
// and 'typename' prefix when we want to use the result of the
// meta-function in templates.
#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 t_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 ----------------------------------