为什么std :: less< Eigen :: VectorXd>无法编译？ [英] Why does std::less<Eigen::VectorXd> fail to compile?
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问题描述
我为 Eigen :: VectorXd 实现了一个比较运算符 operator< ，有时，我需要传递一个将函数与我的另一个函数进行比较，我已经厌倦了将运算符 c包装到 []（const VectorXd& v1，const VectorXd& v2）- > bool {return v1< v2} ，所以我认为 std :: less 类会很有用，因为据我所知，它可以生成lambda函数，只要
I implemented a comparison operator operator< for Eigen::VectorXd, and sometimes, I need to pass a compare function to another of my function, I am tired of wrapping the operator< into [](const VectorXd& v1, const VectorXd& v2)->bool{return v1 < v2}, so I think the std::less class would be useful, as, to my understanding, it can generate the lambda function as long as operator< is defined.
但是，我发现 std :: less< VectorXd> 对我不起作用，例如，以下代码可以正常工作：
However, I found that std::less<VectorXd> didn't work for me, for example, the below code works fine:
#include "Eigen/Dense"
#include <iostream>
#include <functional>
using namespace std;
using namespace Eigen;
struct T
{
int x;
};
bool operator<(const T& t1, const T& t2)
{
return t1.x < t2.x;
}
bool operator<(const VectorXd& v1, const VectorXd& v2)
{
return (v1.array() <= v2.array()).all() and (v1 != v2);
}
int main()
{
T t1, t2;
t1.x = 3;
t2.x = 2;
auto ft = std::less<T>();
cout << ft(t1, t2) << endl;
return EXIT_SUCCESS;
}
但是，如果我使用 std :: less< VectorXd> 像这样：
However, if I use std::less<VectorXd> like this:
#include "Eigen/Dense"
#include <iostream>
#include <functional>
using namespace std;
using namespace Eigen;
struct T
{
int x;
};
bool operator<(const T& t1, const T& t2)
{
return t1.x < t2.x;
}
bool operator<(const VectorXd& v1, const VectorXd& v2)
{
return (v1.array() <= v2.array()).all() and (v1 != v2);
}
int main()
{
T t1, t2;
t1.x = 3;
t2.x = 2;
auto ft = std::less<T>();
cout << ft(t1, t2) << endl;
VectorXd v1(3);
VectorXd v2(3);
v1 << 1, 2, 3;
v2 << 2, 3, 4;
auto fv = std::less<VectorXd>();
cout << fv(v1, v2) << endl;
return EXIT_SUCCESS;
}
代码无法编译，并且出现如下错误消息：
The code fails to compile, and I get error messages like below:
In file included from /usr/include/c++/5/string:48:0,
from /usr/include/c++/5/bits/locale_classes.h:40,
from /usr/include/c++/5/bits/ios_base.h:41,
from /usr/include/c++/5/ios:42,
from /usr/include/c++/5/istream:38,
from /usr/include/c++/5/sstream:38,
from /usr/include/c++/5/complex:45,
from /home/alaya/.softwares/linear_algebra/eigen3.3/include/eigen3/Eigen/Core:80,
from /home/alaya/.softwares/linear_algebra/eigen3.3/include/eigen3/Eigen/Dense:1,
from test.cpp:1:
/usr/include/c++/5/bits/stl_function.h: In instantiation of ‘bool std::less<_Tp>::operator()(const _Tp&, const _Tp&) const [with _Tp = Eigen::Matrix<double, -1, 1>]’:
test.cpp:36:26: required from here
/usr/include/c++/5/bits/stl_function.h:387:20: error: no match for ‘operator<’ (operand types are ‘const Eigen::Matrix<double, -1, 1>’ and ‘const Eigen::Matrix<double, -1, 1>’)
{ return __x < __y; }
^
In file included from /usr/include/c++/5/bits/stl_algobase.h:64:0,
from /usr/include/c++/5/bits/char_traits.h:39,
from /usr/include/c++/5/ios:40,
from /usr/include/c++/5/istream:38,
from /usr/include/c++/5/sstream:38,
from /usr/include/c++/5/complex:45,
from /home/alaya/.softwares/linear_algebra/eigen3.3/include/eigen3/Eigen/Core:80,
from /home/alaya/.softwares/linear_algebra/eigen3.3/include/eigen3/Eigen/Dense:1,
