bslmf_isbitwiseequalitycomparable.h

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/// @file bslmf_isbitwiseequalitycomparable.h
///
/// The content of this file has been pre-processed for Doxygen.
///
 
 
// bslmf_isbitwiseequalitycomparable.h                                -*-C++-*-
#ifndef INCLUDED_BSLMF_ISBITWISEEQUALITYCOMPARABLE
#define INCLUDED_BSLMF_ISBITWISEEQUALITYCOMPARABLE
 
#include <bsls_ident.h>
BSLS_IDENT("$Id: $")
 
/// @defgroup bslmf_isbitwiseequalitycomparable bslmf_isbitwiseequalitycomparable
/// @brief  Provide a type trait for bitwise equality.
/// @addtogroup bsl
/// @{
/// @addtogroup bslmf
/// @{
/// @addtogroup bslmf_isbitwiseequalitycomparable
/// @{
///
/// <h1> Outline </h1>
/// * <a href="#bslmf_isbitwiseequalitycomparable-purpose"> Purpose</a>
/// * <a href="#bslmf_isbitwiseequalitycomparable-classes"> Classes </a>
/// * <a href="#bslmf_isbitwiseequalitycomparable-description"> Description </a>
///   * <a href="#bslmf_isbitwiseequalitycomparable-usage"> Usage </a>
///     * <a href="#bslmf_isbitwiseequalitycomparable-example-1-using-the-trait-to-optimize-range-comparison"> Example 1: Using the trait to optimize range comparison </a>
///     * <a href="#bslmf_isbitwiseequalitycomparable-example-2-associating-a-trait-with-a-class-template"> Example 2: Associating a Trait with a Class Template </a>
///
/// # Purpose {#bslmf_isbitwiseequalitycomparable-purpose}
/// Provide a type trait for bitwise equality.
///
/// # Classes {#bslmf_isbitwiseequalitycomparable-classes}
///
/// -  bslmf::IsBitwiseEqualityComparable: trait metafunction
///
/// # Description {#bslmf_isbitwiseequalitycomparable-description}
/// This component provides a single trait metafunction,
/// `bslmf::IsBitwiseEqualityComparable`, which allows generic code to determine
/// whether objects of the specified `t_TYPE` can be compared using `memcmp`.
/// Such types are said to be bitwise EqualityComparable.  Observe that this
/// trait may be `true` only for object types, even though, for example,
/// reference types may be guaranteed identical bit representations when they
/// refer to the same object, just as the corresponding pointer type would be
/// bitwise EqualityComparable.
///
/// For a type to be bitwise EqalityComparable, each bit of its object
/// representation must be significant in the value representation, and distinct
/// sequences of bits represent different values, i.e., this trait is an
/// assertion that the specified `t_TYPE` has unique representations for each
/// possible value, and no padding bits.  This property is deemed to hold for
/// `bool` and enumerations where, in practice, the compiler will enforce a
/// value representation over all the seemingly unused bits.  For a C++17 tool
/// chain, this trait should be equivalent to the
/// `std::has_unique_object_representation` trait.
///
/// Note that as arrays are not allowed to introduce padding, arrays of a
/// bitwise EqualityComparable `t_TYPE` are also bitwise EqualityComparable,
/// even though they do not provide an overloaded `operator==`.  While
/// transforming comparisons of a single object using this trait into calls to
/// `memcmp` is unlikely to be profitable, transforming comparisons of a whole
/// array into a single `memcmp` call is more likely to be beneficial.
///
/// ## Usage {#bslmf_isbitwiseequalitycomparable-usage}
///
///
///
/// ### Example 1: Using the trait to optimize range comparison {#bslmf_isbitwiseequalitycomparable-example-1-using-the-trait-to-optimize-range-comparison}
///
///
/// Suppose we want to compare two sequences of the same object type to
/// determine whether or not they hold the same values.  The simplest solution
/// would be to iterate over both sequences, comparing each member, and return
/// `false` as soon as any pair of elements do not compare equal; if we walk all
/// the way to the end of both sequences, then they hold the same values.  If we
/// want to perform this comparison most efficiently though, we would rather not
/// invoke `operator==` on each member, and instead defer to the `memcmp`
/// function in the standard library that is highly optimized (often to take
/// advantage of platform-specific instructions) for comparing ranges of raw
