src/gromacs/utility/allocator.h

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  35 /*! \file
  36  * \brief
  37  * Declares gmx::Allocator template whose allocation functionality is
  38  * configured both by type of object allocated and a policy class that
  39  * configures the necessary matching malloc and free operation.
  40  *
  41  * \author Erik Lindahl <erik.lindahl@gmail.com>
  42  * \author Mark Abraham <mark.j.abraham@gmail.com>
  43  * \inpublicapi
  44  * \ingroup module_utility
  45  */
  46 #ifndef GMX_UTILITY_ALLOCATOR_H
  47 #define GMX_UTILITY_ALLOCATOR_H
  48
  49 #include <cstddef>
  50
  51 #include <memory>
  52 #include <new>
  53
  54 #include "gromacs/utility/basedefinitions.h"
  55
  56 namespace gmx
  57 {
  58
  59 /*! \libinternal \brief Policy-based memory allocator.
  60  *
  61  *  \tparam T                 Type of objects to allocate
  62  *  \tparam AllocationPolicy  Policy of (matching) allocation and deallocation functions.
  63  *
  64  * This class can be used for the optional allocator template
  65  * parameter in standard library containers. It must be configured
  66  * with both the type of object to allocate, and an AllocationPolicy
  67  * which effectively wraps a matching pair of malloc and free
  68  * functions. This permits implementing a family of related allocators
  69  * e.g. with SIMD alignment, GPU host-side page locking, or perhaps
  70  * both, in a way that preserves a common programming interface and
  71  * duplicates minimal code.
  72  *
  73  * AllocationPolicy is used as a base class, so that if
  74  * AllocationPolicy is stateless, then the empty base optimization
  75  * will ensure that Allocation is also stateless, and objects made
  76  * with the Allocator will incur no size penalty. (Embedding an
  77  * AllocationPolicy object incurs a size penalty always, even if the
  78  * object is empty.) Normally a stateless allocator will be used.
  79  *
  80  * However, an AllocationPolicy with state might be desirable for
  81  * simplifying writing code that needs to allocate suitably for a
  82  * transfer to a GPU. That code needs to specify an Allocator that can
  83  * do the right job, which can be stateless. However, if we have code
  84  * that will not know until run time whether a GPU transfer will
  85  * occur, then the allocator needs to be aware of the state.  That
  86  * will increase the size of a container that uses the stateful
  87  * allocator.
  88  *
  89  * \throws std::bad_alloc Instead of a GROMACS exception object, we
  90  * throw the standard one on allocation failures to make it as
  91  * compatible as possible with the errors expected by code using the
  92  * standard library containers.
  93  *
  94  * \inlibraryapi
  95  * \ingroup module_utility
  96  */
  97 template <class T, typename AllocationPolicy>
  98 class Allocator : public AllocationPolicy
  99 {
 100     public:
 101         // The standard library specification for a custom allocator
 102         // requires this typedef, with this capitalization/underscoring.
 103         typedef T              value_type;      //!< Type of allocated elements
 104
 105         /*! \brief Constructor
 106          *
 107          * No constructor can be auto-generated in the presence of any
 108          * user-defined constructor, but we want the default constructor.
 109          */
 110         Allocator() = default;
 111
 112         /*! \brief Constructor to accept an AllocationPolicy.
 113          *
 114          * This is useful for AllocationPolicies with state.
 115          */
 116         Allocator(const AllocationPolicy &p) : AllocationPolicy(p) {}
 117
 118         /*! \brief Do the actual memory allocation
 119          *
 120          *  \param n    Number of elements of type T to allocate. n can be
 121          *              0 bytes, which will return a non-null properly aligned
 122          *              and padded pointer that should not be used.
 123          *  \param hint Optional value returned from previous call to allocate.
 124          *              For now this is not used.
 125          *  \return Pointer to allocated memory
 126          *
 127          *  \throws std::bad_alloc if the allocation fails.
 128          */
 129         value_type*
 130         allocate(std::size_t n, typename std::allocator<void>::const_pointer gmx_unused hint = nullptr)
 131         {
 132             void *p = AllocationPolicy::malloc(n*sizeof(T));
 133
 134             if (p == nullptr)
 135             {
 136                 throw std::bad_alloc();
 137             }
 138             else
 139             {
 140                 return static_cast<value_type*>(p);
 141             }
 142         }
 143
 144         /*! \brief Release memory
 145          *
 146          * \param p  Pointer to previously allocated memory returned from allocate()
 147          * \param n  number of objects previously passed to allocate()
 148          */
 149         void
 150         deallocate(value_type* p, std::size_t gmx_unused n)
 151         {
 152             AllocationPolicy::free(p);
 153         }
 154
 155         /*! \brief Return true if two allocators are identical
 156          *
 157          * This is a member function of the left-hand-side allocator.
 158          * Always true for stateless polcies. Has to be defined in the policy for stateful policies.
 159          * FUTURE: Can be removed with C++17 (is_always_equal)
 160          */
 161         template<class T2, class A = AllocationPolicy,
 162                  typename          = typename std::enable_if<std::is_empty<A>::value>::type>
 163         bool operator==(const Allocator<T2, AllocationPolicy> & /*unused*/) const { return true; }
 164
 165         /*! \brief Return true if two allocators are different
 166          *
 167          * \param rhs Other allocator.
 168          *
 169          * This is a member function of the left-hand-side allocator.
 170          */
 171         template<class T2>
 172         bool operator!=(const Allocator<T2, AllocationPolicy> &rhs) const { return !(*this == rhs); }
 173 };
 174
 175 }      // namespace gmx
 176
 177 #endif