bdlb_transparentless.h

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/// @file bdlb_transparentless.h
///
/// The content of this file has been pre-processed for Doxygen.
///
 
 
// bdlb_transparentless.h                                             -*-C++-*-
#ifndef INCLUDED_BDLB_TRANSPARENTLESS
#define INCLUDED_BDLB_TRANSPARENTLESS
 
#include <bsls_ident.h>
BSLS_IDENT("$Id: $")
 
/// @defgroup bdlb_transparentless bdlb_transparentless
/// @brief  Provide a transparent less-than predicate.
/// @addtogroup bdl
/// @{
/// @addtogroup bdlb
/// @{
/// @addtogroup bdlb_transparentless
/// @{
///
/// <h1> Outline </h1>
/// * <a href="#bdlb_transparentless-purpose"> Purpose</a>
/// * <a href="#bdlb_transparentless-classes"> Classes </a>
/// * <a href="#bdlb_transparentless-description"> Description </a>
///   * <a href="#bdlb_transparentless-usage"> Usage </a>
///     * <a href="#bdlb_transparentless-example-1-basic-use-of-bdlb-transparentless"> Example 1: Basic Use of bdlb::TransparentLess </a>
///
/// # Purpose {#bdlb_transparentless-purpose}
/// Provide a transparent less-than predicate.
///
/// # Classes {#bdlb_transparentless-classes}
///
/// -  bdlb::TransparentLess: a transparent less-than predicate.
///
/// @see  bsl_map, bsl_set
///
/// # Description {#bdlb_transparentless-description}
/// This component provides a `struct`, `bdlb::TransparentLess`,
/// that defines a functor that can be used as transparent less-than comparator
/// for heterogeneous lookup.
///
/// ## Usage {#bdlb_transparentless-usage}
///
///
/// This section illustrates intended use of this component.
///
/// ### Example 1: Basic Use of bdlb::TransparentLess {#bdlb_transparentless-example-1-basic-use-of-bdlb-transparentless}
///
///
/// Suppose we need a container to store set of unique `bsl::strings` objects.
/// We use `bsl::set` for our container, which uses `bsl::less` as default
/// comparator.  `bsl::less` is suitable if we want to search an entry using a
/// `bsl::string` object as a key.  However, if we were to try and search using
/// a `bsl::string_view` object the code would not compile, even though the
/// comparison operator between `bsl::string` and `bsl::string_view` exists.  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::TransparentLess` as a comparator for the
/// standard container `set`, in this case to allow a `bsl::set<bsl::string>` to
/// be searched with a `bsl::string_view`.
///
/// First, we create several C-strings:
/// @code
/// const char *newYork          = "NY";
/// const char *losAngeles       = "LA";
/// const char *cityWithLongName = "The abbreviation of really really long "
///                                "name of the city. The name is so long "
///                                "that even its abbreviation definitely "
///                                "exceeds Small String Optimization limit";
/// const char *sanFrancisco     = "SF";
/// @endcode
/// Next, we create several allocators to precisely control memory allocation:
/// @code
/// bslma::TestAllocator         ca("container", veryVeryVeryVerbose);
/// bslma::TestAllocator         sa("strings",   veryVeryVeryVerbose);
/// @endcode
/// And set one of them as default:
/// @code
/// bslma::TestAllocator         da("default",   veryVeryVeryVerbose);
/// bslma::DefaultAllocatorGuard dag(&da);
/// @endcode
/// Then, we create two containers, one with default comparator and another
/// using `bdlb::TransparentLess` as a comparator:
/// @code
/// bsl::set<bsl::string>                        defaultSet(&ca);
/// bsl::set<bsl::string, bdlb::TransparentLess> userSet(&ca);
/// @endcode
/// Now, we fill the containers with the strings:
/// @code
/// bsl::string newYorkStr         (newYork,           &sa);
/// bsl::string losAngelesStr      (losAngeles,        &sa);
/// bsl::string cityWithLongNameStr(cityWithLongName,  &sa);
/// bsl::string sanFranciscoStr    (sanFrancisco,      &sa);
///
/// defaultSet.insert(newYorkStr         );
/// defaultSet.insert(losAngelesStr      );
/// defaultSet.insert(cityWithLongNameStr);
///
/// userSet.insert(newYorkStr         );
/// userSet.insert(losAngelesStr      );
/// userSet.insert(cityWithLongNameStr);
/// @endcode
/// Finally, we observe that the container created with `TransparentLess`
/// container (`userSet`) allows to use @ref string_view  object as a key and does
/// not make any implicit conversions:
/// @code
/// bsl::string_view newYorkStrView         (newYork         );
/// bsl::string_view cityWithLongNameStrView(cityWithLongName);
/// bsl::string_view sanFranciscoStrView    (sanFrancisco    );
///
/// assert(userSet.end() != userSet.find(newYorkStrView         ));
/// assert(userSet.end() != userSet.find(cityWithLongNameStrView));
/// assert(userSet.end() == userSet.find(sanFranciscoStrView    ));
///
/// assert(0 == da.numBytesTotal());
///
/// assert(userSet.end() != userSet.find(newYork         ));
/// assert(userSet.end() != userSet.find(cityWithLongName));
/// assert(userSet.end() == userSet.find(sanFrancisco    ));
///
/// assert(0 == da.numBytesTotal());
/// @endcode
/// While the container using the default comparator implicitly converts
/// C-strings to `bsl::string` objects:
/// @code
/// assert(defaultSet.end() != defaultSet.find(newYork         ));
/// assert(defaultSet.end() != defaultSet.find(cityWithLongName));
/// assert(defaultSet.end() == defaultSet.find(sanFrancisco    ));
///
/// assert(0 < da.numBytesTotal());
/// @endcode
/// @}
/** @} */
/** @} */
 
