bslmf_isbitwisemoveable.h

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/// @file bslmf_isbitwisemoveable.h
///
/// The content of this file has been pre-processed for Doxygen.
///
 
 
// bslmf_isbitwisemoveable.h                                          -*-C++-*-
#ifndef INCLUDED_BSLMF_ISBITWISEMOVEABLE
#define INCLUDED_BSLMF_ISBITWISEMOVEABLE
 
#include <bsls_ident.h>
BSLS_IDENT("$Id: $")
 
/// @defgroup bslmf_isbitwisemoveable bslmf_isbitwisemoveable
/// @brief  Provide a primitive type trait for bitwise moveable classes.
/// @addtogroup bsl
/// @{
/// @addtogroup bslmf
/// @{
/// @addtogroup bslmf_isbitwisemoveable
/// @{
///
/// <h1> Outline </h1>
/// * <a href="#bslmf_isbitwisemoveable-purpose"> Purpose</a>
/// * <a href="#bslmf_isbitwisemoveable-classes"> Classes </a>
/// * <a href="#bslmf_isbitwisemoveable-description"> Description </a>
///   * <a href="#bslmf_isbitwisemoveable-what-classes-are-not-bitwise-moveable"> What Classes are Not Bitwise Moveable? </a>
///     * <a href="#bslmf_isbitwisemoveable-one-byte-objects"> One-Byte Objects </a>
///   * <a href="#bslmf_isbitwisemoveable-usage"> Usage </a>
///     * <a href="#bslmf_isbitwisemoveable-example-1-using-the-trait-to-implement-destructivemovearray"> Example 1: Using the Trait to Implement destructiveMoveArray </a>
///     * <a href="#bslmf_isbitwisemoveable-example-2-associating-a-trait-with-a-class-template"> Example 2: Associating a Trait with a Class Template </a>
///     * <a href="#bslmf_isbitwisemoveable-example-3-avoiding-false-positives-on-one-byte-classes"> Example 3: Avoiding False Positives on One-Byte Classes </a>
///
/// # Purpose {#bslmf_isbitwisemoveable-purpose}
/// Provide a primitive type trait for bitwise moveable classes.
///
/// # Classes {#bslmf_isbitwisemoveable-classes}
///
/// -  bslmf::IsBitwiseMoveable: bitwise moveable trait metafunction
///
/// @see 
///
/// # Description {#bslmf_isbitwisemoveable-description}
/// This component provides a single trait metafunction,
/// `bslmf::IsBitwiseMoveable`, which allows generic code to determine whether
/// `t_TYPE` can be destructively moved using `memcpy`.  Given a pointer, `p1`,
/// to an object of `t_TYPE`, and a pointer `p2` of the same type pointing to
/// allocated but uninitialized storage, a destructive move from `p1` to `p2`
/// comprises the following pair of operations:
/// @code
/// new ((void*) p2) t_TYPE(*p1);// Or new ((void*) p2) t_TYPE(std::move(*p1));
/// p1->~t_TYPE();
/// @endcode
/// An object of a `t_TYPE` is *bitwise* *moveable*, if the above operation can
/// be replaced by the following operation without affecting correctness:
/// @code
/// std::memcpy(p2, p1, sizeof(t_TYPE));
/// @endcode
/// If `IsBitwiseMoveable<t_TYPE>::value` inherits from `true_type` for a given
/// `t_TYPE`, then a generic algorithm can infer that `t_TYPE` is bitwise
/// moveable.
///
/// This trait is used by various components for providing optimizations for
/// types that can be bitwise moved.  The major benefit of this trait is not for
/// a single object but for an array of such types, as a loop of copy/destroy
/// operations can be replaced by a single call to `memcpy`.  This replacement
/// is not only faster, but is guaranteed not to throw an exception.
///
/// `IsBitwiseMoveable<t_TYPE>` will inherit from `true_type` if `t_TYPE` is a
/// fundamental object type, enumeration type, or pointer type.  Most
/// user-defined classes are bitwise moveable, but generic code must assume that
/// an arbitrary `t_TYPE` is not bitwise-moveable, as bitwise moving a type that
/// is not bitwise moveable is likely to result in a dangling pointer.  Thus, it
/// is necessary to explicitly associate the bitwise moveable trait with a class
/// (via template specialization or by use of the `BSLMF_DECLARE_NESTED_TRAIT`
/// macro) in order for generic algorithms to recognize that class as bitwise
