// bslma_isstdallocator.h -*-C++-*- #ifndef INCLUDED_BSLMA_ISSTDALLOCATOR #define INCLUDED_BSLMA_ISSTDALLOCATOR #include <bsls_ident.h> BSLS_IDENT("$Id: $") //@PURPOSE: Provide a compile-time check for determining allocator types. // //@CLASSES: // bsl::IsStdAllocator: standard meta-function for determining allocator types // bsl::IsStdAllocator_v: the result of 'bsl::IsStdAllocator' meta-function // //@DESCRIPTION: This component defines a meta-function, 'bsl::IsStdAllocator' // and a template variable 'bsl::IsStdAllocator_v', that represents the result // value of the 'bsl::IsStdAllocator' meta-function. // // 'bsl::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 implementation has one. // // Two implementations are supplied; one for C++11 (and later) conforming // compilers, and a pre-C++11 compatibility trait that gives different answers // for custom allocator types due to the lack of 'decltype' in the older // language. // // Also note that the template variable 'IsStdAllocator_v' is defined in the // style of the C++17 standard as an inline variable. If the current compiler // supports the inline variable C++17 compiler feature, 'bsl::IsStdAllocator_v' // is defined as an 'inline constexpr bool' variable. Otherwise, if the // compiler supports the variable templates C++14 compiler feature, // 'bsl::IsStdAllocator_v' is defined as a non-inline 'constexpr bool' // variable. See 'BSLS_COMPILERFEATURES_SUPPORT_INLINE_VARIABLES' and // 'BSLS_COMPILERFEATURES_SUPPORT_VARIABLE_TEMPLATES' macros in // bsls_compilerfeatures component for details. // ///Usage ///----- // In this section we show intended use of this component. // ///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': //.. // struct MyAllocator // { // typedef int value_type; // int *allocate(size_t count); // // Allocate some memory for use by the caller. // }; //.. // Now, we instantiate the 'bsl::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. //.. // assert(false == bsl::IsStdAllocator<int>::value); // #ifdef BSLS_COMPILERFEATURES_SUPPORT_DECLTYPE // assert(true == bsl::IsStdAllocator<MyAllocator>::value); // #else // assert(false == bsl::IsStdAllocator<MyAllocator>::value); // #endif //.. // Note that if the current compiler supports the variable the templates C++14 // feature then we can re-write the snippet of code above using the // 'bsl::IsStdAllocator_v' variable as follows: //.. //#ifdef BSLS_COMPILERFEATURES_SUPPORT_VARIABLE_TEMPLATES // assert(false == bsl::IsStdAllocator_v<int>); // assert(true == bsl::IsStdAllocator_v<MyAllocator>); //#endif //.. #include <bslscm_version.h> #include <bslma_stdallocator.h> #include <bslmf_integralconstant.h> #include <bslmf_voidtype.h> #include <bsls_compilerfeatures.h> #include <bsls_keyword.h> #include <memory> // allocator #include <stddef.h> // size_t #ifndef BDE_DONT_ALLOW_TRANSITIVE_INCLUDES #include <bsls_nativestd.h> #endif // BDE_DONT_ALLOW_TRANSITIVE_INCLUDES namespace bsl { // ===================== // struct IsStdAllocator // ===================== template<class ALLOC, class = void, class = void> struct IsStdAllocator : false_type {}; // In general, things are not allocators #ifdef BSLS_COMPILERFEATURES_SUPPORT_DECLTYPE template<class ALLOC> struct IsStdAllocator<ALLOC, void_t<typename ALLOC::value_type>, void_t<decltype(std::declval<ALLOC&>().allocate(size_t(0)))> > : public true_type {}; // if it has a 'value_type` and method named 'allocate' that takes // something convertible from a size_t, then it's an allocator. #else template <class TYPE> struct IsStdAllocator<std::allocator<TYPE> > : public true_type {}; // std::allocator<T> is an allocator template <class TYPE> struct IsStdAllocator< ::bsl::allocator<TYPE> > : public true_type {}; // bsl::Allocator is an allocator #endif #ifdef BSLS_COMPILERFEATURES_SUPPORT_VARIABLE_TEMPLATES template <class TYPE> BSLS_KEYWORD_INLINE_VARIABLE constexpr bool IsStdAllocator_v = IsStdAllocator<TYPE>::value; // This template variable represents the result value of the // 'bsl::IsStdAllocator' meta-function. #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 ----------------------------------