/** @addtogroup bdl
 * @{
 */
/** @addtogroup bdlb
 * @{
 */
/** @addtogroup bdlb_transparentless
 * @{
 */
 
#include <bdlscm_version.h>
 
#include <bslmf_istriviallycopyable.h>
#include <bslmf_istriviallydefaultconstructible.h>
#include <bslmf_nestedtraitdeclaration.h>
 
 
namespace bdlb {
 
                    // ======================
                    // struct TransparentLess
                    // ======================
 
/// This `struct` defines an ordering on objects of different types,
/// enabling them for use for heterogeneous comparison in the standard
/// associative containers such as `bsl::map` and `bsl::set`.  
/// 
/// \note Note that   this class is an empty POD type.
///
/// See @ref bdlb_transparentless 
struct TransparentLess {
 
    // TYPES
 
    /// Type alias indicating this is a transparent comparator.
    typedef void is_transparent;
 
    // CREATORS
 
    /// Create a `TransparentLess` object.
     TransparentLess() = default;
 
    /// Create a `TransparentLess` object.  
    /// \note Note that  as `TransparentLess`
    /// is an empty (stateless) type, this operation has no observable
    /// effect.
     TransparentLess(const TransparentLess& original) = default;
 
    /// Destroy this object.
     ~TransparentLess() = 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 `TransparentLess` is an empty (stateless) type, this
    /// operation has no observable effect.
     TransparentLess& operator=(const TransparentLess& rhs) = default;
 
    // ACCESSORS
 
    /// Return `true` if the specified `lhs` is less than the specified
    /// `rhs` and `false` otherwise.
    template <class LHS, class RHS>
    bool operator()(const LHS& lhs, const RHS& rhs) const;
};
 
// ============================================================================
//                           INLINE DEFINITIONS
// ============================================================================
 
                          // ----------------------
                          // struct TransparentLess
                          // ----------------------
 
// ACCESSORS
template <class LHS, class RHS>
inline
bool TransparentLess::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 ----------------------------------
 
/** @} */
/** @} */
/** @} */