bslh_hashvariant.h

Go to the documentation of this file.

/// @file bslh_hashvariant.h
///
/// The content of this file has been pre-processed for Doxygen.
///
 
 
// bslh_hashvariant.h                                                 -*-C++-*-
#ifndef INCLUDED_BSLH_HASHVARIANT
#define INCLUDED_BSLH_HASHVARIANT
 
#include <bsls_ident.h>
BSLS_IDENT("$Id: $")
 
/// @defgroup bslh_hashvariant bslh_hashvariant
/// @brief  Provide `hashAppend` for `std::variant`.
/// @addtogroup bsl
/// @{
/// @addtogroup bslh
/// @{
/// @addtogroup bslh_hashvariant
/// @{
///
/// <h1> Outline </h1>
/// * <a href="#bslh_hashvariant-purpose"> Purpose</a>
/// * <a href="#bslh_hashvariant-description"> Description </a>
///   * <a href="#bslh_hashvariant-usage"> Usage </a>
///     * <a href="#bslh_hashvariant-example-1-hashing-an-integer-or-floating-point-value"> Example 1:  Hashing an integer or floating-point value </a>
///
/// # Purpose {#bslh_hashvariant-purpose}
/// Provide `hashAppend` for `std::variant`.
///
/// # Description {#bslh_hashvariant-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 {#bslh_hashvariant-usage}
///
///
/// This section illustrates intended usage of this component.
///
/// ### Example 1:  Hashing an integer or floating-point value {#bslh_hashvariant-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.
/// @code
/// typedef std::variant<int, double> id;
/// id variantInt    = 42;
/// id variantDouble = 3.475;
/// @endcode
/// Then, we create a hashing object.
/// @code
/// bslh::Hash<> hasher;
/// @endcode
/// Next, we hash each of our values.
/// @code
/// size_t variantIntHash    = hasher(variantInt);
/// size_t variantDoubleHash = hasher(variantDouble);
/// @endcode
/// 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.
/// @code
/// 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());
/// @endcode
/// Finally, we verify that the `std::variant` hasher produces the same results
/// as the underlying hashers.
/// @code
/// assert(expectedIntHash    == variantIntHash);
/// assert(expectedDoubleHash == variantDoubleHash);
/// @endcode
/// @}
/** @} */
/** @} */
 
/** @addtogroup bsl
 * @{
 */
/** @addtogroup bslh
 * @{
 */
/** @addtogroup bslh_hashvariant
 * @{
 */
 
#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 bslh {
 
// FREE FUNCTIONS
 
/// 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`.
template <class HASH_ALGORITHM>
inline
void
hashAppend(HASH_ALGORITHM& algorithm, const std::monostate&)
{
    hashAppend(algorithm, -7777);
}
 
/// 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.
template <class HASH_ALGORITHM, class ... TYPE>
inline
void
hashAppend(HASH_ALGORITHM& algorithm, const std::variant<TYPE ...> &input)
{
    hashAppend(algorithm, input.index());
    std::visit([&](const auto& x) { hashAppend(algorithm, x); }, input);
}
 
}  // close package 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 ----------------------------------
 
/** @} */
/** @} */
/** @} */