from test.cpp:1:
/usr/include/c++/5/bits/stl_pair.h:220:5: note: candidate: template<class _T1, class _T2> constexpr bool std::operator<(const std::pair<_T1, _T2>&, const std::pair<_T1, _T2>&)
operator<(const pair<_T1, _T2>& __x, const pair<_T1, _T2>& __y)
^
/usr/include/c++/5/bits/stl_pair.h:220:5: note: template argument deduction/substitution failed:
In file included from /usr/include/c++/5/string:48:0,
from /usr/include/c++/5/bits/locale_classes.h:40,
from /usr/include/c++/5/bits/ios_base.h:41,
from /usr/include/c++/5/ios:42,
from /usr/include/c++/5/istream:38,
from /usr/include/c++/5/sstream:38,
from /usr/include/c++/5/complex:45,
from /home/alaya/.softwares/linear_algebra/eigen3.3/include/eigen3/Eigen/Core:80,
from /home/alaya/.softwares/linear_algebra/eigen3.3/include/eigen3/Eigen/Dense:1,
from test.cpp:1:
/usr/include/c++/5/bits/stl_function.h:387:20: note: ‘const Eigen::Matrix<double, -1, 1>’ is not derived from ‘const std::pair<_T1, _T2>’
{ return __x < __y; }
^
In file included from /usr/include/c++/5/bits/stl_algobase.h:67:0,
from /usr/include/c++/5/bits/char_traits.h:39,
from /usr/include/c++/5/ios:40,
from /usr/include/c++/5/istream:38,
from /usr/include/c++/5/sstream:38,
from /usr/include/c++/5/complex:45,
from /home/alaya/.softwares/linear_algebra/eigen3.3/include/eigen3/Eigen/Core:80,
from /home/alaya/.softwares/linear_algebra/eigen3.3/include/eigen3/Eigen/Dense:1,
from test.cpp:1:
/usr/include/c++/5/bits/stl_iterator.h:298:5: note: candidate: template<class _Iterator> bool std::operator<(const std::reverse_iterator<_Iterator>&, const std::reverse_iterator<_Iterator>&)
operator<(const reverse_iterator<_Iterator>& __x,
^
/usr/include/c++/5/bits/stl_iterator.h:298:5: note: template argument deduction/substitution failed:
In file included from /usr/include/c++/5/string:48:0,
from /usr/include/c++/5/bits/locale_classes.h:40,
from /usr/include/c++/5/bits/ios_base.h:41,
from /usr/include/c++/5/ios:42,
from /usr/include/c++/5/istream:38,
from /usr/include/c++/5/sstream:38,
from /usr/include/c++/5/complex:45,
from /home/alaya/.softwares/linear_algebra/eigen3.3/include/eigen3/Eigen/Core:80,
from /home/alaya/.softwares/linear_algebra/eigen3.3/include/eigen3/Eigen/Dense:1,
from test.cpp:1:
/usr/include/c++/5/bits/stl_function.h:387:20: note: ‘const Eigen::Matrix<double, -1, 1>’ is not derived from ‘const std::reverse_iterator<_Iterator>’
{ return __x < __y; }
^
In file included from /usr/include/c++/5/bits/stl_algobase.h:67:0,
from /usr/include/c++/5/bits/char_traits.h:39,
from /usr/include/c++/5/ios:40,
from /usr/include/c++/5/istream:38,
from /usr/include/c++/5/sstream:38,
from /usr/include/c++/5/complex:45,
from /home/alaya/.softwares/linear_algebra/eigen3.3/include/eigen3/Eigen/Core:80,
from /home/alaya/.softwares/linear_algebra/eigen3.3/include/eigen3/Eigen/Dense:1,
from test.cpp:1:
/usr/include/c++/5/bits/stl_iterator.h:348:5: note: candidate: template<class _IteratorL, class _IteratorR> bool std::operator<(const std::reverse_iterator<_Iterator>&, const std::reverse_iterator<_IteratorR>&)
operator<(const reverse_iterator<_IteratorL>& __x,
^
/usr/include/c++/5/bits/stl_iterator.h:348:5: note: template argument deduction/substitution failed:
In file included from /usr/include/c++/5/string:48:0,
from /usr/include/c++/5/bits/locale_classes.h:40,
from /usr/include/c++/5/bits/ios_base.h:41,
