0
我有一个ID类模板，它需要参数T。编译问题：使用元组构建Composite_Key variadic模板类
- 如果
T具有T类型的对象上定义的key_type，ID呼叫get_key()获得用于存储的标识符。 - 如果
T没有定义key_type，则ID将使用该对象的地址作为标识符。
该代码正常工作到这一点。
现在，我想定义一个新的可变参数类模板Composite_Key，它将可变参数模板参数作为std::tuple。我试图让这个新代码与ID一起工作，但是我遇到了编译错误，我很难理解。
错误似乎表明缺少operator<()，这很奇怪，因为它在ID中定义;我不知道我做错了什么。
下面的代码，包括测试代码，直到最后一行（注释）都能正常工作。错误
In file included from sandbox.cpp:2:0:
/usr/include/c++/4.7/tuple: In instantiation of ‘static bool std::__tuple_compare<0ul, __i, __j, _Tp, _Up>::__less(const _Tp&, const _Up&) [with long unsigned int __i = 0ul; long unsigned int __j = 3ul; _Tp = std::tuple<String_Key, Int_Key, Char_Key>; _Up = std::tuple<String_Key, Int_Key, Char_Key>]’:
/usr/include/c++/4.7/tuple:814:62: required from ‘bool std::operator<(const std::tuple<_TElements ...>&, const std::tuple<_Elements ...>&) [with _TElements = {String_Key, Int_Key, Char_Key}; _UElements = {String_Key, Int_Key, Char_Key}]’
sandbox.cpp:50:59: required from ‘bool ID<T>::operator<(const ID<T>&) const [with T = Composite_Key<String_Key, Int_Key, Char_Key>; ID<T> = ID<Composite_Key<String_Key, Int_Key, Char_Key> >]’
/usr/include/c++/4.7/bits/stl_function.h:237:22: required from ‘bool std::less<_Tp>::operator()(const _Tp&, const _Tp&) const [with _Tp = ID<Composite_Key<String_Key, Int_Key, Char_Key> >]’
/usr/include/c++/4.7/bits/stl_tree.h:1285:4: required from ‘std::pair<std::_Rb_tree_iterator<_Val>, bool> std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_M_insert_unique(_Arg&&) [with _Arg = ID<Composite_Key<String_Key, Int_Key, Char_Key> >; _Key = ID<Composite_Key<String_Key, Int_Key, Char_Key> >; _Val = ID<Composite_Key<String_Key, Int_Key, Char_Key> >; _KeyOfValue = std::_Identity<ID<Composite_Key<String_Key, Int_Key, Char_Key> > >; _Compare = std::less<ID<Composite_Key<String_Key, Int_Key, Char_Key> > >; _Alloc = std::allocator<ID<Composite_Key<String_Key, Int_Key, Char_Key> > >]’
/usr/include/c++/4.7/bits/stl_set.h:424:40: required from ‘std::pair<typename std::_Rb_tree<_Key, _Key, std::_Identity<_Key>, _Compare, typename _Alloc::rebind<_Key>::other>::const_iterator, bool> std::set<_Key, _Compare, _Alloc>::insert(std::set<_Key, _Compare, _Alloc>::value_type&&) [with _Key = ID<Composite_Key<String_Key, Int_Key, Char_Key> >; _Compare = std::less<ID<Composite_Key<String_Key, Int_Key, Char_Key> > >; _Alloc = std::allocator<ID<Composite_Key<String_Key, Int_Key, Char_Key> > >; typename std::_Rb_tree<_Key, _Key, std::_Identity<_Key>, _Compare, typename _Alloc::rebind<_Key>::other>::const_iterator = std::_Rb_tree_const_iterator<ID<Composite_Key<String_Key, Int_Key, Char_Key> > >; std::set<_Key, _Compare, _Alloc>::value_type = ID<Composite_Key<String_Key, Int_Key, Char_Key> >]’
sandbox.cpp:121:15: required from here
/usr/include/c++/4.7/tuple:781:63: error: no match for ‘operator<’ in ‘std::get<0ul, {String_Key, Int_Key, Char_Key}>((* & __u)) < std::get<0ul, {String_Key, Int_Key, Char_Key}>((* & __t))’
/usr/include/c++/4.7/tuple:781:63: note: candidates are:
In file included from /usr/include/c++/4.7/bits/stl_algobase.h:65:0,
from /usr/include/c++/4.7/bits/char_traits.h:41,
from /usr/include/c++/4.7/string:42,
from sandbox.cpp:1:
/usr/include/c++/4.7/bits/stl_pair.h:212:5: note: template<class _T1, class _T2> constexpr bool std::operator<(const std::pair<_T1, _T2>&, const std::pair<_T1, _T2>&)
/usr/include/c++/4.7/bits/stl_pair.h:212:5: note: template argument deduction/substitution failed:
In file included from sandbox.cpp:2:0:
/usr/include/c++/4.7/tuple:781:63: note: ‘const String_Key’ is not derived from ‘const std::pair<_T1, _T2>’
In file included from /usr/include/c++/4.7/bits/stl_algobase.h:68:0,
from /usr/include/c++/4.7/bits/char_traits.h:41,
from /usr/include/c++/4.7/string:42,
from sandbox.cpp:1:
/usr/include/c++/4.7/bits/stl_iterator.h:299:5: note: template<class _Iterator> bool std::operator<(const std::reverse_iterator<_Iterator>&, const std::reverse_iterator<_Iterator>&)
/usr/include/c++/4.7/bits/stl_iterator.h:299:5: note: template argument deduction/substitution failed:
In file included from sandbox.cpp:2:0:
/usr/include/c++/4.7/tuple:781:63: note: ‘const String_Key’ is not derived from ‘const std::reverse_iterator<_Iterator>’
In file included from /usr/include/c++/4.7/bits/stl_algobase.h:68:0,
from /usr/include/c++/4.7/bits/char_traits.h:41,
from /usr/include/c++/4.7/string:42,
from sandbox.cpp:1:
/usr/include/c++/4.7/bits/stl_iterator.h:349:5: note: template<class _IteratorL, class _IteratorR> bool std::operator<(const std::reverse_iterator<_IteratorL>&, const std::reverse_iterator<_IteratorR>&)
/usr/include/c++/4.7/bits/stl_iterator.h:349:5: note: template argument deduction/substitution failed:
In file included from sandbox.cpp:2:0:
/usr/include/c++/4.7/tuple:781:63: note: ‘const String_Key’ is not derived from ‘const std::reverse_iterator<_IteratorL>’
In file included from /usr/include/c++/4.7/bits/stl_algobase.h:68:0,
from /usr/include/c++/4.7/bits/char_traits.h:41,
from /usr/include/c++/4.7/string:42,
from sandbox.cpp:1:
/usr/include/c++/4.7/bits/stl_iterator.h:1057:5: note: template<class _IteratorL, class _IteratorR> bool std::operator<(const std::move_iterator<_IteratorL>&, const std::move_iterator<_IteratorR>&)
/usr/include/c++/4.7/bits/stl_iterator.h:1057:5: note: template argument deduction/substitution failed:
In file included from sandbox.cpp:2:0:
/usr/include/c++/4.7/tuple:781:63: note: ‘const String_Key’ is not derived from ‘const std::move_iterator<_IteratorL>’
In file included from /usr/include/c++/4.7/bits/stl_algobase.h:68:0,
from /usr/include/c++/4.7/bits/char_traits.h:41,
from /usr/include/c++/4.7/string:42,
from sandbox.cpp:1:
/usr/include/c++/4.7/bits/stl_iterator.h:1063:5: note: template<class _Iterator> bool std::operator<(const std::move_iterator<_Iterator>&, const std::move_iterator<_Iterator>&)
/usr/include/c++/4.7/bits/stl_iterator.h:1063:5: note: template argument deduction/substitution failed:
In file included from sandbox.cpp:2:0:
/usr/include/c++/4.7/tuple:781:63: note: ‘const String_Key’ is not derived from ‘const std::move_iterator<_Iterator>’
In file included from /usr/include/c++/4.7/string:54:0,
from sandbox.cpp:1:
/usr/include/c++/4.7/bits/basic_string.h:2566:5: note: template<class _CharT, class _Traits, class _Alloc> bool std::operator<(const std::basic_string<_CharT, _Traits, _Alloc>&, const std::basic_string<_CharT, _Traits, _Alloc>&)