/// memory.  We can switch to this other technique only if we know that the
/// value representations of a type are unique, rely on all of the bits in their
/// representation, and do not have strange values like `NaN` that self-compare
/// as `false`.  This property is denoted by the `IsBitwiseEqualityComparable`
/// trait.
///
/// First, we create a simple `struct` that contains a `char` and a `short` as
/// its two data members, and supported comparison with `operator==`.  Note that
/// there will be a byte of padding between the `char` and the `short` members
/// to ensure proper alignment.  We insert telemetry to count the number of
/// times `operator==` is called:
/// @code
/// namespace BloombergLP {
///
/// struct SimpleType {
///     // This 'struct' holds two data members with a byte of padding, and can
///     // be compared using the overloaded 'operator=='.
///
///     char  d_dataC;
///     short d_dataS;
///
///     static int s_comparisons;
///
///     friend bool operator==(const SimpleType& a, const SimpleType& b)
///         // Return 'true' if the specified 'a' has the same value as the
///         // specified 'b'.  Two 'SimpleType' objects have the same value if
///         // their corresponding 'd_dataC' and 'd_dataS' members have the
///         // same value.  The static data member 's_comparisons' is
///         // incremented by one each time this function is called.
///     {
///         ++s_comparisons;
///         return a.d_dataC == b.d_dataC
///             && a.d_dataS == b.d_dataS;
///     }
///
///     friend bool operator!=(const SimpleType& a, const SimpleType& b)
///         // Return 'true' if the specified 'a' does not have the same value
///         // as the specified 'b'.  Two 'SimpleType' objects do not have the
///         // same value if their corresponding 'd_dataC' and 'd_dataS'
///         // members do not have the same value.  The static data member
///         // 's_comparisons' is incremented by one each time this function is
///         // called.
///     {
///         ++s_comparisons;
///         return a.d_dataC != b.d_dataC
///             || a.d_dataS != b.d_dataS;
///     }
/// };
///
/// int SimpleType::s_comparisons = 0;
/// @endcode
/// Then, we create another `struct` that wraps a single `int` as its only data
/// member, and supports comparison with `operator==`, inserting telemetry to
/// count the number of times `operator==` is called:
/// @code
/// struct SecondType {
///     // This 'struct' holds a single 'int' member, 'd_data', and can be
///     // compared using the overloaded 'operator=='.
/// @endcode
/// We associate the bitwise EqualityComparable trait with `SecondType` using
/// the BDE nested trait declaration facility:
/// @code
///     BSLMF_NESTED_TRAIT_DECLARATION(SecondType,
///                                    bslmf::IsBitwiseEqualityComparable);
///
///     int d_data;
///
///     static int s_comparisons;
///
///     friend bool operator==(const SecondType& a, const SecondType& b)
///         // Return 'true' if the specified 'a' has the same value as the
///         // specified 'b'.  Two 'SecondType' objects have the same value if
///         // their corresponding 'd_data' elements have the same value.  The
///         // static data member 's_comparisons' is incremented by one each
///         // time this function is called.
///     {
///         ++s_comparisons;
///         return a.d_data == b.d_data;
///     }
///
///     friend bool operator!=(const SecondType& a, const SecondType& b)
///         // Return 'true' if the specified 'a' does not have the same value
///         // as the specified 'b'.  Two 'SecondType' objects do not have the
///         // same value if their corresponding 'd_data' elements do not have
///         // the same value.  The static data member 's_comparisons' is
///         // incremented by one each time this function is called.
///     {
///         ++s_comparisons;
///         return a.d_data != b.d_data;
///     }
/// };
///
/// int SecondType::s_comparisons = 0;
/// @endcode
/// Next, we create another `struct` that wraps a single `int` as its only data
/// member, and supports comparison with `operator==`, inserting telemetry to