/// moveable.  As a special case, one-byte objects are deduced as bitwise
/// moveable unless explicitly annotated otherwise (see-below).
///
/// ## What Classes are Not Bitwise Moveable? {#bslmf_isbitwisemoveable-what-classes-are-not-bitwise-moveable}
///
///
/// A class that has any of the following attributes is **not** bitwise moveable:
///
/// * Its address is one of the salient attributes that comprise its value.
/// * It contains a pointer that might (directly or indirectly) point to
///   itself or to one of its own members or which stores an encoding of its
///   own address or the address of one of its members.  For example, a list
///   implementation that includes an embedded sentinel node such that the
///   last node in the list points back to the sentinel node within the list
///   class object is not bitwise moveable.
/// * Its constructor registers a pointer to itself in some static registry.
/// * Its constructor or destructor have some side effect that, if omitted
///   during destructive move, would render the program incorrect.
/// * It contains a data member or base class that is not bitwise moveable.
///
/// Because of the destructive nature of a bitwise move (the original object
/// must be treated as uninitialized storage after the move), a class can be
/// bitwise moveable but not also bitwise copyable.  For example, a class that
/// contains a pointer to heap-allocated storage is generally bitwise moveable.
/// The moved object simply refers to the same storage as the (defunct)
/// original.  However a bitwise copy of the same object would incorrectly cause
/// the original and the copy to share the same heap-allocated storage.
///
/// ### One-Byte Objects {#bslmf_isbitwisemoveable-one-byte-objects}
///
///
/// An object whose size does not exceed one byte are deduced to be bitwise
/// moveable.  The validity of this heuristic can be deduced by examining the
/// criteria for non-bitwise moveable classes above:
///
/// * Very few types have their own address as a salient attribute.
/// * It is not possible for an object to store a pointer to itself in only one
///   byte.  It is difficult to conceive of why an object would store a
///   (compressed) encoding of own address in one byte.
/// * Static registries of objects of any size are rare and are almost never a
///   good design.
/// * Constructors and destructors with side effects are also rare, and appear
///   almost entirely within test code (i.e., counting constructions and
///   destructions).  Even in those cases, it is often acceptable or even
///   preferable to skip the balanced side effects of the constructor and
///   destructor during a destructive move.
/// * Any data member or base class of a one-byte class must also be either an
///   empty base class or a one-byte object, so the above rationale applies
///   recursively to them.
///
/// The purpose of this heuristic is to deduce bitwise moveability for an
/// important category of empty classes that are not explicitly annotated as
/// being bitwise moveable: standard predicate classes such as `std::less<T>`.
/// Being able to treat these classes as bitwise moveable means that `bsl::set`
/// and `bsl::map` objects can be deduced as bitwise moveable and that
/// `bsl::function` objects wrapping these classes can use the small-object
/// optimization.  It can be argued that any type with size less than the size
/// of a pointer should be deduced as bitwise moveable by the logic above.
/// However, it is primarily the common case of empty classes that we are trying
/// to handle.  By limiting ourselves to the smallest-possible type, we reduce
/// the chance of false positives (see next paragraph).
///
/// Note that the word "rare" appears several times in the list above.  Rare
/// implies non-zero, so we must provide  a way to annotate non-bitwise moveable
