// bslh_hashvariant.h -*-C++-*- #ifndef INCLUDED_BSLH_HASHVARIANT #define INCLUDED_BSLH_HASHVARIANT #include <bsls_ident.h> BSLS_IDENT("$Id: $") //@PURPOSE: Provide 'hashAppend' for 'std::variant'. // //@DESCRIPTION: This component provides a free function template, // 'bslh::hashAppend', overloaded for the 'std::variant' class template. // Including this function allows for 'std::variant' types (and types that // contain them) to be used as keys in BDE hashed containers. // // Note that use of this component requires that the language standard be 2017 // or later, as that is when 'std::variant' first appears. // ///Usage ///----- // This section illustrates intended usage of this component. // ///Example 1: Hashing an integer or floating-point value /// - - - - - - - - - - - - - - - - - - - - - - - - - - - // Suppose we want to maintain a value as either an integer or a floating-point // number, and have it fit within the BDE hash framework. We can use // 'std::variant<int, double>' for this, and demonstrate that such a value can // be correctly hashed. // // First, we set up a pair of such optional values to represent the two // potential states we wish to represent. //.. // typedef std::variant<int, double> id; // id variantInt = 42; // id variantDouble = 3.475; //.. // Then, we create a hashing object. //.. // bslh::Hash<> hasher; //.. // Next, we hash each of our values. //.. // size_t variantIntHash = hasher(variantInt); // size_t variantDoubleHash = hasher(variantDouble); //.. // Then we hash the underlying values. A variant hashes as a pair of values, // first the 'size_t' index of the held type, then the contained value, so we // need to accumulate the hashes. //.. // bslh::Hash<>::HashAlgorithm haInt; // hashAppend(haInt, size_t(0)); // hashAppend(haInt, 42); // size_t expectedIntHash = size_t(haInt.computeHash()); // // bslh::Hash<>::HashAlgorithm haDouble; // hashAppend(haDouble, size_t(1)); // hashAppend(haDouble, 3.475); // size_t expectedDoubleHash = size_t(haDouble.computeHash()); //.. // Finally, we verify that the 'std::variant' hasher produces the same results // as the underlying hashers. //.. // assert(expectedIntHash == variantIntHash); // assert(expectedDoubleHash == variantDoubleHash); //.. #include <bslscm_version.h> #include <bslh_hash.h> #include <bsls_compilerfeatures.h> #include <bsls_libraryfeatures.h> #include <bsls_platform.h> #include <stddef.h> // for 'size_t' #ifdef BSLS_LIBRARYFEATURES_HAS_CPP17_BASELINE_LIBRARY #include <variant> #define BSLH_HASHVARIANT_DEFINE_HASH_MONOSTATE 1 namespace BloombergLP { namespace bslh { // FREE FUNCTIONS template <class HASH_ALGORITHM> inline void hashAppend(HASH_ALGORITHM& algorithm, const std::monostate&) // Pass the special value corresponding to the 'std::monostate' object to // the specified 'algorithm'. This function integrates with the 'bslh' // modular hashing system and effectively provides a 'bsl::hash' // specialization for 'std::monostate'. { hashAppend(algorithm, -7777); } template <class HASH_ALGORITHM, class ... TYPE> inline void hashAppend(HASH_ALGORITHM& algorithm, const std::variant<TYPE ...> &input) // Pass the index of the active variant of the specified 'input' to the // specified 'algorithm', followed by the active value itself, to be // combined into the internal state of the algorithm that is used to // produce the resulting hash value. { hashAppend(algorithm, input.index()); std::visit([&](const auto& x) { hashAppend(algorithm, x); }, input); } } // close package namespace } // close enterprise namespace #endif // BSLS_LIBRARYFEATURES_HAS_CPP17_BASELINE_LIBRARY #endif // ---------------------------------------------------------------------------- // Copyright 2020 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 ----------------------------------