我正在尝试将迭代器用作std::map中的值，以便我可以通过其id高效地查找对象，或者通过其depth有效地迭代它。
请考虑以下代码：
#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模式下编译>。
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。
发生了什么事？
答案 0 :(得分：5)
clang在给你这些错误时是正确的。我可以帮忙解释一下它们的意思：
1）STL容器不允许不完整的类型。见map with incomplete value type。
这就是其中一条错误消息试图告诉您：definition of 'object' is not complete until the closing '}'。
2）你delete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，这就是您收到以下错误的原因：candidate constructor (the implicit copy constructor) not viable和candidate constructor (the implicit move constructor) not viable。
答案 1 :(得分：3)
正如Jesse提供的链接中所述，最简单的解决方法是使用std :: unique_ptr，它具有额外堆分配的缺点，但我真的看不到更好的选择。
#include <string>
#include <algorithm>
#include <array>
#include <iostream>
#include <random>
#include <map>
#include <unordered_map>
#include <memory>
struct object {
static int next_id;
int id;
int depth;
std::map<int, std::unique_ptr<object>>::iterator id_it;
std::multimap<int, std::map<int, std::unique_ptr<object>>::iterator>::iterator depth_it;
static std::map<int, std::unique_ptr<object>> by_id;
static std::multimap<int, std::map<int, std::unique_ptr<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, std::unique_ptr<object>>::iterator it) {
id_it = it;
depth_it = by_depth.emplace(depth, id_it);
}
static int create() {
auto it = by_id.emplace(next_id, std::unique_ptr<object>(new object(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, std::unique_ptr<object>> object::by_id;
std::multimap<int, std::map<int, std::unique_ptr<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;
});
}
与Jesse声称的移动构造函数和复制构造函数相反，虽然许多容器（如std :: vector）确实需要MoveConstructible或CopyConstructible，但std :: map却没有。