// bdlb_transparentequalto.h                                          -*-C++-*-
#ifndef INCLUDED_BDLB_TRANSPARENTEQUALTO
#define INCLUDED_BDLB_TRANSPARENTEQUALTO

#include <bsls_ident.h>
BSLS_IDENT("$Id: $")

//@PURPOSE: Provide a transparent equality predicate.
//
//@CLASSES:
//  bdlb::TransparentEqualTo: a transparent equality predicate.
//
//@SEE_ALSO: bdlb_transparentless, bdlb_transparenthash
//
//@DESCRIPTION: This component provides a 'struct', 'bdlb::TransparentEqualTo',
// that defines a functor that can be used as transparent equality comparator
// for heterogeneous lookup.
//
///Usage
///-----
// This section illustrates intended use of this component.
//
///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:
//..
//  typedef bsl::unordered_set<bsl::string,
//                             bdlb::TransparentHash,
//                             bdlb::TransparentEqualTo>  TransparentSet;
//
//  TransparentSet  transparentSet;
//..
// Now, we fill the container with the strings:
//..
//  transparentSet.insert("NY");
//  transparentSet.insert("LA");
//..
// Finally, we observe that the container allows to use 'bsl::string_view'
// objects as a key and does not make any implicit conversions:
//..
//  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));
//..

#include <bdlscm_version.h>

#include <bslmf_istriviallycopyable.h>
#include <bslmf_istriviallydefaultconstructible.h>
#include <bslmf_nestedtraitdeclaration.h>

namespace BloombergLP {
namespace bdlb {

                    // =========================
                    // struct TransparentEqualTo
                    // =========================

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 that this
    // class is an empty POD type.

    // TYPES
    typedef void is_transparent;
        // Type alias indicating this is a transparent comparator.

    // CREATORS
    //! TransparentEqualTo() = default;
        // Create a 'TransparentEqualTo' object.

    //! TransparentEqualTo(const TransparentEqualTo& original) = default;
        // Create a 'TransparentEqualTo' object.  Note that as
        // 'TransparentEqualTo' is an empty (stateless) type, this operation
        // has no observable effect.

    //! ~TransparentEqualTo() = default;
        // Destroy this object.

    // MANIPULATORS
    //! TransparentEqualTo& operator=(const TransparentEqualTo& rhs) = default;
        // Assign to this object the value of the specified 'rhs' object, and
        // return a reference providing modifiable access to this object.
        // Note that as 'TransparentEqualTo' is an empty (stateless) type, this
        // operation has no observable effect.

    // ACCESSORS
    template <class LHS, class RHS>
    bool operator()(const LHS& lhs, const RHS& rhs) const;
        // Return 'true' if the specified 'lhs' is equal to the specified 'rhs'
        // and 'false' otherwise.
};

// ============================================================================
//                           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
}  // close enterprise 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 ----------------------------------