C++:使用 clang 编译以迭代器作为值的映射时出现巨大错误

Question

我正在尝试将迭代器用作 std::map 中的值，以便我可以通过对象的 id 高效地查找对象或通过其有效地迭代对象深度。

考虑以下代码:

#include <string>
#include <algorithm>
#include <array>
#include <iostream>
#include <random>
#include <map>
#include <unordered_map>

struct object {
    static int next_id;
    int id;
    int depth;
    std::map<int, object>::iterator id_it;
    std::multimap<int, std::map<int, object>::iterator>::iterator depth_it;
    static std::map<int, object> by_id;
    static std::multimap<int, std::map<int, object>::iterator> by_depth;
    object() = delete;
    object(object const &) = delete;
    object(object &&) = delete;
    object & operator=(object const &) = delete;
    object && operator=(object &&) = delete;
    ~object() = default;
    object(int id) : id(id), depth(rand()) {}
    void init(std::map<int, object>::iterator it) {
        id_it = it;
        depth_it = by_depth.emplace(depth, id_it);
    }
    static int create() {
        auto it = by_id.emplace(next_id, next_id).first;
        it->second.init(it);
        ++next_id;
        return it->second.id;
    }
    static void set_depth(int o, int d) {
        auto it = by_id.find(o);
        if (it == by_id.end())
            return;
        auto & obj = it->second;
        obj.depth = d;
        by_depth.erase(obj.depth_it);
        obj.depth_it = by_depth.emplace(obj.depth, obj.id_it);
    }
    static int get_depth(int o) {
        auto it = by_id.find(o);
        if (it == by_id.end())
            return 0;
        return it->second.depth;
    }
    static int get_id(int o) {
        auto it = by_id.find(o);
        if (it == by_id.end())
            return 0;
        return it->second.id;
    }
    static void destroy(int o) {
        auto it = by_id.find(o);
        if (it == by_id.end())
            return;
        by_depth.erase(it->second.depth_it);
        by_id.erase(it->first);
    }
    template <typename T>
    static void with(T f) {
        for (auto it : by_depth) {
            f(it.second->second.id);
        }
    }
};
int object::next_id = 10000;
std::map<int, object> object::by_id;
std::multimap<int, std::map<int, object>::iterator> object::by_depth;


int main() {
    for (int i = 0; i < 100; ++i) {
        object::create();
    }
    for (int i = 0; i < 100; ++i) {
        object::set_depth(rand() % 100 + 10000, rand());
    }
    object::with([](int obj) {
        std::cout << object::get_id(obj) << "->" << object::get_depth(obj) << std::endl;
    });
    for (int i = 0; i < 100; ++i) {
        object::destroy(10000 + i);
    }
    object::with([](int obj) {
        std::cout << "Object was not deleted!" << std::endl;
    });
}

Clang 发出巨大错误，但 MSVC 在 STRICT 模式下可以很好地编译它。

这里的错误:

In file included from error.cpp:1:
In file included from /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/c++/v1/string:434:
In file included from /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/c++/v1/algorithm:593:
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/c++/v1/utility:221:9: error: 
      field has incomplete type 'object'
    _T2 second;
        ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/c++/v1/__tree:602:16: note: 
      in instantiation of template class 'std::__1::pair<int, object>' requested here
    value_type __value_;
               ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/c++/v1/__tree:624:22: note: 
      in instantiation of template class 'std::__1::__tree_node<std::__1::pair<int, object>,
      void *>' requested here
    typedef typename __node::base                                 __node_base;
                     ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/c++/v1/map:541:19: note: 
      in instantiation of template class 'std::__1::__tree_iterator<std::__1::pair<int, object>,
      std::__1::__tree_node<std::__1::pair<int, object>, void *> *, long>' requested here
    _TreeIterator __i_;
                  ^
error.cpp:13:37: note: in instantiation of template class
      'std::__1::__map_iterator<std::__1::__tree_iterator<std::__1::pair<int, object>,
      std::__1::__tree_node<std::__1::pair<int, object>, void *> *, long> >' requested here
    std::map<int, object>::iterator id_it;
                                    ^
error.cpp:9:8: note: definition of 'object' is not complete until the closing '}'
struct object {
       ^
In file included from error.cpp:6:
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/c++/v1/map:834:44: error: 
      no viable conversion from 'iterator' (aka '__tree_iterator<value_type, __node_pointer,
      difference_type>') to 'iterator' (aka '__map_iterator<typename __base::iterator>')
          iterator end() _NOEXCEPT {return __tree_.end();}
                                           ^~~~~~~~~~~~~
error.cpp:36:25: note: in instantiation of member function 'std::__1::map<int, object,
      std::__1::less<int>, std::__1::allocator<std::__1::pair<const int, object> > >::end'
      requested here
        if (it == by_id.end())
                        ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/c++/v1/map:539:24: note: 
      candidate constructor (the implicit copy constructor) not viable: no known conversion from
      'iterator' (aka '__tree_iterator<value_type, __node_pointer, difference_type>') to 'const
      std::__1::__map_iterator<std::__1::__tree_iterator<std::__1::pair<int, object>,
      std::__1::__tree_node<std::__1::pair<int, object>, void *> *, long> > &' for 1st argument
class _LIBCPP_TYPE_VIS __map_iterator
                       ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/c++/v1/map:539:24: note: 
      candidate constructor (the implicit move constructor) not viable: no known conversion from
      'iterator' (aka '__tree_iterator<value_type, __node_pointer, difference_type>') to
      'std::__1::__map_iterator<std::__1::__tree_iterator<std::__1::pair<int, object>,
      std::__1::__tree_node<std::__1::pair<int, object>, void *> *, long> > &&' for 1st argument
class _LIBCPP_TYPE_VIS __map_iterator
                       ^
In file included from error.cpp:6:
In file included from /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/c++/v1/map:371:
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/c++/v1/__tree:1968:19: error: 
      static_cast from '__node_pointer' (aka 'std::__1::__tree_node<std::__1::pair<int, object>,
      void *> *') to '__node_base_pointer' (aka 'std::__1::__tree_node_base<void *> *') is not
      allowed
                  static_cast<__node_base_pointer>(__np));
                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/c++/v1/__tree:1990:5: note: 
      in instantiation of member function 'std::__1::__tree<std::__1::pair<int, object>,
      std::__1::__map_value_compare<int, object, std::__1::less<int>, true>,
      std::__1::allocator<std::__1::pair<int, object> > >::erase' requested here
    erase(__i);
    ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/c++/v1/map:937:25: note: 
      in instantiation of function template specialization 'std::__1::__tree<std::__1::pair<int,
      object>, std::__1::__map_value_compare<int, object, std::__1::less<int>, true>,
      std::__1::allocator<std::__1::pair<int, object> > >::__erase_unique<int>' requested here
        {return __tree_.__erase_unique(__k);}
                        ^
error.cpp:60:15: note: in instantiation of member function 'std::__1::map<int, object,
      std::__1::less<int>, std::__1::allocator<std::__1::pair<const int, object> > >::erase'
      requested here
        by_id.erase(it->first);
              ^
In file included from error.cpp:6:
In file included from /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/c++/v1/map:371:
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/c++/v1/__tree:650:59: error: 
      static_cast from '__node_pointer' (aka 'std::__1::__tree_node<std::__1::pair<int, object>,
      void *> *') to '__node_base_pointer' (aka 'int') is not allowed
  ...= static_cast<__node_pointer>(__tree_next(static_cast<__node_base_pointer>(__ptr_)));
                                               ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/c++/v1/__tree:1961:5: note: 
      in instantiation of member function 'std::__1::__tree_iterator<std::__1::pair<int,
      object>, std::__1::__tree_node<std::__1::pair<int, object>, void *> *, long>::operator++'
      requested here
    ++__r;
    ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/c++/v1/__tree:1990:5: note: 
      in instantiation of member function 'std::__1::__tree<std::__1::pair<int, object>,
      std::__1::__map_value_compare<int, object, std::__1::less<int>, true>,
      std::__1::allocator<std::__1::pair<int, object> > >::erase' requested here
    erase(__i);
    ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/c++/v1/map:937:25: note: 
      in instantiation of function template specialization 'std::__1::__tree<std::__1::pair<int,
      object>, std::__1::__map_value_compare<int, object, std::__1::less<int>, true>,
      std::__1::allocator<std::__1::pair<int, object> > >::__erase_unique<int>' requested here
        {return __tree_.__erase_unique(__k);}
                        ^
error.cpp:60:15: note: in instantiation of member function 'std::__1::map<int, object,
      std::__1::less<int>, std::__1::allocator<std::__1::pair<const int, object> > >::erase'
      requested here
        by_id.erase(it->first);
              ^
In file included from error.cpp:6:
In file included from /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/c++/v1/map:371:
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/c++/v1/__tree:650:19: error: 
      cannot cast from type 'int' to pointer type '__node_pointer' (aka
      'std::__1::__tree_node<std::__1::pair<int, object>, void *> *')
  ...= static_cast<__node_pointer>(__tree_next(static_cast<__node_base_pointer>(__ptr_)));
       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/c++/v1/__tree:156:14: error: 
      member reference type 'int' is not a pointer
    if (__x->__right_ != nullptr)
        ~~~  ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/c++/v1/__tree:650:47: note: 
      in instantiation of function template specialization 'std::__1::__tree_next<int>'
      requested here
        {__ptr_ = static_cast<__node_pointer>(__tree_next(static_cast<__node_base_pointe...
                                              ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/c++/v1/__tree:1961:5: note: 
      in instantiation of member function 'std::__1::__tree_iterator<std::__1::pair<int,
      object>, std::__1::__tree_node<std::__1::pair<int, object>, void *> *, long>::operator++'
      requested here
    ++__r;
    ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/c++/v1/__tree:1990:5: note: 
      in instantiation of member function 'std::__1::__tree<std::__1::pair<int, object>,
      std::__1::__map_value_compare<int, object, std::__1::less<int>, true>,
      std::__1::allocator<std::__1::pair<int, object> > >::erase' requested here
    erase(__i);
    ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/c++/v1/map:937:25: note: 
      in instantiation of function template specialization 'std::__1::__tree<std::__1::pair<int,
      object>, std::__1::__map_value_compare<int, object, std::__1::less<int>, true>,
      std::__1::allocator<std::__1::pair<int, object> > >::__erase_unique<int>' requested here
        {return __tree_.__erase_unique(__k);}
                        ^
error.cpp:60:15: note: in instantiation of member function 'std::__1::map<int, object,
      std::__1::less<int>, std::__1::allocator<std::__1::pair<const int, object> > >::erase'
      requested here
        by_id.erase(it->first);
              ^
In file included from error.cpp:6:
In file included from /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/c++/v1/map:371:
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/c++/v1/__tree:159:20: error: 
      member reference type 'int' is not a pointer
        __x = __x->__parent_;
              ~~~  ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/c++/v1/__tree:160:17: error: 
      member reference type 'int' is not a pointer
    return __x->__parent_;
           ~~~  ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/c++/v1/__tree:67:24: error: 
      member reference type 'int' is not a pointer
    return __x == __x->__parent_->__left_;
                  ~~~  ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/c++/v1/__tree:158:13: note: 
      in instantiation of function template specialization 'std::__1::__tree_is_left_child<int>'