from /usr/include/c++/5/ios:42,
from /usr/include/c++/5/istream:38,
from /usr/include/c++/5/sstream:38,
from /usr/include/c++/5/complex:45,
from /home/alaya/.softwares/linear_algebra/eigen3.3/include/eigen3/Eigen/Core:80,
from /home/alaya/.softwares/linear_algebra/eigen3.3/include/eigen3/Eigen/Dense:1,
from test.cpp:1:
/usr/include/c++/5/bits/stl_function.h:387:20: note: ‘const Eigen::Matrix<double, -1, 1>’ is not derived from ‘const std::reverse_iterator<_Iterator>’
{ return __x < __y; }
^
In file included from /usr/include/c++/5/bits/stl_algobase.h:67:0,
from /usr/include/c++/5/bits/char_traits.h:39,
from /usr/include/c++/5/ios:40,
from /usr/include/c++/5/istream:38,
from /usr/include/c++/5/sstream:38,
from /usr/include/c++/5/complex:45,
from /home/alaya/.softwares/linear_algebra/eigen3.3/include/eigen3/Eigen/Core:80,
from /home/alaya/.softwares/linear_algebra/eigen3.3/include/eigen3/Eigen/Dense:1,
from test.cpp:1:
/usr/include/c++/5/bits/stl_iterator.h:1089:5: note: candidate: template<class _IteratorL, class _IteratorR> bool std::operator<(const std::move_iterator<_Iterator>&, const std::move_iterator<_IteratorR>&)
operator<(const move_iterator<_IteratorL>& __x,
^
/usr/include/c++/5/bits/stl_iterator.h:1089:5: note: template argument deduction/substitution failed:
In file included from /usr/include/c++/5/string:48:0,
from /usr/include/c++/5/bits/locale_classes.h:40,
from /usr/include/c++/5/bits/ios_base.h:41,
from /usr/include/c++/5/ios:42,
from /usr/include/c++/5/istream:38,
from /usr/include/c++/5/sstream:38,
from /usr/include/c++/5/complex:45,
from /home/alaya/.softwares/linear_algebra/eigen3.3/include/eigen3/Eigen/Core:80,
from /home/alaya/.softwares/linear_algebra/eigen3.3/include/eigen3/Eigen/Dense:1,
from test.cpp:1:
/usr/include/c++/5/bits/stl_function.h:387:20: note: ‘const Eigen::Matrix<double, -1, 1>’ is not derived from ‘const std::move_iterator<_Iterator>’
{ return __x < __y; }
^
In file included from /usr/include/c++/5/bits/stl_algobase.h:67:0,
from /usr/include/c++/5/bits/char_traits.h:39,
from /usr/include/c++/5/ios:40,
from /usr/include/c++/5/istream:38,
from /usr/include/c++/5/sstream:38,
from /usr/include/c++/5/complex:45,
from /home/alaya/.softwares/linear_algebra/eigen3.3/include/eigen3/Eigen/Core:80,
from /home/alaya/.softwares/linear_algebra/eigen3.3/include/eigen3/Eigen/Dense:1,
from test.cpp:1:
/usr/include/c++/5/bits/stl_iterator.h:1095:5: note: candidate: template<class _Iterator> bool std::operator<(const std::move_iterator<_Iterator>&, const std::move_iterator<_Iterator>&)
operator<(const move_iterator<_Iterator>& __x,
^
/usr/include/c++/5/bits/stl_iterator.h:1095:5: note: template argument deduction/substitution failed:
In file included from /usr/include/c++/5/string:48:0,
from /usr/include/c++/5/bits/locale_classes.h:40,
from /usr/include/c++/5/bits/ios_base.h:41,
from /usr/include/c++/5/ios:42,
from /usr/include/c++/5/istream:38,
from /usr/include/c++/5/sstream:38,
from /usr/include/c++/5/complex:45,
from /home/alaya/.softwares/linear_algebra/eigen3.3/include/eigen3/Eigen/Core:80,
from /home/alaya/.softwares/linear_algebra/eigen3.3/include/eigen3/Eigen/Dense:1,
from test.cpp:1:
/usr/include/c++/5/bits/stl_function.h:387:20: note: ‘const Eigen::Matrix<double, -1, 1>’ is not derived from ‘const std::move_iterator<_Iterator>’
{ return __x < __y; }