/// count the number of times `operator==` is called:
/// @code
/// struct ThirdType {
///     // This 'struct' holds a single 'int' member, 'd_data', and can be
///     // compared using the overloaded 'operator=='.
///
///     int d_data;
///
///     static int s_comparisons;
///
///     friend bool operator==(const ThirdType& a, const ThirdType& b)
///         // Return 'true' if the specified 'a' has the same value as the
///         // specified 'b'.  Two 'SecondType' objects have the same value if
///         // their corresponding 'd_data' elements have the same value.  The
///         // static data member 's_comparisons' is incremented by one each
///         // time this function is called.
///     {
///         ++s_comparisons;
///         return a.d_data == b.d_data;
///     }
///
///     friend bool operator!=(const ThirdType& a, const ThirdType& b)
///         // Return 'true' if the specified 'a' does not have the same value
///         // as the specified 'b'.  Two 'ThirdType' objects do not have the
///         // same value if their corresponding 'd_data' elements do not have
///         // the same value.  The static data member 's_comparisons' is
///         // incremented by one each time this function is called.
///     {
///         ++s_comparisons;
///         return a.d_data != b.d_data;
///     }
/// };
///
/// int ThirdType::s_comparisons = 0;
/// @endcode
/// We associate the bitwise EqualityComparable trait with `ThirdType` by
/// explicitly specializing the trait:
/// @code
/// namespace bslmf {
///     template <>
///     struct IsBitwiseEqualityComparable<ThirdType> : bsl::true_type {};
/// }  // close namespace bslmf
/// @endcode
/// Now, we write a function template to compare two arrays of the same type:
/// @code
/// template <class t_TYPE>
/// bool rangeCompare(const t_TYPE *start, size_t length, const t_TYPE *other)
/// {
/// @endcode
/// If we detect the bitwise EqualityComparable trait, we rely on the optimized
/// `memcmp` function:
/// @code
///     if (bslmf::IsBitwiseEqualityComparable<t_TYPE>::value) {
///         return 0 == memcmp(start,
///                            other,
///                            length * sizeof(t_TYPE));              // RETURN
///     }
/// @endcode
/// Otherwise we iterate over the range directly until we find a pair of
/// elements that do not have the same value, and return `true` if we reach the
/// end of the range.
/// @code
///     if (0 != length)  {
///         while (*start++ == *other++) {
///             if (!--length) {
///                 return true;                                      // RETURN
///             }
///         }
///     }
///     return false;
/// }
/// @endcode
/// Finally, we write a test to confirm that two arrays containing different
/// values do not compare as equal (using our array comparison function), and
/// that an array compares equal to itself, as it does comprise elements all
/// having the same value.  By inspecting the static data members provided for
/// telemetry, we can confirm that `operator==` is called only for `SimpleType`
/// as the other two `struct`s dispatch to `memcmp` instead:
/// @code
/// int usageExample1()
/// {
/// @endcode
/// We confirm the initial state of the telemetry:
/// @code
///     assert(0 == SimpleType::s_comparisons);
///     assert(0 == SecondType::s_comparisons);
///     assert(0 == ThirdType ::s_comparisons);
/// @endcode
/// Then we create zero-initialized arrays for each of the types to be tested,
/// and a second array for each type with a set of values distinct from all
/// zeroes:
/// @code
///     const SimpleType simpleZeroes[10] = { };
///     const SecondType secondZeroes[10] = { };
///     const ThirdType  thirdZeroes [10] = { };
///
///     const SimpleType simpleValues[10] = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
///     const SecondType secondValues[10] = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
///     const ThirdType  thirdValues [10] = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
/// @endcode
/// Next we confirm that the two arrays (of each type) do not compare equal, and
/// inspect the telemetry to confirm that the comparison operator was called for
/// only the `SimpleType` without the bitwise EqualityComparable trait:
