// bslmf_isreference.h -*-C++-*- #ifndef INCLUDED_BSLMF_ISREFERENCE #define INCLUDED_BSLMF_ISREFERENCE #include <bsls_ident.h> BSLS_IDENT("$Id: $") //@PURPOSE: Provide a meta-function to test reference types. // //@CLASSES: // bsl::is_reference: standard meta-function for testing reference types // bsl::is_reference_v: the result value of the standard meta-function // //@SEE_ALSO: // //@DESCRIPTION: This component defines a meta-function, 'bsl::is_reference' and // a template variable 'bsl::is_reference_v', that represents the result value // of the 'bsl::is_class' meta-function, that may be used to query whether a // type is an (lvalue or rvalue) reference type. // // 'bsl::is_reference' meets the requirements of the 'is_reference' template // defined in the C++11 standard [meta.unary.comp]. // // Note that the template variable 'is_reference_v' is defined in the C++17 // standard as an inline variable. If the current compiler supports the inline // variable C++17 compiler feature, 'bsl::is_reference_v' is defined as an // 'inline constexpr bool' variable. Otherwise, if the compiler supports the // variable templates C++14 compiler feature, 'bsl::is_reference_v' is defined // as a non-'const' '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 Reference Types ///- - - - - - - - - - - - - - - - - // Suppose that we want to assert whether a set of types are (lvalue or rvalue) // reference types. // // Now, we instantiate the 'bsl::is_reference' template for a non-reference // type, an lvalue reference type, and an rvalue reference type, and assert the // 'value' static data member of each instantiation: //.. // assert(false == bsl::is_reference<int>::value); // assert(true == bsl::is_reference<int&>::value); //#if defined(BSLS_COMPILERFEATURES_SUPPORT_RVALUE_REFERENCES) // assert(true == bsl::is_reference<int&&>::value); //#endif //.. // Note that rvalue reference is a feature introduced in the C++11 standand, // and may not be supported by all compilers. // // Also note that if the current compiler supports the variable templates C++14 // feature then we can re-write the snippet of code above using the // 'bsl::is_reference_v' variable as follows: //.. //#ifdef BSLS_COMPILERFEATURES_SUPPORT_VARIABLE_TEMPLATES // assert(false == bsl::is_reference_v<int>); // assert(true == bsl::is_reference_v<int&>); // assert(true == bsl::is_reference_v<int&&>); //#endif //.. #include <bslscm_version.h> #include <bslmf_integralconstant.h> #include <bsls_compilerfeatures.h> #include <bsls_keyword.h> #ifndef BDE_DONT_ALLOW_TRANSITIVE_INCLUDES #include <bslmf_islvaluereference.h> #include <bslmf_isrvaluereference.h> #endif // BDE_DONT_ALLOW_TRANSITIVE_INCLUDES namespace bsl { // =================== // struct is_reference // =================== // IMPLEMENTATION NOTE: We originally implemented this trait in terms of // 'is_lvalue_reference' and 'is_rvalue_reference' but ran into an issue on // Sun where the compiler was reporting that the maximum depth of template // expansion was being exceeded. template <class t_TYPE> struct is_reference : false_type { // This 'struct' template implements the 'is_reference' meta-function // defined in the C++11 standard [meta.unary.comp] to determine if the // (template parameter) 't_TYPE' is a (lvalue or rvalue) reference type. // // This 'struct' derives from 'bsl::false_type' with specializations to // follow for lvalue and rvalue references. }; template <class t_TYPE> struct is_reference<t_TYPE&> : true_type { // This 'struct' template implements the 'is_reference' meta-function // defined in the C++11 standard [meta.unary.comp] to determine if the // (template parameter) 't_TYPE' is a (lvalue or rvalue) reference type. // // This specialization for lvalue references derives from 'bsl::true_type'. }; #if defined(BSLS_COMPILERFEATURES_SUPPORT_RVALUE_REFERENCES) template <class t_TYPE> struct is_reference<t_TYPE&&> : true_type { // This 'struct' template implements the 'is_reference' meta-function // defined in the C++11 standard [meta.unary.comp] to determine if the // (template parameter) 't_TYPE' is a (lvalue or rvalue) reference type. // // This specialization for rvalue references derives from 'bsl::true_type. }; #endif #ifdef BSLS_COMPILERFEATURES_SUPPORT_VARIABLE_TEMPLATES template <class t_TYPE> BSLS_KEYWORD_INLINE_VARIABLE constexpr bool is_reference_v = is_reference<t_TYPE>::value; // This template variable represents the result value of the // 'bsl::is_reference' meta-function. #endif } // close namespace bsl #endif // ---------------------------------------------------------------------------- // Copyright 2013 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 ----------------------------------