c++ - 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。常规选项适用于 '''local_copy()'''。我想知道在下面描述的这种情况下使用 std::copy 的过程 -

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);
                        ~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~

有一些 vector 和整型数据类型的例子。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使用代替 for循环，而不是一个。

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

本质上与

相同

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

此外，不要忘记 #include <algorithm>对于 std::copy

---

std::copy 参数的解释:

std::copy 将 2 个匹配 InputIterator 类型的参数作为参数OutputIteretor 类型的要求和单个参数.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，所以我们可以将其作为参数直接传递给 std::sort。

---

Chef Gladiator 建议的更好版本:

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