/// @code
///     assert(!rangeCompare(simpleZeroes, 10u, simpleValues) );
///     assert(!rangeCompare(secondZeroes, 10u, secondValues) );
///     assert(!rangeCompare(thirdZeroes,  10u, thirdValues)  );
///
///     assert(0 <  SimpleType::s_comparisons);
///     assert(0 == SecondType::s_comparisons);
///     assert(0 == ThirdType ::s_comparisons);
///
/// @endcode
/// Then we reset the telemetry and confirm that an array of each type compares
/// equal to itself, and inspect the telemetry to confirm that the comparison
/// operator was called for only the `SimpleType` without the bitwise
/// EqualityComparable trait:
/// @code
///     SimpleType::s_comparisons = 0;
///
///     assert( rangeCompare(simpleValues, 10u, simpleValues) );
///     assert( rangeCompare(secondValues, 10u, secondValues) );
///     assert( rangeCompare(thirdZeroes,  10u, thirdZeroes)  );
///
///     assert(0 <  SimpleType::s_comparisons);
///     assert(0 == SecondType::s_comparisons);
///     assert(0 == ThirdType ::s_comparisons);
///
///     return 0;
/// }
///
/// }  // close enterprise namespace
/// @endcode
///
/// ### Example 2: Associating a Trait with a Class Template {#bslmf_isbitwiseequalitycomparable-example-2-associating-a-trait-with-a-class-template}
///
///
/// In this example, we associate a trait not with a class, but with a class
/// *template*.  We create a "control" template that is not bitwise
/// EqualityComparable, and two class templates, each of which uses a different
/// mechanisms for being associated with the `IsBitwiseEqualityComparable`
/// trait.  First, we define a class template that is not bitwise
/// EqualityComparable, `NotComparable`:
/// @code
/// namespace BloombergLP {
///
/// template <class t_TYPE>
/// struct NotComparable
/// {
///     t_TYPE d_value;
/// };
/// @endcode
/// Then, we define the class template `PotentiallyComparable1`, which uses
/// partial template specialization to associate the
/// `IsBitwiseEqualityComparable` trait with each instantiation on a `t_TYPE`
/// that is itself bitwise EqualityComparable:
/// @code
/// template <class t_TYPE>
/// struct PotentiallyComparable1
/// {
///     t_TYPE d_value;
/// };
///
/// namespace bslmf {
///     template <class t_TYPE>
///     struct IsBitwiseEqualityComparable<PotentiallyComparable1<t_TYPE> >
///         : IsBitwiseEqualityComparable<t_TYPE>::type {
///     };
/// }  // close namespace bslmf
/// @endcode
/// Next, we define the class template'PotentiallyComparable2', which uses the
/// `BSLMF_NESTED_TRAIT_DECLARATION` macro to associate the
/// `IsBitwiseEqualityComparable` trait with each instantiation on a `t_TYPE`
/// that is itself bitwise EqualityComparable:
/// @code
/// template <class t_TYPE>
/// struct PotentiallyComparable2
/// {
///     t_TYPE d_value;
///
///     BSLMF_NESTED_TRAIT_DECLARATION_IF(
///                         PotentiallyComparable2,
///                         bslmf::IsBitwiseEqualityComparable,
///                         bslmf::IsBitwiseEqualityComparable<t_TYPE>::value);
/// };
/// @endcode
/// Finally, we check that the traits are correctly associated by instantiating
/// each template with types that are bitwise EqualityComparable and with types
/// that are not not bitwise EqualityComparable, verifying the value of
/// `IsBitwiseEqualityComparable<T>::value` in each case:
/// @code
/// int usageExample2()
/// {
///     using namespace bslmf;
///
///     assert(!IsBitwiseEqualityComparable<NotComparable<int> >::value);
///     assert(!IsBitwiseEqualityComparable<
///                           NotComparable<NotComparable<int> > >::value);
///
///     assert( IsBitwiseEqualityComparable<
///                                PotentiallyComparable1<int> >::value);
///     assert(!IsBitwiseEqualityComparable<
///                  PotentiallyComparable1<NotComparable<int> > >::value);
///
///     assert( IsBitwiseEqualityComparable<
///                                PotentiallyComparable2<int> >::value);
///     assert(!IsBitwiseEqualityComparable<
///                  PotentiallyComparable2<NotComparable<int> > >::value);
///
///     return 0;
/// }
///
/// }  // close enterprise namespace
/// @endcode
/// @}
/** @} */
/** @} */
 
