bdlb_transparentequalto.h

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/// @file bdlb_transparentequalto.h
///
/// The content of this file has been pre-processed for Doxygen.
///
 
 
// bdlb_transparentequalto.h                                          -*-C++-*-
#ifndef INCLUDED_BDLB_TRANSPARENTEQUALTO
#define INCLUDED_BDLB_TRANSPARENTEQUALTO
 
#include <bsls_ident.h>
BSLS_IDENT("$Id: $")
 
/// @defgroup bdlb_transparentequalto bdlb_transparentequalto
/// @brief  Provide a transparent equality predicate.
/// @addtogroup bdl
/// @{
/// @addtogroup bdlb
/// @{
/// @addtogroup bdlb_transparentequalto
/// @{
///
/// <h1> Outline </h1>
/// * <a href="#bdlb_transparentequalto-purpose"> Purpose</a>
/// * <a href="#bdlb_transparentequalto-classes"> Classes </a>
/// * <a href="#bdlb_transparentequalto-description"> Description </a>
///   * <a href="#bdlb_transparentequalto-usage"> Usage </a>
///     * <a href="#bdlb_transparentequalto-example-1-basic-use-of-bdlb-transparentequalto"> Example 1: Basic Use of bdlb::TransparentEqualTo </a>
///
/// # Purpose {#bdlb_transparentequalto-purpose}
/// Provide a transparent equality predicate.
///
/// # Classes {#bdlb_transparentequalto-classes}
///
/// -  bdlb::TransparentEqualTo: a transparent equality predicate.
///
/// @see  bdlb_transparentless, bdlb_transparenthash
///
/// # Description {#bdlb_transparentequalto-description}
/// This component provides a `struct`, `bdlb::TransparentEqualTo`,
/// that defines a functor that can be used as transparent equality comparator
/// for heterogeneous lookup.
///
/// ## Usage {#bdlb_transparentequalto-usage}
///
///
/// This section illustrates intended use of this component.
///
/// ### Example 1: Basic Use of bdlb::TransparentEqualTo {#bdlb_transparentequalto-example-1-basic-use-of-bdlb-transparentequalto}
///
///
/// Suppose we need a container to store set of unique `bsl::string` 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::equal_to` as default comparator.
/// The problem is that even though the equality operator between `bsl::string`
/// and `bsl::string_view` exists, the compiler will try to convert
/// the `bsl::string_view` objects to `bsl::string` since `bsl::equal_to` is
/// parameterized by `bsl::string`.  The compilation will fail because there is
/// no such implicit conversion.  Even if the compilation were to succeed
/// (because an implicit conversion were available), such an operation can lead
/// to additional memory allocation for temporary objects.  The following code
/// illustrates how to use `bdlb::TransparentEqualTo` as a comparator 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 create a container that uses `bdlb::TransparentEqualTo` as a
/// comparator.  Note that to avoid implicit conversions we also have to use
/// transparent hash functor:
/// @code
/// typedef bsl::unordered_set<bsl::string,
///                            bdlb::TransparentStringHash,
///                            bdlb::TransparentEqualTo>  TransparentSet;
///
/// TransparentSet  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");
/// bsl::string_view 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_transparentequalto
 * @{
 */
 
#include <bdlscm_version.h>
 
#include <bslmf_istriviallycopyable.h>
#include <bslmf_istriviallydefaultconstructible.h>
#include <bslmf_nestedtraitdeclaration.h>
 
 
namespace bdlb {
 
                    // =========================
                    // struct TransparentEqualTo
                    // =========================
 
/// 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 Note that   this
/// class is an empty POD type.
///
/// See @ref bdlb_transparentequalto 
struct TransparentEqualTo {
 
    // TYPES
 
    /// Type alias indicating this is a transparent comparator.
    typedef void is_transparent;
 
    // CREATORS
 
    /// Create a `TransparentEqualTo` object.
     TransparentEqualTo() = default;
 
    /// Create a `TransparentEqualTo` object.  
    /// \note Note that  as
    /// `TransparentEqualTo` is an empty (stateless) type, this operation
    /// has no observable effect.
     TransparentEqualTo(const TransparentEqualTo& original) = default;
 
    /// Destroy this object.
     ~TransparentEqualTo() = 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 `TransparentEqualTo` is an empty (stateless) type, this
    /// operation has no observable effect.
     TransparentEqualTo& operator=(const TransparentEqualTo& rhs) = default;
 
    // 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;
};
 
// ============================================================================
//                           INLINE DEFINITIONS
// ============================================================================
 
                          // -------------------------
                          // struct TransparentEqualTo
                          // -------------------------
 
// ACCESSORS
template <class LHS, class RHS>
inline
bool TransparentEqualTo::operator()(const LHS& lhs, const RHS& rhs) const
{
    return lhs == rhs;
}
 
}  // 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 ----------------------------------
 
/** @} */
/** @} */
/** @} */