bslma_isstdallocator.h

Go to the documentation of this file.

/// @file bslma_isstdallocator.h
///
/// The content of this file has been pre-processed for Doxygen.
///
 
 
// bslma_isstdallocator.h                                             -*-C++-*-
#ifndef INCLUDED_BSLMA_ISSTDALLOCATOR
#define INCLUDED_BSLMA_ISSTDALLOCATOR
 
#include <bsls_ident.h>
BSLS_IDENT("$Id: $")
 
/// @defgroup bslma_isstdallocator bslma_isstdallocator
/// @brief  Provide a compile-time check for determining allocator types.
/// @addtogroup bsl
/// @{
/// @addtogroup bslma
/// @{
/// @addtogroup bslma_isstdallocator
/// @{
///
/// <h1> Outline </h1>
/// * <a href="#bslma_isstdallocator-purpose"> Purpose</a>
/// * <a href="#bslma_isstdallocator-classes"> Classes </a>
/// * <a href="#bslma_isstdallocator-description"> Description </a>
///   * <a href="#bslma_isstdallocator-usage"> Usage </a>
///     * <a href="#bslma_isstdallocator-example-1-verify-if-a-class-meets-the-requirements-for-an-allocator"> Example 1: Verify if a class meets the requirements for an allocator. </a>
///
/// # Purpose {#bslma_isstdallocator-purpose}
/// Provide a compile-time check for determining allocator types.
///
/// # Classes {#bslma_isstdallocator-classes}
///
/// -  bslma::IsStdAllocator: meta-function to determine if a type is an allocator
/// -  bslma::IsStdAllocator_v: Boolean result of `bslma::IsStdAllocator`
///
/// # Description {#bslma_isstdallocator-description}
/// This component defines a meta-function, `bslma::IsStdAllocator`
/// and a variable template, `bslma::IsStdAllocator_v`, that represents the
/// result value of the `bslma::IsStdAllocator` meta-function.
///
/// `bslma::IsStdAllocator` is used to determine if a type meets the
/// requirements for an allocator, as specified in
/// [container.requirements.general].  Note that there is no `is_allocator`
/// trait specified in the C++ standard, even though every C++ implementation
/// has a private equivalent.
///
/// In C++03, it is impossible to automatically detect conformance to the
/// allocator requirements, owing to limitations in SFINAE capabilities; even
/// elaborate detection metaprogramming will break if a type has a private
/// `allocate` method.  Therefore, a portable allocator type, *some-alloc*, must
/// have `bslma::IsStdAllocator<some-alloc>` specified directly.  There are two
/// ways to specify this trait:
///
/// 1. Specialize `IsStdAllocator<some-alloc>` to derive from `bsl::true_type`
///    in namespace `BloombergLP::bslma`.
/// 2. Add 'BSLMF_NESTED_TRAIT_DECLARATION(some-alloc,
///    bslma::IsStdAllocator)' within the public portion of the class definition
///    for *some-alloc*.
///
/// The first option will bypass any automatic-detection metalogic.  The second
/// option is preferred because it will be checked for correctness -- failing to
/// compile if the allocator is missing a critical member.
///
/// In C++11 and later, the `bslma::IsStdAllocator` trait is detected
/// automatically: for any type `A` having a `value_type` and `allocate` method
/// that meet the allocator requirements, `bslma::IsStdAllocator<A>::value` will
/// be `true`.  However, to prevent inadvertantly declaring an allocator that is
/// not detected in a C++03 build, using this trait in C++11 or later build will
/// yield a compilation error if it is detected that a type is an allocator does
/// not have `IsStdAllocator` explicitly specified.  The simplest way to avoid
/// that error is to specify `IsStdAllocator` deliberately for *every*
/// allocator, as described above.  Alternatively, defining the
/// `BSLMA_ISALLOCATOR_IGNORE_CPP03_COMPATIBILITY` macro will suppress the error
/// in C++11 and later builds, defering to automatic allocator detection.
///
/// If C++14 variable templates and constexpr variables are supported, the
/// variable template `IsStdAllocator_v` is defined to be the value
/// `IsStdAllocator<T>::value`.  If C++17 inline variables are supported, it is
/// inline.
///
/// ## Usage {#bslma_isstdallocator-usage}
///
///
/// This section shows the intended use of this component.
///
/// ### Example 1: Verify if a class meets the requirements for an allocator. {#bslma_isstdallocator-example-1-verify-if-a-class-meets-the-requirements-for-an-allocator}
///
///
/// Suppose that we want to assert whether a set of types meet the requirements
/// for allocators.
///
/// First, we create a struct type `MyAllocator`:
/// @code
/// struct MyAllocator
/// {
///     BSLMF_NESTED_TRAIT_DECLARATION(MyAllocator, bslma::IsStdAllocator);
///     typedef int value_type;
///
///     /// Allocate some memory for use by the caller.
///     int *allocate(size_t);
/// };
/// @endcode
/// Now, we instantiate the `bslma::IsStdAllocator` template for both a type
/// that does not meet the allocator requirements and the defined type
/// `MyClass`, that does, asserting the `value` static data member of each
/// instantiation.
/// @code
/// int main()
/// {
///     assert(false == bslma::IsStdAllocator<int>::value);
///     assert(true  == bslma::IsStdAllocator<MyAllocator>::value);
/// @endcode
/// Note that if the current compiler supports C++14 variable templates then we
/// can re-write the snippet of code above using the `bslma::IsStdAllocator_v`
/// variable:
/// @code
/// #ifdef BSLS_COMPILERFEATURES_SUPPORT_VARIABLE_TEMPLATES
///     assert(false == bslma::IsStdAllocator_v<int>);
///     assert(true  == bslma::IsStdAllocator_v<MyAllocator>);
/// #endif
/// }
/// @endcode
/// @}
/** @} */
/** @} */
 
