src/gromacs/utility/arrayref.h

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  35 /*! \file
  36  * \brief
  37  * Declares gmx::ArrayRef
  38  *
  39  * \author Teemu Murtola <teemu.murtola@gmail.com>
  40  * \inpublicapi
  41  * \ingroup module_utility
  42  */
  43 #ifndef GMX_UTILITY_ARRAYREF_H
  44 #define GMX_UTILITY_ARRAYREF_H
  45
  46 #include <cstddef>
  47
  48 #include <array>
  49 #include <iterator>
  50 #include <stdexcept>
  51 #include <utility>
  52 #include <vector>
  53
  54 #include "gromacs/utility/gmxassert.h"
  55
  56 namespace gmx
  57 {
  58
  59 /*! \brief
  60  * Tag type to initialize empty array references.
  61  *
  62  * This type (together with appropriate constructors in ArrayRef)
  63  * allows initializing any array reference to an empty value
  64  * without explicitly specifying its type.  This is convenient when calling
  65  * a function that takes an array reference, where constructing an empty
  66  * reference explicitly would otherwise require specifying the full array
  67  * reference type, including the template parameter.
  68  */
  69 struct EmptyArrayRef {};
  70
  71 /*! \brief STL-like container for an interface to a C array of T (or part
  72  * of a std::vector<T, A> or std::array<T>).
  73  *
  74  * \tparam T  Value type of elements.
  75  *
  76  * This class provides an interface similar to \c std::vector<T, A>, with the
  77  * following main differences:
  78  *  - This class does not have its own storage.  Instead, it references an
  79  *    existing array of values (either a C-style array or part of an existing
  80  *    std::vector<T, A> or std::array<T>).
  81  *  - It is only possible to modify the values themselves through ArrayRef;
  82  *    it is not possible to add or remove values.
  83  *  - Copying objects of this type is cheap, and the copies behave identically
  84  *    to the original object: the copy references the same set of values.
  85  *
  86  * This class is useful for writing wrappers that expose a view of the
  87  * internal data stored as a single vector/array, which can be a whole
  88  * or part of the underlying storage.
  89  *
  90  * Methods in this class do not throw, except where indicated.
  91  *
  92  * Note that due to a Doxygen limitation, the constructor that takes a C array
  93  * whose size is known at compile time does not appear in the documentation.
  94  *
  95  * To refer to const data of type T, ArrayRef<const T> is used. For both const
  96  * and non-const std::vector and std::array an ArrayRef view can be created.
  97  * Attempting to create a non-const ArrayRef of a const vector/array will result
  98  * in a compiler error in the respective constructor.
  99  *
 100  * For SIMD types there is template specialization available
 101  * (e.g. ArrayRef<SimdReal>) in gromacs/simd/simd_memory.h which should have
 102  * the same functionality as much as possible.
 103  *
 104  * \todo
 105  * This class is not complete. There are likely also methods missing (not
 106  * required for current usage).
 107  *
 108  * \inpublicapi
 109  * \ingroup module_utility
 110  */
 111 template <typename T>
 112 class ArrayRef
 113 {
 114     public:
 115         //! Type of values stored in the container.
 116         typedef T         value_type;
 117         //! Type for representing size of the container.
 118         typedef size_t    size_type;
 119         //! Type for representing difference between two container indices.
 120         typedef ptrdiff_t difference_type;
 121         //! Const reference to a container element.
 122         typedef const T  &const_reference;
 123         //! Const pointer to a container element.
 124         typedef const T  *const_pointer;
 125         //! Const iterator type for the container.
 126         typedef const T  *const_iterator;
 127         //! Reference to a container element.
 128         typedef T        &reference;
 129         //! Pointer to a container element.
 130         typedef T        *pointer;
 131         //! Iterator type for the container.
 132         typedef T        *iterator;
 133         //! Standard reverse iterator.
 134         typedef std::reverse_iterator<iterator>       reverse_iterator;
 135         //! Standard reverse iterator.
 136         typedef std::reverse_iterator<const_iterator> const_reverse_iterator;
 137
 138         /*! \brief
 139          * Constructs an empty reference.
 140          */
 141         ArrayRef() : begin_(NULL), end_(NULL) {}
 142         /*! \brief
 143          * Constructs an empty reference.
 144          *
 145          * This is provided for convenience, such that EmptyArrayRef can be
 146          * used to initialize any ArrayRef, without specifying the template
 147          * type.  It is not explicit to enable that usage.
 148          */
 149         ArrayRef(const EmptyArrayRef &) : begin_(nullptr), end_(nullptr) {}
 150         /*! \brief
 151          * Constructs a reference to a container or reference
 152          *
 153          * \param[in] o container to reference.
 154          *
 155          * Can be used to create a reference to a whole vector, std::array or
 156          * an ArrayRef. The destination has to have a convertible pointer type
 157          * (identical besides const or base class).
 158          *
 159          * Passed container must remain valid and not be reallocated for the
 160          * lifetime of this object.
 161          *
 162          * This constructor is not explicit to allow directly passing
 163          * a container to a method that takes ArrayRef.
 164          */
 165         template<typename U,
 166                  typename = typename std::enable_if<
 167                          std::is_convertible<typename std::remove_reference<U>::type::pointer,
 168                                              pointer>::value>::type>
 169         ArrayRef(U &&o) : begin_(o.data()), end_(o.data()+o.size()) {}
 170         /*! \brief
 171          * Constructs a reference to a particular range.
 172          *
 173          * \param[in] begin  Pointer to the beginning of a range.
