bdlb_transparenthash.h

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/// @file bdlb_transparenthash.h
///
/// The content of this file has been pre-processed for Doxygen.
///
 
 
// bdlb_transparenthash.h                                             -*-C++-*-
#ifndef INCLUDED_BDLB_TRANSPARENTHASH
#define INCLUDED_BDLB_TRANSPARENTHASH
 
#include <bsls_ident.h>
BSLS_IDENT("$Id: $")
 
/// @defgroup bdlb_transparenthash bdlb_transparenthash
/// @brief  Provide a transparent hash functor.
/// @addtogroup bdl
/// @{
/// @addtogroup bdlb
/// @{
/// @addtogroup bdlb_transparenthash
/// @{
///
/// <h1> Outline </h1>
/// * <a href="#bdlb_transparenthash-purpose"> Purpose</a>
/// * <a href="#bdlb_transparenthash-classes"> Classes </a>
/// * <a href="#bdlb_transparenthash-description"> Description </a>
///   * <a href="#bdlb_transparenthash-usage"> Usage </a>
///     * <a href="#bdlb_transparenthash-example-1-basic-use-of-bdlb-transparenthash"> Example 1: Basic Use of bdlb::TransparentHash </a>
///
/// # Purpose {#bdlb_transparenthash-purpose}
/// Provide a transparent hash functor.
///
/// # Classes {#bdlb_transparenthash-classes}
///
/// -  bdlb::TransparentHash: a transparent hash functor
///
/// @see  bdlb::TransparentStringHash, bsl_map, bsl_set
///
/// # Description {#bdlb_transparenthash-description}
/// This component provides a `struct`, `bdlb::TransparentHash`,
/// that defines a functor to generate a hash code for different types and can
/// be used as transparent hash functor for heterogeneous lookup.
///
/// `TransparentHash` should not be used for hashing strings, however, because
/// it gives unexpected results for `char *`s.  `TransparentStringHash` should
/// be used instead.
///
///
/// ## Usage {#bdlb_transparenthash-usage}
///
///
/// This section illustrates intended use of this component.
///
/// ### Example 1: Basic Use of bdlb::TransparentHash {#bdlb_transparenthash-example-1-basic-use-of-bdlb-transparenthash}
///
///
/// Suppose we need a container to store set of `bsl::string` unique objects.
/// `bsl::unordered_set` is designed exactly for this purpose.  But imagine that
/// we want to use `bsl::string_view` objects for search operations within our
/// container.  `bsl::unordered_set` uses `bsl::hash` as default hash functor.
/// The problem is that even though the hash function for `bsl::string_view`
/// exists, compiler tries to convert `bsl::string_view` objects to the
/// `bsl::string` since `bsl::hash` is parameterized by `bsl::string`.  And
/// compilation fails, because there is no such implicit conversion.  In
/// addition, implicit conversions where they are available, may lead to
/// additional memory allocation for temporary objects.  The following code
/// illustrates how to use `bdlb::TransparentHash` as a hash functor for the
/// standard container `unordered_set`, in this case to allow a
/// `bsl::unordered_set<bsl::string>` to be searched with a `bsl::string_view`.
///
/// First, we define a transparent equality predicate, that is required by the
/// `bsl::unordered_set` along with the transparent hash:
/// @code
///                  // =============================
///                  // struct TestTransparentEqualTo
///                  // =============================
///
/// /// This `struct` defines an equality of objects of different types,
/// /// enabling them for use for heterogeneous comparison in the standard
/// /// associative containers such as `bsl::unordered_map`.  Note that this
/// /// class is an empty POD type.
/// struct TestTransparentEqualTo {
///
///     // TYPES
///
///     // Type alias indicating this is a transparent comparator.
///     typedef void is_transparent;
///
///     // ACCESSORS
///
///     /// Return `true` if the specified `lhs` is equal to the specified
///     /// `rhs` and `false` otherwise.
///     template <class LHS, class RHS>
///     bool operator()(const LHS& lhs, const RHS& rhs) const
///     {
///         return lhs == rhs;
///     }
/// };
/// @endcode
/// Note that this struct is defined only to avoid cycle dependencies between
/// BDE components.  In real code for these purposes it is recommended to use
/// `bdlb::TransparentEqualTo`.
///
/// Then, we create a container that uses `bdlb::TransparentHash`.  We use the
/// transparent comparator defined above to avoid implicit conversions:
/// @code
/// typedef bsl::unordered_set<bsl::optional<int>,
///                            bdlb::TransparentHash,
///                            TestTransparentEqualTo> TransparentHashSet;
///
/// TransparentHashSet transparentSet;
/// @endcode
/// Now, we fill the container with the strings:
/// @code
/// transparentSet.insert(10001);   // New York
/// transparentSet.insert(90001);   // Los Angeles
/// @endcode
/// Finally, we observe that the container allows to use `bsl::string_view`
/// objects as a key and does not make any implicit conversions:
/// @code
/// int                newYork      = 10001;
/// int                losAngeles   = 90001;
/// bsl::optional<int> sanFrancisco = 94102;
/// bsl::optional<int> nowhere;
///
/// ASSERT(transparentSet.end() != transparentSet.find(newYork     ));
/// ASSERT(transparentSet.end() != transparentSet.find(losAngeles  ));
/// ASSERT(transparentSet.end() == transparentSet.find(sanFrancisco));
/// ASSERT(transparentSet.end() == transparentSet.find(nowhere     ));
/// @endcode
/// @}
/** @} */
/** @} */
 