/** @addtogroup bsl
 * @{
 */
/** @addtogroup bslma
 * @{
 */
/** @addtogroup bslma_isstdallocator
 * @{
 */
 
#include <bslscm_version.h>
 
#include <bslmf_assert.h>
#include <bslmf_detectnestedtrait.h>
#include <bslmf_enableif.h>
#include <bslmf_integralconstant.h>
#include <bslmf_voidtype.h>
 
#include <bsls_compilerfeatures.h>
#include <bsls_keyword.h>
 
#include <cstddef>  // 'std::size_t'
#include <memory>   // 'std::allocator'
 
 
namespace bslma {
 
// FORWARD DECLARATIONS
template <class TYPE, class SIZE_T, class = void> struct IsStdAllocator_Imp;
 
                  // =======================================
                  // struct template IsStdAllocator_SizeType
                  // =======================================
 
/// This component-private metafunction determines the appopriate size type
/// for a specified template parameter `TYPE`.  The nested `type` typedef is
/// `TYPE::size_type` if such a type exists and `std::size_t` otherwise.
/// Note that this metafunction produces the same type as
/// `bsl::allocator_traits<TYPE>::size_type`, but avoids a dependency on
/// @ref bslma_allocatortraits  and is guaranteed to compile even if `TYPE` is
/// not an allocator type.
template <class TYPE, class = void>
struct IsStdAllocator_SizeType
{
 
    typedef std::size_t type;
};
 
/// This specialization is selected when `TYPE` has a nested `size_type`.
template <class TYPE>
struct IsStdAllocator_SizeType<
                    TYPE,
                    typename bslmf::VoidType<typename TYPE::size_type>::type> {
 
    typedef typename TYPE::size_type type;
};
 
                      // ==============================
                      // struct template IsStdAllocator
                      // ==============================
 
/// Metafunction to determine whether the specified template parameter
/// `TYPE` meets the minimum requirements for a C++11 allocator.
/// Specifically, this `struct` is derived from `true_type` if `TYPE` has a
/// nested `value_type` and supports the operation `a.allocate(bytes)`,
/// where `a` has type `TYPE` and `bytes` has type
/// `allocator_traits<TYPE>::size_type`; otherwise it is derived from
/// `false_type`.
template <class TYPE>
struct IsStdAllocator
    : IsStdAllocator_Imp<TYPE,
                         typename IsStdAllocator_SizeType<TYPE>::type>::type
{
};
 
#ifdef BSLS_COMPILERFEATURES_SUPPORT_VARIABLE_TEMPLATES
/// This template variable represents the result value of the
/// `bslma::IsStdAllocator` meta-function.
template <class TYPE>
BSLS_KEYWORD_INLINE_VARIABLE
constexpr bool IsStdAllocator_v = IsStdAllocator<TYPE>::value;
#endif
 
/// Specialization for lvalue reference types.
template <class TYPE>
struct IsStdAllocator<TYPE&> : bsl::false_type {
};
 
#ifdef BSLS_COMPILERFEATURES_SUPPORT_RVALUE_REFERENCES
/// Specialization for rvalue reference types.
template <class TYPE>
struct IsStdAllocator<TYPE&&> : bsl::false_type {
};
#endif
 
/// Specialization for `std::allocator`.
template <class TYPE>
struct IsStdAllocator<std::allocator<TYPE> > : bsl::true_type {
};
 
/// `std::allocator<void>` is not an allocator type, even though all other
/// specializations are allocator types.
template <>
struct IsStdAllocator<std::allocator<void> > : bsl::false_type {
};
 
// ============================================================================
//                         TEMPLATE IMPLEMENTATIONS
// ============================================================================
 