 174          * \param[in] end    Pointer to the end of a range.
 175          *
 176          * Passed pointers must remain valid for the lifetime of this object.
 177          */
 178         ArrayRef(pointer begin, pointer end)
 179             : begin_(begin), end_(end)
 180         {
 181             GMX_ASSERT(end >= begin, "Invalid range");
 182         }
 183         //! \cond
 184         // Doxygen 1.8.5 doesn't parse the declaration correctly...
 185         /*! \brief
 186          * Constructs a reference to a C array.
 187          *
 188          * \param[in] array  C array to reference.
 189          * \tparam    count  Deduced number of elements in \p array.
 190          *
 191          * This constructor can only be used with a real array (not with a
 192          * pointer).  It constructs a reference to the whole array, without
 193          * a need to pass the number of elements explicitly.  The compiler
 194          * must be able to deduce the array size.
 195          *
 196          * Passed array must remain valid for the lifetime of this object.
 197          *
 198          * This constructor is not explicit to allow directly passing
 199          * a C array to a function that takes an ArrayRef parameter.
 200          */
 201         template <size_t count>
 202         ArrayRef(value_type (&array)[count])
 203             : begin_(array), end_(array + count)
 204         {
 205         }
 206         //! \endcond
 207
 208         //! Returns a reference to part of the container.
 209         ArrayRef subArray(size_type start, size_type count) const
 210         {
 211             return {begin_+start, begin_+start+count};
 212         }
 213         //! Returns an iterator to the beginning of the container.
 214         iterator begin() const { return begin_; }
 215         //! Returns an iterator to the end of the container.
 216         iterator end() const { return end_; }
 217         //! Returns an iterator to the reverse beginning of the container.
 218         reverse_iterator rbegin() const { return reverse_iterator(end()); }
 219         //! Returns an iterator to the reverse end of the container.
 220         reverse_iterator rend() const { return reverse_iterator(begin()); }
 221
 222         //! Returns the size of the container.
 223         size_type size() const { return end_ - begin_; }
 224         //! Identical to size().
 225         size_type capacity() const { return end_ - begin_; }
 226         //! Whether the container is empty.
 227         bool empty() const { return begin_ == end_; }
 228
 229         //! Access container element.
 230         reference operator[](size_type n) const { return begin_[n]; }
 231         //! Access container element (throws on out-of-range error).
 232         reference at(size_type n) const
 233         {
 234             if (n >= size())
 235             {
 236                 throw std::out_of_range("Vector index out of range");
 237             }
 238             return begin_[n];
 239         }
 240         //! Returns the first element in the container.
 241         reference front() const { return *begin_; }
 242         //! Returns the first element in the container.
 243         reference back() const { return *(end_ - 1); }
 244
 245         //! Returns a raw pointer to the contents of the array.
 246         pointer data() const { return begin_; }
 247
 248         /*! \brief
 249          * Swaps referenced memory with the other object.
 250          *
 251          * The actual memory areas are not modified, only the references are
 252          * swapped.
 253          */
 254         void swap(ArrayRef<T> &other)
 255         {
 256             std::swap(begin_, other.begin_);
 257             std::swap(end_, other.end_);
 258         }
 259
 260     private:
 261         pointer           begin_;
 262         pointer           end_;
 263 };
 264
 265 //! \copydoc ArrayRef::fromArray()
 266 //! \related ArrayRef
 267 template <typename T>
 268 ArrayRef<T> arrayRefFromArray(T *begin, size_t size)
 269 {
 270     return ArrayRef<T>(begin, begin+size);
 271 }
 272
 273 //! \copydoc ArrayRef::fromArray()
 274 //! \related ArrayRef
 275 template <typename T>
 276 ArrayRef<const T> constArrayRefFromArray(const T *begin, size_t size)
 277 {
 278     return ArrayRef<const T>(begin, begin+size);
 279 }
 280
 281 /*! \brief
 282  * Create ArrayRef from container with type deduction
 283  *
 284  * \see ArrayRef
 285  */
 286 template <typename T>
 287 ArrayRef<typename std::conditional<std::is_const<T>::value,
 288                                    const typename T::value_type,
 289                                    typename T::value_type>::type>
 290 makeArrayRef(T &c)
 291 {
 292     return c;
 293 }
 294
 295 /*! \brief
 296  * Create ArrayRef to const T from container with type deduction
 297  *
 298  * \see ArrayRef
 299  */
 300 template <typename T>
 301 ArrayRef<const typename T::value_type> makeConstArrayRef(const T &c)
 302 {
 303     return c;
 304 }
 305
 306 /*! \brief
 307  * Simple swap method for ArrayRef objects.
 308  *
 309  * \see ArrayRef::swap()
 310  *
 311  * \ingroup module_utility
 312  */
 313 template <typename T>
 314 void swap(ArrayRef<T> &a, ArrayRef<T> &b)
 315 {
 316     a.swap(b);
 317 }
 318
 319 /*! \brief Return a vector that is a copy of an ArrayRef.
 320  *
 321  * This makes it convenient, clear, and performant (the compiler will
 322  * either do RVO to elide the temporary, or invoke the move constructor
 323  * taking the unnamed temporary) to write a declaration like
 324  *
 325  *   auto v = copyOf(arrayRef);
 326  *
 327  * \ingroup module_utility
 328  */
 329 template <typename T>
 330 std::vector<T> copyOf(const ArrayRef<const T> &arrayRef)
 331 {
 332     return std::vector<T>(arrayRef.begin(), arrayRef.end());
 333 }
 334
 335 } // namespace gmx
 336
 337 #endif