/** @addtogroup bdl
 * @{
 */
/** @addtogroup bdlb
 * @{
 */
/** @addtogroup bdlb_transparenthash
 * @{
 */
 
#include <bdlscm_version.h>
 
#include <bslmf_istriviallycopyable.h>
#include <bslmf_istriviallydefaultconstructible.h>
#include <bslmf_nestedtraitdeclaration.h>
 
#include <bsl_functional.h>
 
 
namespace bdlb {
 
                    // ======================
                    // struct TransparentHash
                    // ======================
 
/// This `struct` defines a hash operation for different types, enabling
/// them for use for heterogeneous lookup in the standard associative containers such as `bsl::unordered_map`.  
/// 
/// \note Note that   this class is an
/// empty POD type.
///
/// See @ref bdlb_transparenthash 
struct TransparentHash {
 
    // TYPES
 
    /// Type alias indicating this is a transparent hash functor.
    typedef void is_transparent;
 
    // CREATORS
 
    /// Create a `TransparentHash` object.
     TransparentHash() = default;
 
    /// Create a `TransparentHash` object.  
    /// \note Note that  as `TransparentHash`
    /// is an empty (stateless) type, this operation has no observable
    /// effect.
     TransparentHash(const TransparentHash& original) = default;
 
    /// Destroy this object.
     ~TransparentHash() = default;
 
    // MANIPULATORS
 
    /// Assign to this object the value of the specified `rhs` object, and
    /// return a reference providing modifiable access to this object. 
    /// 
    /// \note Note that   as `TransparentHash` is an empty (stateless) type, this
    /// operation has no observable effect.
     TransparentHash& operator=(const TransparentHash& rhs) = default;
 
    // ACCESSORS
 
    /// Return a hash code generated from the contents of the specified
    /// `value`.
    template <class TYPE>
    std::size_t operator()(const TYPE &value) const;
};
 
// ============================================================================
//                           INLINE DEFINITIONS
// ============================================================================
 
                          // ---------------------
                          // struct TransparentHash
                          // ---------------------
 
// ACCESSORS
template <class TYPE>
inline
std::size_t TransparentHash::operator()(const TYPE& value) const
{
    return bsl::hash<TYPE>().operator()(value);
}
 
}  // close package namespace
 
 
#endif
 
// ----------------------------------------------------------------------------
// 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 ----------------------------------
 
/** @} */
/** @} */
/** @} */