      requested here
    while (!__tree_is_left_child(__x))
            ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/c++/v1/__tree:650:47: note: 
      in instantiation of function template specialization 'std::__1::__tree_next<int>'
      requested here
        {__ptr_ = static_cast<__node_pointer>(__tree_next(static_cast<__node_base_pointe...
                                              ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/c++/v1/__tree:1961:5: note: 
      in instantiation of member function 'std::__1::__tree_iterator<std::__1::pair<int,
      object>, std::__1::__tree_node<std::__1::pair<int, object>, void *> *, long>::operator++'
      requested here
    ++__r;
    ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/c++/v1/__tree:1990:5: note: 
      in instantiation of member function 'std::__1::__tree<std::__1::pair<int, object>,
      std::__1::__map_value_compare<int, object, std::__1::less<int>, true>,
      std::__1::allocator<std::__1::pair<int, object> > >::erase' requested here
    erase(__i);
    ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/c++/v1/map:937:25: note: 
      in instantiation of function template specialization 'std::__1::__tree<std::__1::pair<int,
      object>, std::__1::__map_value_compare<int, object, std::__1::less<int>, true>,
      std::__1::allocator<std::__1::pair<int, object> > >::__erase_unique<int>' requested here
        {return __tree_.__erase_unique(__k);}
                        ^
error.cpp:60:15: note: in instantiation of member function 'std::__1::map<int, object,
      std::__1::less<int>, std::__1::allocator<std::__1::pair<const int, object> > >::erase'
      requested here
        by_id.erase(it->first);
              ^
In file included from error.cpp:6:
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/c++/v1/map:1586:44: error: 
      no viable conversion from 'iterator' (aka '__tree_iterator<value_type, __node_pointer,
      difference_type>') to 'iterator' (aka '__map_iterator<typename __base::iterator>')
          iterator end() _NOEXCEPT {return __tree_.end();}
                                           ^~~~~~~~~~~~~
error.cpp:64:22: note: in instantiation of member function 'std::__1::multimap<int,
      std::__1::__map_iterator<std::__1::__tree_iterator<std::__1::pair<int, object>,
      std::__1::__tree_node<std::__1::pair<int, object>, void *> *, long> >,
      std::__1::less<int>, std::__1::allocator<std::__1::pair<const int,
      std::__1::__map_iterator<std::__1::__tree_iterator<std::__1::pair<int, object>,
      std::__1::__tree_node<std::__1::pair<int, object>, void *> *, long> > > > >::end'
      requested here
        for (auto it : by_depth) {
                     ^
error.cpp:81:5: note: in instantiation of function template specialization 'object::with<<lambda
      at error.cpp:81:18> >' requested here
    object::with([](int obj) {
    ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/c++/v1/map:539:24: note: 
      candidate constructor (the implicit copy constructor) not viable: no known conversion from
      'iterator' (aka '__tree_iterator<value_type, __node_pointer, difference_type>') to 'const
      std::__1::__map_iterator<std::__1::__tree_iterator<std::__1::pair<int,
      std::__1::__map_iterator<std::__1::__tree_iterator<std::__1::pair<int, object>,
      std::__1::__tree_node<std::__1::pair<int, object>, void *> *, long> > >,
      std::__1::__tree_node<std::__1::pair<int,
      std::__1::__map_iterator<std::__1::__tree_iterator<std::__1::pair<int, object>,
      std::__1::__tree_node<std::__1::pair<int, object>, void *> *, long> > >, void *> *, long>
      > &' for 1st argument
class _LIBCPP_TYPE_VIS __map_iterator
                       ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/c++/v1/map:539:24: note: 
      candidate constructor (the implicit move constructor) not viable: no known conversion from
      'iterator' (aka '__tree_iterator<value_type, __node_pointer, difference_type>') to
      'std::__1::__map_iterator<std::__1::__tree_iterator<std::__1::pair<int,
      std::__1::__map_iterator<std::__1::__tree_iterator<std::__1::pair<int, object>,
      std::__1::__tree_node<std::__1::pair<int, object>, void *> *, long> > >,
      std::__1::__tree_node<std::__1::pair<int,
      std::__1::__map_iterator<std::__1::__tree_iterator<std::__1::pair<int, object>,
      std::__1::__tree_node<std::__1::pair<int, object>, void *> *, long> > >, void *> *, long>
      > &&' for 1st argument
class _LIBCPP_TYPE_VIS __map_iterator
                       ^
10 errors generated.

调用:g++ -std=c++11 file.cpp 其中 g++ 是 clang。

这是怎么回事？

Answer 1

clang 给你这些错误是正确的。我可以帮助解释它们的含义:

1) STL 容器不允许不完整的类型。参见 map with incomplete value type .这就是其中一条错误消息试图告诉您的内容:“object”的定义在结束“}”之前未完成。

2) 您删除移动构造函数和复制构造函数:

object(object const &) = delete;
object(object &&) = delete;

当两者都被删除时，您的使用变得非常有限，这就是为什么您会收到以下错误:候选构造函数(隐式复制构造函数)不可行和候选构造函数(隐式移动构造函数) ) 不可行。