                      // ----------------------------------
                      // struct template IsStdAllocator_Imp
                      // ----------------------------------
 
#ifdef BSLS_COMPILERFEATURES_SUPPORT_DECLTYPE
 
/// This C++11 primary template yields `false_type`.  It is selected when
/// either `TYPE::value_type` does not exists or `a.allocate(bytes)` is
/// ill-formed, where `a` has type `TYPE` and `bytes` has type `SIZE_T`.
template <class TYPE, class SIZE_T, class>
struct IsStdAllocator_Imp : bsl::false_type {
 
    // If this assert fails, it means that 'bslma::IsStdAllocator' is declared
    // as a nested trait within 'TYPE' but 'TYPE' does not meet the C++11
    // allocator requirements.
    BSLMF_ASSERT(! (bslmf::DetectNestedTrait<TYPE, IsStdAllocator>::value));
};
 
/// This C++11 specialization yields `true_type` and is selected when
/// `TYPE::value_type` exists and `a.allocate(bytes)` is well-formed, where
/// `a` has type `TYPE` and `bytes` has type `SIZE_T`.
template <class TYPE, class SIZE_T>
struct IsStdAllocator_Imp<
             TYPE,
             SIZE_T,
             bsl::void_t<typename TYPE::value_type,
                         decltype(std::declval<TYPE&>().allocate(SIZE_T()))> >
    : public bsl::true_type {
 
#ifndef BSLMA_ISALLOCATOR_IGNORE_CPP03_COMPATIBILITY
    // If this assert fails, it means that 'TYPE' meets the C++11 allocator
    // requirements but cannot be detected as being an allocator using a C++03
    // compiler.  Specifically, 'bslma::IsStdAllocator' is neither specialized
    // for 'TYPE' nor declared as a nested trait within 'TYPE'.  Although
    // explicitly declaring this trait would not be necessary for a C++11 or
    // later compiler, this assertion prevents portability errors whereby real
    // allocator types that would not be recognized as such in C++03.  To
    // suppress this compatibility check, '#define'
    // 'BSLMA_ISALLOCATOR_IGNORE_CPP03_COMPATIBILITY' before the '#include' for
    // this header.
    BSLMF_ASSERT((bslmf::DetectNestedTrait<TYPE, IsStdAllocator>::value));
#endif
};
 
#else // if ! defined(BSLS_COMPILERFEATURES_SUPPORT_DECLTYPE)
 
/// This C++03 primary template metafunction is derived from `false_type`.
/// It is selected when the specified template paramter `TYPE` does not
/// declare itself to be an allocator, i.e., when
/// `bslmf::DetectNestedTrait<TYPE, IsStdAllocator>::value` is `false`.
template <class TYPE, class SIZE_T, class>
struct IsStdAllocator_Imp : bsl::false_type {
};
 
/// This C++03 specialization is derived from `true_type`.  It is selected
/// when the specified template parameter `TYPE` is a class that declares
/// the nested `IsStdAllocator` trait.  This specialization contains
/// compile-time correctness checks to ensure that `TYPE` really does have
/// `value_type` and `allocator()` members.
template <class TYPE, class SIZE_T>
struct IsStdAllocator_Imp<
               TYPE,
               SIZE_T,
               typename bsl::enable_if<
                   bslmf::DetectNestedTrait<TYPE, IsStdAllocator>::value>::type
    >
    : bsl::true_type {
 
  private:
    // TYPES
    template <class T> struct check { };
 
    // NOT DEFINED
    static TYPE& allocObj;
 
    // COMPILE-TIME CORRECTNESS CHECK:
    // If 'TYPE' is declared as being an allocator, the following assertions
    // will fail even to compile unless 'TYPE' also meets the minimum
    // requirements for a C++11 allocator.  These tests prevent non-allocators
    // from being declared as allocators.
    BSLMF_ASSERT(sizeof(check<typename TYPE::value_type>) > 0);
    BSLMF_ASSERT(sizeof(allocObj.allocate(SIZE_T())) > 0);
};
 
#endif // ! BSLS_COMPILERFEATURES_SUPPORT_DECLTYPE
 
}  // close package namespace
 
 
// For backwards compatibility:
namespace bsl {
 
using BloombergLP::bslma::IsStdAllocator;
 
#ifdef BSLS_COMPILERFEATURES_SUPPORT_VARIABLE_TEMPLATES
using BloombergLP::bslma::IsStdAllocator_v;
#endif
 
}  // close namespace bsl
 
#endif // INCLUDED_BSLMA_ISSTDALLOCATOR
 
// ----------------------------------------------------------------------------
// Copyright 2021 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 ----------------------------------
 
/** @} */
/** @} */
/** @} */