/usr/include/c++/4.7/bits/basic_string.h:2566:5: note: template argument deduction/substitution failed:
In file included from sandbox.cpp:2:0:
/usr/include/c++/4.7/tuple:781:63: note: ‘const String_Key’ is not derived from ‘const std::basic_string<_CharT, _Traits, _Alloc>’
In file included from /usr/include/c++/4.7/string:54:0,
from sandbox.cpp:1:
/usr/include/c++/4.7/bits/basic_string.h:2578:5: note: template<class _CharT, class _Traits, class _Alloc> bool std::operator<(const std::basic_string<_CharT, _Traits, _Alloc>&, const _CharT*)
/usr/include/c++/4.7/bits/basic_string.h:2578:5: note: template argument deduction/substitution failed:
In file included from sandbox.cpp:2:0:
/usr/include/c++/4.7/tuple:781:63: note: ‘const String_Key’ is not derived from ‘const std::basic_string<_CharT, _Traits, _Alloc>’
In file included from /usr/include/c++/4.7/string:54:0,
from sandbox.cpp:1:
/usr/include/c++/4.7/bits/basic_string.h:2590:5: note: template<class _CharT, class _Traits, class _Alloc> bool std::operator<(const _CharT*, const std::basic_string<_CharT, _Traits, _Alloc>&)
/usr/include/c++/4.7/bits/basic_string.h:2590:5: note: template argument deduction/substitution failed:
In file included from sandbox.cpp:2:0:
/usr/include/c++/4.7/tuple:781:63: note: mismatched types ‘const _CharT*’ and ‘String_Key’
/usr/include/c++/4.7/tuple:808:5: note: template<class ... _TElements, class ... _UElements> bool std::operator<(const std::tuple<_TElements ...>&, const std::tuple<_Elements ...>&)
/usr/include/c++/4.7/tuple:808:5: note: template argument deduction/substitution failed:
/usr/include/c++/4.7/tuple:781:63: note: ‘const String_Key’ is not derived from ‘const std::tuple<_TElements ...>’
In file included from /usr/include/c++/4.7/set:60:0,
from sandbox.cpp:3:
/usr/include/c++/4.7/bits/stl_tree.h:873:5: note: template<class _Key, class _Val, class _KeyOfValue, class _Compare, class _Alloc> bool std::operator<(const std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>&, const std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>&)
/usr/include/c++/4.7/bits/stl_tree.h:873:5: note: template argument deduction/substitution failed:
In file included from sandbox.cpp:2:0:
/usr/include/c++/4.7/tuple:781:63: note: ‘const String_Key’ is not derived from ‘const std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>’
In file included from /usr/include/c++/4.7/set:61:0,
from sandbox.cpp:3:
/usr/include/c++/4.7/bits/stl_set.h:721:5: note: template<class _Key, class _Compare, class _Alloc> bool std::operator<(const std::set<_Key, _Compare, _Alloc>&, const std::set<_Key, _Compare, _Alloc>&)
/usr/include/c++/4.7/bits/stl_set.h:721:5: note: template argument deduction/substitution failed:
In file included from sandbox.cpp:2:0:
/usr/include/c++/4.7/tuple:781:63: note: ‘const String_Key’ is not derived from ‘const std::set<_Key, _Compare, _Alloc>’
In file included from /usr/include/c++/4.7/set:62:0,
from sandbox.cpp:3:
/usr/include/c++/4.7/bits/stl_multiset.h:702:5: note: template<class _Key, class _Compare, class _Alloc> bool std::operator<(const std::multiset<_Key, _Compare, _Alloc>&, const std::multiset<_Key, _Compare, _Alloc>&)
/usr/include/c++/4.7/bits/stl_multiset.h:702:5: note: template argument deduction/substitution failed:
