c++ - 如何在类具有 `unsigned char &operator[]` 时使用 std::copy

Question

我正在使用一个名为 Buffer 的类，它具有以下运算符:

unsigned char &operator[](size_t index)

我正在尝试像这样从中复制:

std::copy(buf[96], buf[96 + 32], my_uint32_t);

我觉得不行

/usr/include/c++/7/bits/stl_algobase.h: In instantiation of '_OI std::__copy_move_a(_II, _II, _OI) [with bool _IsMove = false; _II = unsigned char; _OI = unsigned int]':
/usr/include/c++/7/bits/stl_algobase.h:422:45:   required from '_OI std::__copy_move_a2(_II, _II, _OI) [with bool _IsMove = false; _II = unsigned char; _OI = unsigned int]'
/usr/include/c++/7/bits/stl_algobase.h:455:8:   required from '_OI std::copy(_II, _II, _OI) [with _II = unsigned char; _OI = unsigned int]'
/home/project/SimplePacketCrafter.h:36:95:   required from here
/usr/include/c++/7/bits/stl_algobase.h:377:57: error: no type named 'value_type' in 'struct std::iterator_traits<unsigned char>'
       typedef typename iterator_traits<_II>::value_type _ValueTypeI;
                                                         ^~~~~~~~~~~
/usr/include/c++/7/bits/stl_algobase.h:378:57: error: no type named 'value_type' in 'struct std::iterator_traits<unsigned int>'
       typedef typename iterator_traits<_OI>::value_type _ValueTypeO;
                                                         ^~~~~~~~~~~
/usr/include/c++/7/bits/stl_algobase.h:379:64: error: no type named 'iterator_category' in 'struct std::iterator_traits<unsigned char>'
       typedef typename iterator_traits<_II>::iterator_category _Category;
                                                                ^~~~~~~~~
/usr/include/c++/7/bits/stl_algobase.h:383:9: error: no type named 'value_type' in 'struct std::iterator_traits<unsigned char>'
       const bool __simple = (__is_trivial(_ValueTypeI)
                             ~~~~~~~~~~~~~~~~~~~~~~~~~~
                       && __is_pointer<_II>::__value
                       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                       && __is_pointer<_OI>::__value
                       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
         && __are_same<_ValueTypeI, _ValueTypeO>::__value);
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/7/bits/stl_algobase.h:386:44: error: no type named 'iterator_category' in 'struct std::iterator_traits<unsigned char>'
       return std::__copy_move<_IsMove, __simple,
              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                        _Category>::__copy_m(__first, __last, __result);

因为我认为我应该在前两个中传递一个地址。还有

unsigned char *index(size_t index)

但是还是不行

方法可以看here

Answer 1

std:copy() 采用迭代器，而不是引用。这就是您收到错误的原因。

原始指针可以用作迭代器，前提是指向的元素按顺序存储在内存中。因此，假设缓冲区的 operator[] 返回对单个内存缓冲区中元素的引用，您可以使用 & 地址运算符来获取 的内存地址char 被 operator[] 引用，然后 std::copy() 可以在这两个地址之间迭代:

std::copy(&buf[96], &buf[96 + 32], my_uint32_t);

或者，缓冲区的 index() 和 c_index() 方法返回等效指针:

std::copy(buf.index(96), buf.index(96 + 32), my_uint32_t);

std::copy(buf.c_index(96), buf.c_index(96 + 32), my_uint32_t);