/// one-byte classes so that the `IsBitwiseMoveable` trait is not deduced for
/// them.  This annotation is accomplished simply by specializing
/// `IsBitwiseMoveable` to inherit from `false_type` for these rare classes.
///
/// In C++11 and later, it is possible to accurately deduce a class is bitwise
/// moveable without relying on the one-byte heuristic.  If the deduction with
/// the one-byte heuristic yields true and the deduction without the one-byte
/// heuristic yields false, then a static assert fires and the program is
/// ill-formed.  This error can be corrected by specializing the trait to false
/// for the type in question.
///
/// ## Usage {#bslmf_isbitwisemoveable-usage}
///
///
/// This section illustrates intended use of this component.
///
/// ### Example 1: Using the Trait to Implement destructiveMoveArray {#bslmf_isbitwisemoveable-example-1-using-the-trait-to-implement-destructivemovearray}
///
///
/// Here, we use this trait in a simple algorithm called `destructiveMoveArray`,
/// which moves elements from one array to another.  The algorithm is
/// implemented using two implementation functions, one for types that are known
/// to be bit-wise moveable, and one for other types.  The first takes an extra
/// function argument of type `true_type`, the second takes and extra function
/// argument of type `false_type`:
/// @code
/// namespace BloombergLP {
///
/// template <class t_TYPE>
/// void destructiveMoveArrayImp(t_TYPE *to,
///                              t_TYPE *from,
///                              int   size,
///                              bsl::true_type)
/// {
///     // Bitwise moveable types can be moved using memcpy
///     memcpy(static_cast<void *>(to), from, size * sizeof(t_TYPE));
/// }
///
/// template <class t_TYPE>
/// void destructiveMoveArrayImp(t_TYPE *to,
///                              t_TYPE *from,
///                              int   size,
///                              bsl::false_type)
/// {
///     for (int i = 0; i < size; ++i) {
///         ::new(to + i) t_TYPE(from[i]);
///         from[i].~t_TYPE();
///     }
/// }
/// @endcode
/// Now we can dispatch between the two Imp functions, using the
/// `IsBitwiseMoveable` trait metafunction to determine at compile time which of
/// the implementations should be used:
/// @code
/// template <class t_TYPE>
/// void destructiveMoveArray(t_TYPE *to, t_TYPE *from, int size)
/// {
///     destructiveMoveArrayImp(to, from, size,
///                             bslmf::IsBitwiseMoveable<t_TYPE>());
/// }
/// @endcode
/// Next, to check our work, we create three classes that we will use to
/// instantiate `destructiveMoveArray`.  All of the classes will log the number
/// of constructor and destructor calls.  The first class will not be decorated
/// with the `IsBitwiseMoveable` trait:
/// @code
/// class NonMoveableClass
/// {
///   private:
///     int d_value;
///
///     static int d_ctorCount;
///     static int d_dtorCount;
///
///   public:
///     static int ctorCount() { return d_ctorCount; }
///     static int dtorCount() { return d_dtorCount; }
///
///     NonMoveableClass(int val = 0) : d_value(val) { ++d_ctorCount; }
///     NonMoveableClass(const NonMoveableClass& other)
///         : d_value(other.d_value) { ++d_ctorCount; }
///     ~NonMoveableClass() { d_dtorCount++; }
///
///     int value() const { return d_value; }
/// };
///
/// int NonMoveableClass::d_ctorCount = 0;
/// int NonMoveableClass::d_dtorCount = 0;
/// @endcode
/// The second class is similar except that we declare it to be bit-wise
/// moveable by specializing `IsBitwiseMoveable`:
/// @code
/// class MoveableClass1
/// {
///   private:
///     int d_value;
///
///     static int d_ctorCount;
///     static int d_dtorCount;
///
///   public:
///     static int ctorCount() { return d_ctorCount; }
///     static int dtorCount() { return d_dtorCount; }
///