In file included from sandbox.cpp:2:0:
/usr/include/c++/4.7/tuple:781:63: note: ‘const String_Key’ is not derived from ‘const std::multiset<_Key, _Compare, _Alloc>’
/usr/include/c++/4.7/tuple:781:63: error: no match for ‘operator<’ in ‘std::get<0ul, {String_Key, Int_Key, Char_Key}>((* & __t)) < std::get<0ul, {String_Key, Int_Key, Char_Key}>((* & __u))’
/usr/include/c++/4.7/tuple:781:63: note: candidates are:
In file included from /usr/include/c++/4.7/bits/stl_algobase.h:65:0,
from /usr/include/c++/4.7/bits/char_traits.h:41,
from /usr/include/c++/4.7/string:42,
from sandbox.cpp:1:
/usr/include/c++/4.7/bits/stl_pair.h:212:5: note: template<class _T1, class _T2> constexpr bool std::operator<(const std::pair<_T1, _T2>&, const std::pair<_T1, _T2>&)
/usr/include/c++/4.7/bits/stl_pair.h:212:5: note: template argument deduction/substitution failed:
In file included from sandbox.cpp:2:0:
/usr/include/c++/4.7/tuple:781:63: note: ‘const String_Key’ is not derived from ‘const std::pair<_T1, _T2>’
In file included from /usr/include/c++/4.7/bits/stl_algobase.h:68:0,
from /usr/include/c++/4.7/bits/char_traits.h:41,
from /usr/include/c++/4.7/string:42,
from sandbox.cpp:1:
/usr/include/c++/4.7/bits/stl_iterator.h:299:5: note: template<class _Iterator> bool std::operator<(const std::reverse_iterator<_Iterator>&, const std::reverse_iterator<_Iterator>&)
/usr/include/c++/4.7/bits/stl_iterator.h:299:5: note: template argument deduction/substitution failed:
In file included from sandbox.cpp:2:0:
/usr/include/c++/4.7/tuple:781:63: note: ‘const String_Key’ is not derived from ‘const std::reverse_iterator<_Iterator>’
In file included from /usr/include/c++/4.7/bits/stl_algobase.h:68:0,
from /usr/include/c++/4.7/bits/char_traits.h:41,
from /usr/include/c++/4.7/string:42,
from sandbox.cpp:1:
/usr/include/c++/4.7/bits/stl_iterator.h:349:5: note: template<class _IteratorL, class _IteratorR> bool std::operator<(const std::reverse_iterator<_IteratorL>&, const std::reverse_iterator<_IteratorR>&)
/usr/include/c++/4.7/bits/stl_iterator.h:349:5: note: template argument deduction/substitution failed:
In file included from sandbox.cpp:2:0:
/usr/include/c++/4.7/tuple:781:63: note: ‘const String_Key’ is not derived from ‘const std::reverse_iterator<_IteratorL>’
In file included from /usr/include/c++/4.7/bits/stl_algobase.h:68:0,
from /usr/include/c++/4.7/bits/char_traits.h:41,
from /usr/include/c++/4.7/string:42,
from sandbox.cpp:1:
/usr/include/c++/4.7/bits/stl_iterator.h:1057:5: note: template<class _IteratorL, class _IteratorR> bool std::operator<(const std::move_iterator<_IteratorL>&, const std::move_iterator<_IteratorR>&)
/usr/include/c++/4.7/bits/stl_iterator.h:1057:5: note: template argument deduction/substitution failed:
In file included from sandbox.cpp:2:0:
/usr/include/c++/4.7/tuple:781:63: note: ‘const String_Key’ is not derived from ‘const std::move_iterator<_IteratorL>’
In file included from /usr/include/c++/4.7/bits/stl_algobase.h:68:0,
from /usr/include/c++/4.7/bits/char_traits.h:41,
from /usr/include/c++/4.7/string:42,
from sandbox.cpp:1:
/usr/include/c++/4.7/bits/stl_iterator.h:1063:5: note: template<class _Iterator> bool
ANSWER
正如jmetcalf below指明，
CODE
#include <string>