/** @addtogroup bsl
 * @{
 */
/** @addtogroup bslmf
 * @{
 */
/** @addtogroup bslmf_isbitwiseequalitycomparable
 * @{
 */
 
#include <bslscm_version.h>
 
#include <bslmf_detectnestedtrait.h>
#include <bslmf_integralconstant.h>
#include <bslmf_isconst.h>
#include <bslmf_voidtype.h>
 
#include <bsls_platform.h>
 
#include <stddef.h>
 
#ifndef BDE_DONT_ALLOW_TRANSITIVE_INCLUDES
#include <bslmf_isenum.h>
#endif
 
 
 
namespace bslmf {
 
template <class t_TYPE>
struct IsBitwiseEqualityComparable;
 
#if defined(BSLS_PLATFORM_CMP_IBM)
                     // =========================================
                     // struct IsBitwiseEqualityComparable_Scalar
                     // =========================================
 
template <class t_TYPE, class = void>
struct IsBitwiseEqualityComparable_Imp2 : bsl::false_type {
};
template <class t_TYPE>
struct IsBitwiseEqualityComparable_Imp2<t_TYPE, BSLMF_VOIDTYPE(t_TYPE[])>
: bsl::true_type {
    // This implementation-detail trait determines whether 't_TYPE' is a scalar
    // type (an arithmetic type, enumeration, pointer, or pointer-to-member).
    // This implementation takes advantage of a previous layer of filtering
    // handling all class-types, so any remaining types that are valid as array
    // elements must be scalar types, i.e., the 'BSLMF_VOIDTYPE' test will
    // filter function types and reference types.
};
 
                     // ======================================
                     // struct IsBitwiseEqualityComparable_Imp
                     // ======================================
 
template <class t_TYPE, class = void>
struct IsBitwiseEqualityComparable_Imp
: IsBitwiseEqualityComparable_Imp2<t_TYPE>::type {
};
template <class t_TYPE>
struct IsBitwiseEqualityComparable_Imp<t_TYPE, BSLMF_VOIDTYPE(int t_TYPE::*)>
: DetectNestedTrait<t_TYPE, IsBitwiseEqualityComparable>::type {
    // This trait 'struct' derives from 'bsl::true_type' if (the template
    // paramter) 't_TYPE' is a scalar type or a class with a nested trait
    // declaration for the 'bslmf::IsBitwiseEqualityComparable' trait, and from
    // 'bsl::false_type' otherwise.  Note that this template handles only the
    // class-type cases, delegating the final filtering for scalar types to a
    // further trait template.
};
#else
                     // ======================================
                     // struct IsBitwiseEqualityComparable_Imp
                     // ======================================
 
/// This trait `struct` derives from `bsl::true_type` if (the template
/// paramter) `t_TYPE` is a scalar type or a class with a nested trait
/// declaration for the `bslmf::IsBitwiseEqualityComparable` trait, and from
/// `bsl::false_type` otherwise.  Note that this implementation relies on
/// the fact that reference and function types cannot be cv-qualified, and
/// any other non-class type (that is not handled by specializations of the
/// primary template) will be a scalar type.
template <class t_TYPE, class = void>
struct IsBitwiseEqualityComparable_Imp : bsl::is_const<const t_TYPE>::type {
};
template <class t_TYPE>
struct IsBitwiseEqualityComparable_Imp<t_TYPE, BSLMF_VOIDTYPE(int t_TYPE::*)>
: DetectNestedTrait<t_TYPE, IsBitwiseEqualityComparable>::type {
};
#endif
 
                     // ==================================
                     // struct IsBitwiseEqualityComparable
                     // ==================================
 