///     MoveableClass1(int val = 0) : d_value(val) { ++d_ctorCount; }
///     MoveableClass1(const MoveableClass1& other)
///         : d_value(other.d_value) { ++d_ctorCount; }
///     ~MoveableClass1() { d_dtorCount++; }
///
///     int value() const { return d_value; }
/// };
///
/// int MoveableClass1::d_ctorCount = 0;
/// int MoveableClass1::d_dtorCount = 0;
///
/// namespace bslmf {
///     template <> struct IsBitwiseMoveable<MoveableClass1> : bsl::true_type {
///     };
/// }  // close namespace bslmf
/// @endcode
/// The third class is also declared to be bitwise moveable, but this time we do
/// it using the `BSLMF_NESTED_TRAIT_DECLARATION` macro:
/// @code
/// class MoveableClass2
/// {
///   private:
///     int d_value;
///
///     static int d_ctorCount;
///     static int d_dtorCount;
///
///   public:
///     BSLMF_NESTED_TRAIT_DECLARATION(MoveableClass2,
///                                    bslmf::IsBitwiseMoveable);
///
///     static int ctorCount() { return d_ctorCount; }
///     static int dtorCount() { return d_dtorCount; }
///
///     MoveableClass2(int val = 0) : d_value(val) { ++d_ctorCount; }
///     MoveableClass2(const MoveableClass2& other)
///         : d_value(other.d_value) { ++d_ctorCount; }
///     ~MoveableClass2() { d_dtorCount++; }
///
///     int value() const { return d_value; }
/// };
///
/// int MoveableClass2::d_ctorCount = 0;
/// int MoveableClass2::d_dtorCount = 0;
/// @endcode
/// Finally, invoke `destructiveMoveArray` on arrays of all three classes:
/// @code
/// enum MoveableEnum { A_VALUE };
///
/// int usageExample1()
/// {
///     using namespace bslmf;
///
///     // First, check the basic operation of 'IsBitwiseMoveable':
///     assert(  IsBitwiseMoveable<int>::value);
///     assert(  IsBitwiseMoveable<int*>::value);
///     assert(  IsBitwiseMoveable<const int*>::value);
///     assert(  IsBitwiseMoveable<MoveableEnum>::value);
///     assert(! IsBitwiseMoveable<int&>::value);
///     assert(! IsBitwiseMoveable<const int&>::value);
///     assert(  IsBitwiseMoveable<MoveableClass1>::value);
///     assert(  IsBitwiseMoveable<const MoveableClass1>::value);
///     assert(  IsBitwiseMoveable<MoveableClass2>::value);
///     assert(  IsBitwiseMoveable<volatile MoveableClass2>::value);
///     assert(! IsBitwiseMoveable<NonMoveableClass>::value);
///     assert(! IsBitwiseMoveable<const NonMoveableClass>::value);
///
///     // For each of our test classes, allocate an array, construct three
///     // objects into it, then move it into another array.
///     const int nObj = 3;
///
///     {
///         NonMoveableClass *p1 = (NonMoveableClass*)
///             ::operator new(nObj * sizeof(NonMoveableClass));
///         NonMoveableClass *p2 =  (NonMoveableClass*)
///             ::operator new(nObj * sizeof(NonMoveableClass));
///
///         for (int i = 0; i < nObj; ++i) {
///             new(p1 + i) NonMoveableClass(i);
///         }
///
///         assert(nObj == NonMoveableClass::ctorCount());
///         assert(0    == NonMoveableClass::dtorCount());
///
///         assert(! IsBitwiseMoveable<NonMoveableClass>::value);
///         destructiveMoveArray(p2, p1, nObj);
///
///         // Verify that constructor and destructor were called on each move
///         assert(2 * nObj == NonMoveableClass::ctorCount());
///         assert(nObj     == NonMoveableClass::dtorCount());
///
///         // Verify contents
///         for (int i = 0; i < nObj; ++i) {
///             assert(i == p2[i].value());
///         }
///
///         // Destroy and deallocate
///         for (int i = 0; i < nObj; ++i) {
///             p2[i].~NonMoveableClass();
///         }
///         ::operator delete(p1);
///         ::operator delete(p2);
///     }
///
///     {
///         MoveableClass1 *p1 = (MoveableClass1*)
///             ::operator new(nObj * sizeof(MoveableClass1));
///         MoveableClass1 *p2 = (MoveableClass1*)
///             ::operator new(nObj * sizeof(MoveableClass1));
///
///         for (int i = 0; i < nObj; ++i) {
///             ::new(p1 + i) MoveableClass1(i);
///         }
///
///         assert(nObj == MoveableClass1::ctorCount());
///         assert(0    == MoveableClass1::dtorCount());
///
///         assert(IsBitwiseMoveable<MoveableClass1>::value);
///         destructiveMoveArray(p2, p1, nObj);
///
///         // Verify that constructor and destructor were NOT called on each
///         // move
///         assert(nObj == MoveableClass1::ctorCount());
///         assert(0    == MoveableClass1::dtorCount());
///
///         // Verify contents
///         for (int i = 0; i < nObj; ++i) {
///             assert(i == p2[i].value());
///         }
///
///         // Destroy and deallocate
///         for (int i = 0; i < nObj; ++i) {
///             p2[i].~MoveableClass1();
///         }
///         ::operator delete(p1);
///         ::operator delete(p2);
///     }
///
///     {
///         MoveableClass2 *p1 = (MoveableClass2*)
///             ::operator new(nObj * sizeof(MoveableClass2));
///         MoveableClass2 *p2 = (MoveableClass2*)
///             ::operator new(nObj * sizeof(MoveableClass2));
///
///         for (int i = 0; i < nObj; ++i) {
///             ::new(p1 + i) MoveableClass2(i);
///         }
///
///         assert(nObj == MoveableClass2::ctorCount());
///         assert(0    == MoveableClass2::dtorCount());
///
///         assert(IsBitwiseMoveable<MoveableClass2>::value);
///         destructiveMoveArray(p2, p1, nObj);
///
///         // Verify that constructor and destructor were NOT called on each
///         // move
///         assert(nObj == MoveableClass2::ctorCount());
///         assert(0    == MoveableClass2::dtorCount());
///
///         // Verify contents
///         for (int i = 0; i < nObj; ++i) {
///             assert(i == p2[i].value());
///         }
///
///         // Destroy and deallocate
///         for (int i = 0; i < nObj; ++i) {
///             p2[i].~MoveableClass2();
///         }
///         ::operator delete(p1);
///         ::operator delete(p2);
///     }
///
///     return 0;
/// }
///
/// }  // close enterprise namespace
/// @endcode
///
/// ### Example 2: Associating a Trait with a Class Template {#bslmf_isbitwisemoveable-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 three class templates, each of which uses a different
/// mechanisms for being associated with the `IsBitwiseMoveable` trait, plus a
/// "control" template that is not bit-wise moveable.  First, we define the
/// non-bit-wise-moveable template, `NonMoveableTemplate`:
/// @code
/// namespace BloombergLP {
///
/// template <class t_TYPE>
/// struct NonMoveableTemplate
/// {
///     t_TYPE d_p;
/// };
/// @endcode
/// Second, we define a `MoveableTemplate1`, which uses partial template
/// specialization to associate the `IsBitwiseMoveable` trait with each
/// instantiation:
/// @code
/// template <class t_TYPE>
/// struct MoveableTemplate1
/// {
///     t_TYPE *d_p;
/// };
///
/// namespace bslmf {
///     template <class t_TYPE>
///     struct IsBitwiseMoveable<MoveableTemplate1<t_TYPE> > : bsl::true_type {
///     };
/// }  // close namespace bslmf
/// @endcode
/// Third, we define `MoveableTemplate2`, which uses the
/// `BSLMF_NESTED_TRAIT_DECLARATION` macro to associate the `IsBitwiseMoveable`
/// trait with each instantiation:
/// @code
/// template <class t_TYPE>
/// struct MoveableTemplate2
/// {
///     t_TYPE *d_p;
///
///     BSLMF_NESTED_TRAIT_DECLARATION(MoveableTemplate2,
///                                    bslmf::IsBitwiseMoveable);
/// };
/// @endcode
/// Fourth, we define `MoveableTemplate3`, which is bit-wise moveable iff its
/// `t_TYPE` template parameter is bit-wise moveable.  There is no way to get
/// this effect using `BSLMF_NESTED_TRAIT_DECLARATION`, so we use partial