#include <tuple>
#include <set>
#include <cassert>
template<typename T>
struct void_ {
using type = void;
};
// -----------------------------------------------------------------------------
template<typename T, typename = void>
struct ptr_or_key_type {
using type = T const*; // our default key_type : a ptr
static type get_key(T const& t) { return &t; }
};
template<typename T>
struct ptr_or_key_type<T, typename void_<typename T::key_type>::type> {
using type = typename T::key_type; // the specialised key_type
static type get_key(T const& t) { return t.get_key(); }
};
// -----------------------------------------------------------------------------
template<typename T>
class ID
{
private:
typename ptr_or_key_type<T>::type m_id;
public:
ID(T const& t) :
m_id(ptr_or_key_type<T>::get_key(t))
{ }
ID(ID const& rhs) :
m_id(rhs.m_id)
{ }
~ID() { }
ID& operator=(ID const& rhs)
{
if (&rhs!=this)
m_id = rhs.m_id;
return *this;
}
public:
bool operator==(ID const& rhs) const { return m_id==rhs.m_id; }
bool operator!=(ID const& rhs) const { return !(*this==rhs); }
bool operator<(ID const& rhs) const { return m_id<rhs.m_id; }
bool operator<=(ID const& rhs) const { return m_id<=rhs.m_id; }
bool operator>(ID const& rhs) const { return m_id>rhs.m_id; }
bool operator>=(ID const& rhs) const { return m_id>=rhs.m_id; }
};
// -----------------------------------------------------------------------------
struct Plain_Class { };
struct String_Key {
using key_type = std::string;
std::string m_key;
String_Key(std::string const& key) : m_key(key) { }
std::string const& get_key() const { return m_key; }
};
struct Char_Key {
using key_type = char;
char m_key;
Char_Key(char key) : m_key(key) { }
char get_key() const { return m_key; }
};
struct Int_Key {
using key_type = int;
int m_key;
Int_Key(int key) : m_key(key) { }
int get_key() const { return m_key; }
};
template<typename... Args>
struct Composite_Key
{
using key_type = std::tuple<Args...>;
key_type m_key;
Composite_Key(key_type const& key) : m_key(key) { }
key_type const& get_key() const { return m_key; }
};
// -----------------------------------------------------------------------------
int main(int argc, char* argv[])
{
// Plain_Class will use address of object as key
Plain_Class f,g;
ID<Plain_Class> id_f(f), id_g(g);
assert(id_f!=id_g);
std::set<ID<Plain_Class>> s;
s.insert(f);
s.insert(g);
assert(s.size()==2u); // two unique addresses, so two in the set
// String_Key will use std::string as the key
String_Key h("abc"), i("abc");
std::set<ID<String_Key>> s2;
s2.insert(h);
s2.insert(i);
assert(s2.size()==1u); // since sets must have unique values
// attempt a composite key type
using My_Composite = Composite_Key<String_Key,Int_Key,Char_Key>;
My_Composite
j(std::make_tuple(String_Key{ "foo" }, Int_Key{ 1 }, Char_Key{ 'c' })),
k(std::make_tuple(String_Key{ "foo" }, Int_Key{ 1 }, Char_Key{ 'c' }))
;
std::set<ID<My_Composite>> s3;
s3.insert(j); // FAILURE: everything above this line compiles fine
#if 0
s3.insert(k);
assert(s3.size()==1u); // since sets must have unique values
#endif
}
墙，所有的重点班也必须实现operator<()。这是有道理的，因为operator<()在ID实际上取决于std::tuple的operator<()，这取决于所有的个人类型'operator<()。
这是一个痛苦的很多代码来通读。你真的不能减少这个更简单的例子？ – 2013-02-13 18:43:01