bdlb_transparentstringhash.h

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/// @file bdlb_transparentstringhash.h
///
/// The content of this file has been pre-processed for Doxygen.
///
 
 
// bdlb_transparentstringhash.h                                       -*-C++-*-
#ifndef INCLUDED_BDLB_TRANSPARENTSTRINGHASH
#define INCLUDED_BDLB_TRANSPARENTSTRINGHASH
 
#include <bsls_ident.h>
BSLS_IDENT("$Id: $")
 
/// @defgroup bdlb_transparentstringhash bdlb_transparentstringhash
/// @brief  Provide a transparent hash functor.
/// @addtogroup bdl
/// @{
/// @addtogroup bdlb
/// @{
/// @addtogroup bdlb_transparentstringhash
/// @{
///
/// <h1> Outline </h1>
/// * <a href="#bdlb_transparentstringhash-purpose"> Purpose</a>
/// * <a href="#bdlb_transparentstringhash-classes"> Classes </a>
/// * <a href="#bdlb_transparentstringhash-description"> Description </a>
///   * <a href="#bdlb_transparentstringhash-usage"> Usage </a>
///     * <a href="#bdlb_transparentstringhash-example-1-basic-use-of-bdlb-transparentstringhash"> Example 1: Basic Use of bdlb::TransparentStringHash </a>
///
/// # Purpose {#bdlb_transparentstringhash-purpose}
/// Provide a transparent hash functor.
///
/// # Classes {#bdlb_transparentstringhash-classes}
///
/// -  bdlb::TransparentStringHash: a transparent hash functor for strings
///
/// @see  bsl_unordered_map, bsl_unordered_set
///
/// # Description {#bdlb_transparentstringhash-description}
/// This component provides a `struct`,
/// `bdlb::TransparentStringHash`, that defines a functor to generate a hash
/// code for different 'string-like' types and can be used as transparent hash
/// functor for heterogeneous lookup.
///
/// ## Usage {#bdlb_transparentstringhash-usage}
///
///
/// This section illustrates intended use of this component.
///
/// ### Example 1: Basic Use of bdlb::TransparentStringHash {#bdlb_transparentstringhash-example-1-basic-use-of-bdlb-transparentstringhash}
///
///
/// 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::TransparentStringHash` 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::TransparentStringHash`.  We use
/// the transparent comparator defined above to avoid implicit conversions:
/// @code
/// typedef bsl::unordered_set<bsl::string,
///                            bdlb::TransparentStringHash,
///                            TestTransparentEqualTo> TransparentHashSet;
///
/// TransparentHashSet transparentSet;
/// @endcode
/// Now, we fill the container with the strings:
/// @code
/// transparentSet.insert("NY");
/// transparentSet.insert("LA");
/// @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
/// bsl::string_view  newYork       ("NY");
/// bsl::string_view  losAngeles    ("LA");
/// const char       *sanFrancisco = "SF";
///
/// assert(transparentSet.end() != transparentSet.find(newYork     ));
/// assert(transparentSet.end() != transparentSet.find(losAngeles  ));
/// assert(transparentSet.end() == transparentSet.find(sanFrancisco));
/// @endcode
/// @}
/** @} */
/** @} */
 
/** @addtogroup bdl
 * @{
 */
/** @addtogroup bdlb
 * @{
 */
/** @addtogroup bdlb_transparentstringhash
 * @{
 */
 
#include <bdlscm_version.h>
 
#include <bsl_string.h>      // 'bsl::basic_string'
#include <bsl_string_view.h> // 'bsl::basic_string_view'
 
#include <bsl_functional.h>  // 'bsl::hash'
 
#include <string>            // 'std::string'
#ifdef BSLS_LIBRARYFEATURES_HAS_CPP17_BASELINE_LIBRARY
#include <string_view>       // 'std::string_view'
#endif
 
 
namespace bdlb {
 
                    // ============================
                    // struct TransparentStringHash
                    // ============================
 
/// This `struct` defines a hash operation for different string 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_transparentstringhash 
struct TransparentStringHash {
 
