// bslmf_isvoid.h -*-C++-*- #ifndef INCLUDED_BSLMF_ISVOID #define INCLUDED_BSLMF_ISVOID #include <bsls_ident.h> BSLS_IDENT("$Id: $") //@PURPOSE: Provide a compile-time check for 'void' types. // //@CLASSES: // bsl::is_void: standard meta-function for determining 'void' types // bsl::is_void_v: the result value of the 'bsl::is_void' meta-function // bslmf::IsVoid: meta-function for determining 'void' types // //@SEE_ALSO: bslmf_integralconstant // //@DESCRIPTION: This component defines two meta-functions, 'bsl::is_void' and // 'BloombergLP::bslmf::IsVoid' and a template variable 'bsl::is_void_v', that // represents the result value of 'bsl::is_void' meta-function. All these // meta-functions may be used to query whether a type is the (possibly // cv-qualified) 'void' type. // // 'bsl::is_void' meets the requirements of the 'is_void' template defined in // the C++11 standard [meta.unary.cat], while 'bslmf::IsVoid' was devised // before 'is_void' was standardized. // // The two meta-functions are functionally equivalent. The major difference // between them is that the result for 'bsl::is_void' is indicated by the // class member 'value', while the result for 'bslmf::IsVoid' is indicated // by the class member 'VALUE'. // // Note that 'bsl::is_void' should be preferred over 'bslmf::IsVoid', and in // general, should be used by new components. // // Also note that the template variable 'is_void_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_void_v' is defined as an // 'inline constexpr bool' variable. Otherwise, if the compiler supports the // variable templates C++14 compiler feature, 'bsl::is_void_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: Determine Whether a Type is the 'void' Type /// - - - - - - - - - - - - - - - - - - - - - - - - - - - // Suppose that we want to assert whether a particular type is the 'void' type. // // First, we create two 'typedef's -- the 'void' type and another type: //.. // typedef int MyType; // typedef void MyVoidType; //.. // Now, we instantiate the 'bsl::is_void' template for each of the 'typedef's // and assert the 'value' static data member of each instantiation: //.. // assert(false == bsl::is_void<MyType>::value); // assert(true == bsl::is_void<MyVoidType>::value); //.. // Note that if the current compiler supports the variable templates C++14 // feature, then we can re-write the snippet of code above as follows: //.. //#ifdef BSLS_COMPILERFEATURES_SUPPORT_VARIABLE_TEMPLATES // assert(false == bsl::is_void_v<MyType>); // assert(true == bsl::is_void_v<MyVoidType>); //#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_metaint.h> #include <bslmf_removecv.h> #endif // BDE_DONT_ALLOW_TRANSITIVE_INCLUDES namespace bsl { // ============== // struct is_void // ============== template <class t_TYPE> struct is_void : false_type { // This 'struct' template implements the 'is_void' meta-function defined in // the C++11 standard [meta.unary.cat] to determine if the (template // parameter) 't_TYPE' is the (possibly cv-qualified) 'void' type. This // 'struct' derives from 'bsl::true_type' if 't_TYPE' is the 'void' type, // and 'bsl::false_type' otherwise. }; template <> struct is_void<void> : true_type { // This partial specialization of 'is_void' derives from 'bsl::true_type' // for when the (template parameter) 't_TYPE' is 'void'. }; template <> struct is_void<const void> : true_type { // This partial specialization of 'is_void' derives from 'bsl::true_type' // for when the (template parameter) 't_TYPE' is 'const void'. }; template <> struct is_void<volatile void> : true_type { // This partial specialization of 'is_void' derives from 'bsl::true_type' // for when the (template parameter) 't_TYPE' is 'volatile void'. }; template <> struct is_void<const volatile void> : bsl::true_type { // This partial specialization of 'is_void' derives from 'bsl::true_type' // for when the (template parameter) 't_TYPE' is 'const volatile void'. }; #ifdef BSLS_COMPILERFEATURES_SUPPORT_VARIABLE_TEMPLATES template <class t_TYPE> BSLS_KEYWORD_INLINE_VARIABLE constexpr bool is_void_v = is_void<t_TYPE>::value; // This template variable represents the result value of the 'bsl::is_void' // meta-function. #endif } // close namespace bsl namespace BloombergLP { namespace bslmf { // =================== // struct bslmf_IsVoid // =================== template <class t_TYPE> struct IsVoid : bsl::is_void<t_TYPE>::type { // This 'struct' template implements a meta-function to determine if the // (template parameter) 't_TYPE' is the (possibly cv-qualified) 'void' // type. This 'struct' derives from 'bslmf::MetaInt<1>' if 't_TYPE' is the // 'void' type, and 'bslmf::MetaInt<0>' otherwise. // // Note that although this 'struct' is functionally equivalent to // 'bsl::is_void', and the use of 'bsl::is_void' should be preferred. }; } // close package namespace } // close enterprise namespace #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 ----------------------------------