/// specialization combined with inheritance to "inherit" the trait from
/// `t_TYPE`:
/// @code
/// template <class t_TYPE>
/// struct MoveableTemplate3
/// {
///     t_TYPE d_p;
/// };
///
/// namespace bslmf {
///     template <class t_TYPE>
///     struct IsBitwiseMoveable<MoveableTemplate3<t_TYPE> > :
///         IsBitwiseMoveable<t_TYPE>::type { };
/// }  // close namespace bslmf
/// @endcode
/// Now, we check that the traits are correctly associated by instantiating each
/// class with both bit-wise moveable and non-moveable types and verifying the
/// value of `IsBitwiseMoveable<T>::value`:
/// @code
/// int usageExample2()
/// {
///     using namespace bslmf;
///
///     assert(! IsBitwiseMoveable<
///            NonMoveableTemplate<NonMoveableClass> >::value);
///     assert(! IsBitwiseMoveable<
///            NonMoveableTemplate<MoveableClass1> >::value);
///
///     assert(  IsBitwiseMoveable<
///            MoveableTemplate1<NonMoveableClass> >::value);
///     assert(  IsBitwiseMoveable<
///            MoveableTemplate1<MoveableClass1> >::value);
///
///     assert(  IsBitwiseMoveable<
///            MoveableTemplate2<NonMoveableClass> >::value);
///     assert(  IsBitwiseMoveable<
///            MoveableTemplate2<MoveableClass1> >::value);
///
///     assert(! IsBitwiseMoveable<
///            MoveableTemplate3<NonMoveableClass> >::value);
///     assert(  IsBitwiseMoveable<
///            MoveableTemplate3<MoveableClass1> >::value);
///
///     return 0;
/// }
///
/// }  // close enterprise namespace
/// @endcode
///
/// ### Example 3: Avoiding False Positives on One-Byte Classes {#bslmf_isbitwisemoveable-example-3-avoiding-false-positives-on-one-byte-classes}
///
///
/// In this example, we define an empty class that has a non-trivial copy
/// constructor that has a global side effect.  The side effect should not be
/// omitted, even in a destructive-move situation, so `IsBitwiseMoveable` should
/// be false.  However, the heuristic described above would deduce any one-byte
/// class (including an empty class) as bitwise-moveable by default, so we must
/// take specific action to set the trait to false in this (rare) case.
///
/// First, we declare a normal empty class that *is* bitwise moveable:
/// @code
/// namespace BloombergLP {
/// namespace xyza {
///
/// class MoveableEmptyClass
/// {
///     // This class is implicitly moveable by virtue of being only one byte
///     // in size.
/// };
/// @endcode
/// The class above requires no special treatment.  Next, we define an empty
/// class that is not bitwise moveable:
/// @code
/// class NonMoveableEmptyClass
/// {
///     // This class is empty, which normally would imply bitwise moveability.
///     // However, because it has a non-trivial move/copy constructor, it
///     // should not be bitwise moved.
///
///     static int d_count;
///
///   public:
///     NonMoveableEmptyClass() { ++d_count; }
///     NonMoveableEmptyClass(const NonMoveableEmptyClass&) { ++d_count; }
/// };
///
/// int NonMoveableEmptyClass::d_count = 0;
///
/// }  // close package namespace
/// @endcode
/// Next, we specialize the `IsBitwiseMoveable` trait so that
/// `NonMoveableEmptyClass` is not incorrectly flagged by trait deduction as
/// having the `IsBitwiseMoveable` trait:
/// @code
/// namespace bslmf {
///
/// template <>
/// struct IsBitwiseMoveable<xyza::NonMoveableEmptyClass> : bsl::false_type
/// {
/// };
///
/// }  // close namespace bslmf
/// @endcode
/// Finally, we show that the first class has the `IsBitwiseMoveable` trait and
/// the second class does not:
/// @code
/// int main()
/// {
///     using namespace bslmf;
///
///     assert(  IsBitwiseMoveable<xyza::MoveableEmptyClass>::value);
///     assert(! IsBitwiseMoveable<xyza::NonMoveableEmptyClass>::value);
/// }
///
/// }  // close enterprise namespace
/// @endcode
/// @}
/** @} */
/** @} */
 
