Question

看看链表的这种实现方式：

#include <memory>
#include <type_traits>
#include <iostream>

using namespace std;

template<typename D>
class List {
    struct Node {
        shared_ptr<D> data;
        Node* next;
        Node(shared_ptr<D> d, Node* p, Node* n) : data(d), next(n) {}
        ~Node() {
            data.reset();
            delete next;
        }
    };
    template <bool isconst = false> 
    struct iterator : public std::iterator<std::forward_iterator_tag, shared_ptr<D>> {
        typedef std::forward_iterator_tag iterator_category;
        typedef shared_ptr<D> value_type;
        typedef std::ptrdiff_t Distance;
        typedef typename conditional<isconst, const value_type&, value_type&>::type
                Reference;
        typedef typename conditional<isconst, const value_type*, value_type*>::type
                Pointer;
        typedef typename conditional<isconst, const Node*, Node*>::type
                nodeptr;
        iterator(nodeptr x = nullptr) : curr_node(x) {}
        iterator(const iterator<false>& i) : curr_node(i.curr_node) {}
        Reference operator*() const { return curr_node->data; }
        Pointer operator->() const { return &(curr_node->data); }
        template<bool A>
        friend bool operator==(const iterator<A>& a, const iterator<A>& b) {
            return a.curr_node == b.curr_node;
        }
        template<bool A>
        friend bool operator!=(const iterator<A>& a, const iterator<A>& b) {
            return !(a.curr_node == b.curr_node);
        }
        friend class List<D>;
        iterator& operator++() { 
            curr_node = curr_node->next; 
            return *this; 
        }
        private:
            nodeptr curr_node;
    };
    public:
        List() {
            head = nullptr;
        }
        int len() const {
            int ret = 0;
            for (const auto& n : *this) {
                ret++;
            }
            return ret;
        }
        ~List() {
            delete head;
        }
        std::ostream& dump(std::ostream &strm) const {
            for (const auto s : *this) {
                strm << *s << std::endl;
            }
            return strm;
        }
        iterator<false> begin() {
            return iterator<false>(head);
        }
        iterator<false> end() {
            return iterator<false>(nullptr);
        }
        iterator<true> begin() const {
            return iterator<true>(head);
        }
        iterator<true> end() const {
            return iterator<true>(nullptr);
        }
    private:
        Node* head;
};

给我带来麻烦的部分是此列表的iterator实现。迭代器模板应该提供可变的和const迭代器。

这是一个使用此实现的程序：

#include "List.h"
#include <iostream>

int main( int argc, const char *argv[] ) {
    List<int> l;

    std::cout << l.len() << std::endl;
    return 0;
}

如果我使用clang++，程序可以编译并运行良好，但是g++的编译失败，并出现以下错误：

In file included from t.cpp:1:
List.h: In instantiation of ‘struct List<int>::iterator<false>’:
List.h:136:5:   required from ‘int List<D>::len() const [with D = int]’
t.cpp:7:24:   required from here
List.h:64:21: error: redefinition of ‘template<bool A> bool operator==(const List<int>::iterator<isconst>&, const List<int>::iterator<isconst>&)’                                                
         friend bool operator==(const iterator<A>& a, const iterator<A>& b) {
                     ^~~~~~~~
List.h:64:21: note: ‘template<bool A> bool operator==(const List<int>::iterator<isconst>&, const List<int>::iterator<isconst>&)’ previously declared here                                        
List.h:69:21: error: redefinition of ‘template<bool A> bool operator!=(const List<int>::iterator<isconst>&, const List<int>::iterator<isconst>&)’                                                
         friend bool operator!=(const iterator<A>& a, const iterator<A>& b) {
                     ^~~~~~~~
List.h:69:21: note: ‘template<bool A> bool operator!=(const List<int>::iterator<isconst>&, const List<int>::iterator<isconst>&)’ previously declared here

此错误的原因是什么？我怎样才能解决这个问题？

Answer 1

问题似乎在这里：

function theScore (students) {
  const highScoresByClass = {};
  const scores = {A: 0, B: 0, C: 0};
  for (const student of students){
    const classHighScore = scores[student.class];
    if(classHighScore < student.score) {
      scores[student.class] = student.score;
      highScoresByClass[student.class] = {
        name: student.name,
        score: student.score
      };
    };
  };
  return highScoresByClass;
};


console.log(theScore([
  {
    name: 'Sara',
    score: 90,
    class: 'A'
  },
  {
    name: 'Poyi',
    score: 85,
    class: 'B'
  },
  {
    name: 'Adert',
    score: 74,
    class: 'A'
  },
  {
    name: 'Shynta',
    score: 78,
    class: 'B'
  }
]));

这是说：对于template <bool isconst = false> struct iterator : public std::iterator<std::forward_iterator_tag, shared_ptr<D>> { template<bool A> friend bool operator==(const iterator<A>& a, const iterator<A>& b) { return a.curr_node == b.curr_node; }（外部模板参数）的所有值，请定义模板函数isconst。

因此实例化template<bool A> bool operator==将定义iterator<true>，然后实例化template<bool A> bool operator==将再次定义iterator<false>，从而导致重新定义错误。

解决方案：卸下内部模板。让template<bool A> bool operator==的每个实例仅定义自己的iterator：

operator==

（template <bool isconst = false> struct iterator : public std::iterator<std::forward_iterator_tag, shared_ptr<D>> { friend bool operator==(const iterator& a, const iterator& b) { return a.curr_node == b.curr_node; }会自动引用iterator，即当前实例。）

C ++：模板化代码可使用clang ++编译并正常运行，但对于g ++则失败

1 个答案: