在结构数组上使用 c++ std::copy

Question

我想使用 std::copy 将现有结构数组复制到新结构数组。常规选项可以使用“'local_copy()'”。我想知道在下面描述的这种情况下使用 std::copy 的程序 -

我尝试了代码并在编译时得到以下错误

#include <iostream>

class BigClass{
public:
struct Astruct{
    double x[2], v[2];
    int rank;
}one_struct;

};

void allocate(struct BigClass::Astruct& one_struct, int i)
{
one_struct.x[0] = 1.1;
one_struct.x[1] = 1.2;
one_struct.v[0] = 2.1;
one_struct.v[1] = 2.2;
one_struct.rank = i;
}

void local_copy(struct BigClass::Astruct& dest, struct BigClass::Astruct& source)
{
dest.x[0] = source.x[0];
dest.x[1] = source.x[1];
dest.v[0] = source.v[0];
dest.v[1] = source.v[1];
dest.rank = source.rank;
}

void print(struct BigClass::Astruct one_struct)
{
std::cout << one_struct.rank << " " << one_struct.x[0] << " " << one_struct.x[1] << " " << one_struct.v[0] << " " << one_struct.v[1] << "\n";
}



int main(int argc, char *argv[]) {

int size = 10;
struct BigClass::Astruct BCobj[size];
for(int i = 0; i < size; i++) allocate(BCobj[i], i);
for(int i = 0; i < size; i++) print(BCobj[i]);

struct BigClass::Astruct second_BCobj[size];
//for(int i = 0; i < size; i++) local_copy(second_BCobj[i], BCobj[i]); // this works
for(int i = 0; i < size; i++) std::copy(BCobj[i+1], BCobj[i], second_BCobj[i]); // not working

for(int i = 0; i < size; i++) print(BCobj[i]);

}

编译时错误如下-

/usr/include/c++/7/bits/stl_algobase.h:377:57: error: no type named ‘value_type’ in ‘struct std::iterator_traits<BigClass::Astruct>’
   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<BigClass::Astruct>’
   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<BigClass::Astruct>’
   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<BigClass::Astruct>’
   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<BigClass::Astruct>’
   return std::__copy_move<_IsMove, __simple,
          ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                    _Category>::__copy_m(__first, __last, __result);
                        ~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~

提供了向量和 int 数据类型的一些示例。谁能为这种代码结构详细说明 std::copy 的适当顺序？谢谢

Answer 1

std::copy 用于 而不是 for 循环，而不是一个

std::copy(BCobj, BCobj + size, second_BCobj);

与

基本相同

for(int i = 0; i < size; ++i) {
    second_BCobj[i] = BCobj[i];
}

此外，别忘了 #include <algorithm> for std::copy

std::copy 参数的解释：

std::copy takes as arguments 2 arguments of type matching InputIterator requirements and single argument of type OutputIteretor。幸运的是，指针符合这些要求，所以我们可以直接使用它们。因为数组名称被解释为指向其第一个元素的指针，我们可以将它直接作为参数传递给 std::sort

Chef Gladiator 建议的更好版本：

std::copy(std::begin(BCobj), std::end(BCobj), std::begin(second_BCobj))

Answer 2

欢迎来到 Stack Overflow。您已经提交了完整的代码。下次还请描述您正在构建的开发环境。更好的是，尝试提供与操作系统和编译器无关的标准 C++ 代码。

这是C++论坛，所以我们喜欢在这里使用标准的C++。使用 std::array. That makes it simple. Please study and enjoy the standard C++.

重写您的代码

  #include <array>
  #include <iostream>
  #include <algorithm>

  namespace stack_overflow {

      using namespace std;

      struct BigClass final {
          struct Astruct final {
              double x[2], v[2];
              int rank;
          }one_struct;

          friend ostream& operator << (ostream& os, Astruct const & one_struct)
          {
              return os << one_struct.rank << " " << one_struct.x[0] << " "
                  << one_struct.x[1] << " " << one_struct.v[0] << " "
                  << one_struct.v[1] ;
          }
      };

      constexpr int size = 10;
      using bcobj_arr = std::array<BigClass::Astruct, size>;

      void populate( bcobj_arr& bcobjects_)
      {
          int j{ 0 };
          for (auto& one_struct : bcobjects_) {
              one_struct.x[0] = 1.1;
              one_struct.x[1] = 1.2;
              one_struct.v[0] = 2.1;
              one_struct.v[1] = 2.2;
              one_struct.rank = j++;
          }
      }

      void print(const char prompt[BUFSIZ], bcobj_arr const & bcobjects_ )
      {
          cout << "\n\n" << prompt << "\n\n" ;
          for (auto& one_struct : bcobjects_) {
              cout << one_struct << "\n";
          }
      }

      bool test (int argc, const char* argv[]) 
      {
          bcobj_arr BCobj;
          populate(BCobj);
          print("BCobj", BCobj);
            // std::array instances can be copied
            bcobj_arr second_BCobj = BCobj ;
          print("second_BCobj", second_BCobj);
          return true;
      }
  }

 int main(const int argc, const char * argv[])
 {
  stack_overflow::test(argc, argv);
  return 42;
 }

代码完全没有注释。我想你有很多问题，请在下面的评论中提问。我会通过指向您在线的相关文档来尝试回答所有问题。

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