/** @addtogroup bsl
 * @{
 */
/** @addtogroup bslmf
 * @{
 */
/** @addtogroup bslmf_isbitwisemoveable
 * @{
 */
 
#include <bslscm_version.h>
 
#include <bslmf_conditional.h>
#include <bslmf_detectnestedtrait.h>
#include <bslmf_integralconstant.h>
#include <bslmf_isempty.h>
#include <bslmf_isbitwisecopyable.h>
#include <bslmf_isfunction.h>
#include <bslmf_isreference.h>
 
#include <bsls_compilerfeatures.h>
#include <bsls_platform.h>
 
#include <stddef.h>
 
#ifdef BSLS_COMPILERFEATURES_SUPPORT_TRAITS_HEADER
# include <type_traits>
#endif // BSLS_COMPILERFEATURES_SUPPORT_TRAITS_HEADER
 
#ifndef BDE_DONT_ALLOW_TRANSITIVE_INCLUDES
#include <bsls_nativestd.h>
#endif // BDE_DONT_ALLOW_TRANSITIVE_INCLUDES
 
#if !defined(BSLS_PLATFORM_CMP_IBM)
// Last checked with the xlC 12.1 compiler.  The IBM xlC compiler has problems
// correctly handling arrays of unknown bound as template parameters.
# define BSLMF_ISBITWISEMOVEABLE_NO_SUPPORT_FOR_ARRAY_OF_UNKNOWN_BOUND 1
#endif
 
 
 
namespace bslmf {
 
template <class t_TYPE>
struct IsBitwiseMoveable;
 
#ifdef BSLS_COMPILERFEATURES_SUPPORT_VARIABLE_TEMPLATES
/// This template variable represents the result value of the
/// `bslmf::IsBitwiseMoveable` meta-function.
template <class t_TYPE>
BSLS_KEYWORD_INLINE_VARIABLE constexpr bool IsBitwiseMoveable_v =
                                              IsBitwiseMoveable<t_TYPE>::value;
#endif
 
/// Function types and reference types are not object types, and so are not
/// bitwise-movable.  Both categories of types must be explicitly handled by
/// a distinct template specialization to avoid attempting to instantiate
/// invalid code, such as `sizeof(FUNCTION_TYPE)`, when computing the trait
/// result for other (object) types.
template <class t_TYPE,
          bool = bsl::is_reference<t_TYPE>::value ||
                 bsl::is_function<t_TYPE>::value>
struct IsBitwiseMoveable_Imp : bsl::false_type {
};
 
/// Core implementation of the `IsBitwiseMoveable` trait.  A class is
/// detected as being bitwise moveable iff it is trivially copyable or it
/// has a nested trait declaration for the `IsBitwiseMoveable` trait.  In
/// C++03 however, detection of trivially copyable classes is imperfect and
/// depends on programmer intervention.  As many empty classes (including
/// standard classes like `std::less<T>` would not be detected as being
/// trivially copyable and, therefore, bitwise moveable, a heuristic is put
/// in place whereby any type of one byte size is assumed to be bitwise
/// moveable.  See component-level documentation for this component for more
/// details on this heuristic and how to avoid false positives.
template <class t_TYPE>
struct IsBitwiseMoveable_Imp<t_TYPE, false> {
 
  private:
    static const bool k_NestedBitwiseMoveableTrait =
                           DetectNestedTrait<t_TYPE, IsBitwiseMoveable>::value;
 
  public:
    static const bool value = IsBitwiseCopyable<t_TYPE>::value
                                                || k_NestedBitwiseMoveableTrait
                                                || sizeof(t_TYPE) == 1;
 
    typedef bsl::integral_constant<bool, value> type;
 
#if defined(BSLS_COMPILERFEATURES_SUPPORT_STATIC_ASSERT) && \
    defined(BSLMF_ISTRIVIALLYCOPYABLE_NATIVE_IMPLEMENTATION)
    // In C++11 and beyond, we can accurately detect trivial-copiable types
    // which would allow us to remove the one-byte heuristic used above.
    // Testing with gcc-5 on Bloomberg production software indicates that there
    // are many 1-byte types, which are conceptually bitwise moveable, that are
    // not correctly marked bitwise moveable (for example, any 1-byte
    // code-generated type).  For the moment we have decided not to enable more
    // conservative logic for automatically deducing the
    // 'IsBitwiseMoveableTrait'.  The more conservative logic is shown below
    // for future reference:
 
    static_assert(!bsl::is_reference<t_TYPE>::value,
                  "This imp-detail instantiation should not be selected for "
                  "reference types");
    static_assert(!bsl::is_function<t_TYPE>::value,
                  "This imp-detail instantiation should not be selected for "
                  "function types");
 
    static const bool k_ValueWithoutOnebyteHeuristic =
                        bsl::is_trivially_copyable<t_TYPE>::value ||
                        std::is_empty<t_TYPE>::value  // required for gcc < 5.0
                        || k_NestedBitwiseMoveableTrait;
#endif
};
 