/// This trait `struct` is a metafunction that determines whether the
/// specified parameter `t_TYPE` is bitwise EqualityComparable.  If
/// `IsBitwiseEqualityComparable<t_TYPE>` is derived from `true_type` then
/// `t_TYPE` is bitwise EqualityComparable.  Otherwise, bitwise equality
/// comparability cannot be inferred for `t_TYPE`.  This trait can be
/// associated with a bitwise EqualityComparable user-defined class by
/// specializing this class or by using the `BSLMF_NESTED_TRAIT_DECLARATION`
/// macro.
template <class t_TYPE>
struct IsBitwiseEqualityComparable
: IsBitwiseEqualityComparable_Imp<t_TYPE>::type {
};
 
template <class t_TYPE>
struct IsBitwiseEqualityComparable<const t_TYPE>
: IsBitwiseEqualityComparable<t_TYPE>::type {
};
template <class t_TYPE>
struct IsBitwiseEqualityComparable<volatile t_TYPE>
: IsBitwiseEqualityComparable<t_TYPE>::type {
};
template <class t_TYPE>
struct IsBitwiseEqualityComparable<const volatile t_TYPE>
: IsBitwiseEqualityComparable<t_TYPE>::type {
};
    // Partial specializations for cv-qualified types channel to a single
    // instantiation of the implementation type.  Note that we cannot derive
    // through the 'Imp' type directly as we would not correctly handle
    // cv-qualified types that have been explicitly specialized by our users.
 
template <class t_TYPE, size_t t_LEN>
struct IsBitwiseEqualityComparable<t_TYPE[t_LEN]>
: IsBitwiseEqualityComparable<t_TYPE>::type {
};
template <class t_TYPE, size_t t_LEN>
struct IsBitwiseEqualityComparable<const t_TYPE[t_LEN]>
: IsBitwiseEqualityComparable<t_TYPE>::type {
};
template <class t_TYPE, size_t t_LEN>
struct IsBitwiseEqualityComparable<volatile t_TYPE[t_LEN]>
: IsBitwiseEqualityComparable<t_TYPE>::type {
};
template <class t_TYPE, size_t t_LEN>
struct IsBitwiseEqualityComparable<const volatile t_TYPE[t_LEN]>
: IsBitwiseEqualityComparable<t_TYPE>::type {
};
 
template <class t_TYPE>
struct IsBitwiseEqualityComparable<t_TYPE[]>
: IsBitwiseEqualityComparable<t_TYPE>::type {
};
template <class t_TYPE>
struct IsBitwiseEqualityComparable<const t_TYPE[]>
: IsBitwiseEqualityComparable<t_TYPE>::type {
};
template <class t_TYPE>
struct IsBitwiseEqualityComparable<volatile t_TYPE[]>
: IsBitwiseEqualityComparable<t_TYPE>::type {
};
template <class t_TYPE>
struct IsBitwiseEqualityComparable<const volatile t_TYPE[]>
: IsBitwiseEqualityComparable<t_TYPE>::type {
};
    // Partial specializations for array types, as arrays have a contiguity
    // guarantee (that can inferred from pointer arithmetic rules) so this if a
    // type is bitwise equality comparable, we can be sure there is no padding
    // introduced by an array, so arrays of this type should have the same
    // property.
 
/// Explicit specialization to confirm that `void` types are never bitwise
/// EqualityComparable.  Note that cv-`void` types are covered by the
/// partial specialization for any cv-qualified type.
template <>
struct IsBitwiseEqualityComparable<void> : bsl::false_type {};
 
/// Revert of {DRQS 143286899}. Once clients are fixed, change to false.
/// Explicit specialization to confirm that floating point types are not
/// bitwise EqualityComparable, as they typically have specific problematic
/// values: NaNs do not compare equal with themselves, and there may be
/// multiple representations for zero (with negative zero).
template <>
struct IsBitwiseEqualityComparable<float> : bsl::true_type {};
template <>
struct IsBitwiseEqualityComparable<double> : bsl::true_type {};
template <>
struct IsBitwiseEqualityComparable<long double> : bsl::true_type {};
 
}  // close package namespace
 
 
#endif // ! defined(INCLUDED_BSLMF_ISBITWISEEQUALITYCOMPARABLE)
 
// ----------------------------------------------------------------------------
// Copyright 2019 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 ----------------------------------
 
/** @} */
/** @} */
/** @} */