[英]No type named 'iterator_category' in 'struct std::iterator_traits<int* const>'
I'm writing a custom vector class:我正在编写一个自定义向量 class:
#include <iterator>
template <typename T>
class vector {
public:
using value_type = T;
using pointer = value_type*;
using iterator = pointer;
using const_iterator = const iterator;
using const_reverse_iterator = std::reverse_iterator<const_iterator>;
auto crbegin() const -> const_reverse_iterator {
return const_reverse_iterator(data_);
}
private:
pointer data_{};
};
int main(int argc, char* argv[]) {
vector<int> v;
auto i = v.crbegin();
return 0;
}
When compiling the code above, I get this error (on GCC and MSVC):编译上面的代码时,我收到此错误(在 GCC 和 MSVC 上):
error: no type named ‘iterator_category’ in ‘struct std::iterator_traits<int* const>’
The error goes away when I change the reverse iterator definition to std::reverse_iterator<const T*>
.当我将反向迭代器定义更改为
std::reverse_iterator<const T*>
时,错误消失了。 What's the difference compared to std::reverse_iterator<const_iterator>
?与
std::reverse_iterator<const_iterator>
相比有什么区别?
The problem can be reduced to this:问题可以简化为:
#include <iterator>
template <typename T>
class vector {
public:
using value_type = T;
using pointer = value_type*;
using const_reverse_iterator = std::reverse_iterator<const pointer>;
auto crbegin() const -> const_reverse_iterator {
return const_reverse_iterator(data_);
}
private:
pointer data_{};
};
int main() {
vector<int> v;
auto i = v.crbegin();
(void) i;
return 0;
}
And the error message from clang makes the nature of the problem clearer:而来自 clang 的错误信息让问题的本质更加清晰:
no type named 'reference' in 'std::iterator_traits<int *const>'
^^^^^^^^^^
And so we see that what you thought was a const *int
(ie the pointed-to object is const
) is in fact a, int *const
(ie the pointer itself is const
).因此我们看到您认为的
const *int
(即指向的 object 是const
)实际上是一个int *const
(即指针本身是const
)。
Here's a fix:这是一个修复:
#include <iterator>
template <typename T>
class vector {
public:
using value_type = T;
using pointer = value_type*;
using const_pointer = const value_type*;
using const_reverse_iterator = std::reverse_iterator<const_pointer>;
auto crbegin() const -> const_reverse_iterator {
return const_reverse_iterator(data_);
}
private:
pointer data_{};
};
int main() {
vector<int> v;
auto i = v.crbegin();
(void) i;
return 0;
}
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