                        // ========================
                        // struct IsBitwiseMoveable
                        // ========================
 
/// Trait metafunction that determines whether the specified parameter
/// `t_TYPE` is bitwise moveable.  If `IsBitwiseMoveable<t_TYPE>` is derived
/// from `bsl::true_type` then `t_TYPE` is bitwise moveable.  Otherwise,
/// bitwise moveability cannot be inferred for `t_TYPE`.  This trait can be
/// associated with a bitwise moveable user-defined class by specializing
/// this class or by using the `BSLMF_NESTED_TRAIT_DECLARATION` macro.
template <class t_TYPE>
struct IsBitwiseMoveable : IsBitwiseMoveable_Imp<t_TYPE>::type {
};
 
/// Traits metafunction explicit specialization to indicate that the type `void` is not bitwise movable.  
/// 
/// \note Note that   this specialization is used
/// as a simpler option than making the compile-time logic of the
/// `IsBitwiseMoveable_Imp` template robust for `void` types.
template <>
struct IsBitwiseMoveable<void> : bsl::false_type {
};
 
/// Trait metafunction that determines whether the specified parameter
/// `t_TYPE` is bitwise moveable by stripping off the `const` qualifier and
/// forwarding to the base-case of `IsBitwiseMoveable`.
template <class t_TYPE>
struct IsBitwiseMoveable<const t_TYPE> : IsBitwiseMoveable<t_TYPE>::type {
};
 
/// Trait metafunction that determines whether the specified parameter
/// `t_TYPE` is bitwise moveable by stripping off the `volatile` qualifier
/// and forwarding to the base-case of `IsBitwiseMoveable`.
template <class t_TYPE>
struct IsBitwiseMoveable<volatile t_TYPE> : IsBitwiseMoveable<t_TYPE>::type {
};
 
/// Trait metafunction that determines whether the specified parameter
/// `t_TYPE` is bitwise moveable by stripping off the `const` and `volatile`
/// qualifiers and forwarding to the base-case of `IsBitwiseMoveable`.
template <class t_TYPE>
struct IsBitwiseMoveable<const volatile t_TYPE>
: IsBitwiseMoveable<t_TYPE>::type {
};
 
template <class t_TYPE, size_t t_LEN>
struct IsBitwiseMoveable<t_TYPE[t_LEN]> : IsBitwiseMoveable<t_TYPE>::type {
};
 
template <class t_TYPE, size_t t_LEN>
struct IsBitwiseMoveable<const t_TYPE[t_LEN]>
: IsBitwiseMoveable<t_TYPE>::type {
};
 
template <class t_TYPE, size_t t_LEN>
struct IsBitwiseMoveable<volatile t_TYPE[t_LEN]>
: IsBitwiseMoveable<t_TYPE>::type {
};
 
template <class t_TYPE, size_t t_LEN>
struct IsBitwiseMoveable<const volatile t_TYPE[t_LEN]>
: IsBitwiseMoveable<t_TYPE>::type {
};
 
#if !defined(BSLMF_ISBITWISEMOVEABLE_NO_SUPPORT_FOR_ARRAY_OF_UNKNOWN_BOUND)
template <class t_TYPE>
struct IsBitwiseMoveable<t_TYPE[]> : IsBitwiseMoveable<t_TYPE>::type {
};
 
template <class t_TYPE>
struct IsBitwiseMoveable<const t_TYPE[]> : IsBitwiseMoveable<t_TYPE>::type {
};
 
template <class t_TYPE>
struct IsBitwiseMoveable<volatile t_TYPE[]> : IsBitwiseMoveable<t_TYPE>::type {
};
 
template <class t_TYPE>
struct IsBitwiseMoveable<const volatile t_TYPE[]>
: IsBitwiseMoveable<t_TYPE>::type {
};
#endif // BSLMF_ISBITWISEMOVEABLE_NO_SUPPORT_FOR_ARRAY_OF_UNKNOWN_BOUND
 
}  // close package namespace
 
 
 
#endif // ! defined(INCLUDED_BSLMF_ISBITWISEMOVEABLE)
 
// ----------------------------------------------------------------------------
// Copyright 2013 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 ----------------------------------
 
/** @} */
/** @} */
/** @} */