    // TYPES
 
    /// Type alias indicating this is a transparent hash functor.
    typedef void is_transparent;
 
    // CREATORS
 
    /// Create a `TransparentStringHash` object.
     TransparentStringHash() = default;
 
    /// Create a `TransparentStringHash` object.  
    /// \note Note that  as
    /// `TransparentStringHash` is an empty (stateless) type, this operation
    /// has no observable effect.
     TransparentStringHash(const TransparentStringHash& original) = default;
 
    /// Destroy this object.
     ~TransparentStringHash() = 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 `TransparentStringHash` is an empty (stateless) type, this
    /// operation has no observable effect.
     TransparentStringHash& operator=(const TransparentStringHash& rhs) = default;
 
    // ACCESSORS
 
    /// Return a hash code generated from the contents of the specified
    /// `str`.
    template <class CHAR>
    std::size_t operator()(const CHAR *str) const;
 
    template <class CHAR, class TRAITS, class ALLOCATOR>
    std::size_t operator()(
                  const bsl::basic_string<CHAR, TRAITS, ALLOCATOR>& str) const;
 
    template <class CHAR, class TRAITS, class ALLOCATOR>
    std::size_t operator()(
                  const std::basic_string<CHAR, TRAITS, ALLOCATOR>& str) const;
 
#ifdef BSLS_LIBRARYFEATURES_HAS_CPP17_BASELINE_LIBRARY
    template <class CHAR, class TRAITS>
    std::size_t operator()(
                        const std::basic_string_view<CHAR, TRAITS>& str) const;
#endif
 
#ifndef BSLSTL_STRING_VIEW_IS_ALIASED
    template <class CHAR, class TRAITS>
    std::size_t operator()(
                        const bsl::basic_string_view<CHAR, TRAITS>& str) const;
#endif
};
 
// ============================================================================
//                           INLINE DEFINITIONS
// ============================================================================
 
                    // ----------------------------
                    // struct TransparentStringHash
                    // ----------------------------
 
template <class CHAR>
inline std::size_t
TransparentStringHash::operator()(const CHAR *str) const
{
    bsl::basic_string_view<CHAR> sv(str);
    return bsl::hash<bsl::basic_string_view<CHAR> >().operator()(sv);
}
 
template <class CHAR, class TRAITS, class ALLOCATOR>
inline std::size_t
TransparentStringHash::operator()(
                   const bsl::basic_string<CHAR, TRAITS, ALLOCATOR>& str) const
{
    bsl::basic_string_view<CHAR> sv(str.data(), str.size());
    return bsl::hash<bsl::basic_string_view<CHAR> >().operator()(sv);
}
 
template <class CHAR, class TRAITS, class ALLOCATOR>
inline std::size_t
TransparentStringHash::operator()(
                   const std::basic_string<CHAR, TRAITS, ALLOCATOR>& str) const
{
    bsl::basic_string_view<CHAR> sv(str.data(), str.size());
    return bsl::hash<bsl::basic_string_view<CHAR> >().operator()(sv);
}
 
#ifdef BSLS_LIBRARYFEATURES_HAS_CPP17_BASELINE_LIBRARY
template <class CHAR, class TRAITS>
inline std::size_t
TransparentStringHash::operator()(
                         const std::basic_string_view<CHAR, TRAITS>& str) const
{
    bsl::basic_string_view<CHAR> sv(str.data(), str.size());
    return bsl::hash<bsl::basic_string_view<CHAR> >().operator()(sv);
}
#endif
 
#ifndef BSLSTL_STRING_VIEW_IS_ALIASED
template <class CHAR, class TRAITS>
inline std::size_t
TransparentStringHash::operator()(
                         const bsl::basic_string_view<CHAR, TRAITS>& str) const
{
    return bsl::hash<bsl::basic_string_view<CHAR> >().operator()(str);
}
#endif
 
}  // close package namespace
 
 
#endif
 
// ----------------------------------------------------------------------------
// Copyright 2024 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 ----------------------------------
 
/** @} */
/** @} */
/** @} */