^
In file included from /usr/include/c++/5/string:52:0,
from /usr/include/c++/5/bits/locale_classes.h:40,
from /usr/include/c++/5/bits/ios_base.h:41,
from /usr/include/c++/5/ios:42,
from /usr/include/c++/5/istream:38,
from /usr/include/c++/5/sstream:38,
from /usr/include/c++/5/complex:45,
from /home/alaya/.softwares/linear_algebra/eigen3.3/include/eigen3/Eigen/Core:80,
from /home/alaya/.softwares/linear_algebra/eigen3.3/include/eigen3/Eigen/Dense:1,
from test.cpp:1:
/usr/include/c++/5/bits/basic_string.h:4987:5: note: candidate: template<class _CharT, class _Traits, class _Alloc> bool std::operator<(const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&, const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&)
operator<(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
^
/usr/include/c++/5/bits/basic_string.h:4987:5: note: template argument deduction/substitution failed:
In file included from /usr/include/c++/5/string:48:0,
from /usr/include/c++/5/bits/locale_classes.h:40,
from /usr/include/c++/5/bits/ios_base.h:41,
from /usr/include/c++/5/ios:42,
from /usr/include/c++/5/istream:38,
from /usr/include/c++/5/sstream:38,
from /usr/include/c++/5/complex:45,
from /home/alaya/.softwares/linear_algebra/eigen3.3/include/eigen3/Eigen/Core:80,
from /home/alaya/.softwares/linear_algebra/eigen3.3/include/eigen3/Eigen/Dense:1,
from test.cpp:1:
/usr/include/c++/5/bits/stl_function.h:387:20: note: ‘const Eigen::Matrix<double, -1, 1>’ is not derived from ‘const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>’
{ return __x < __y; }
^
In file included from /usr/include/c++/5/string:52:0,
from /usr/include/c++/5/bits/locale_classes.h:40,
from /usr/include/c++/5/bits/ios_base.h:41,
from /usr/include/c++/5/ios:42,
from /usr/include/c++/5/istream:38,
from /usr/include/c++/5/sstream:38,
from /usr/include/c++/5/complex:45,
from /home/alaya/.softwares/linear_algebra/eigen3.3/include/eigen3/Eigen/Core:80,
from /home/alaya/.softwares/linear_algebra/eigen3.3/include/eigen3/Eigen/Dense:1,
from test.cpp:1:
/usr/include/c++/5/bits/basic_string.h:4999:5: note: candidate: template<class _CharT, class _Traits, class _Alloc> bool std::operator<(const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&, const _CharT*)
operator<(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
^
/usr/include/c++/5/bits/basic_string.h:4999:5: note: template argument deduction/substitution failed:
In file included from /usr/include/c++/5/string:48:0,
from /usr/include/c++/5/bits/locale_classes.h:40,
from /usr/include/c++/5/bits/ios_base.h:41,
from /usr/include/c++/5/ios:42,
from /usr/include/c++/5/istream:38,
from /usr/include/c++/5/sstream:38,
from /usr/include/c++/5/complex:45,
from /home/alaya/.softwares/linear_algebra/eigen3.3/include/eigen3/Eigen/Core:80,
from /home/alaya/.softwares/linear_algebra/eigen3.3/include/eigen3/Eigen/Dense:1,
from test.cpp:1:
/usr/include/c++/5/bits/stl_function.h:387:20: note: ‘const Eigen::Matrix<double, -1, 1>’ is not derived from ‘const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>’
{ return __x < __y; }
^
In file included from /usr/include/c++/5/string:52:0,
from /usr/include/c++/5/bits/locale_classes.h:40,
from /usr/include/c++/5/bits/ios_base.h:41,
from /usr/include/c++/5/ios:42,
from /usr/include/c++/5/istream:38,
from /usr/include/c++/5/sstream:38,
from /usr/include/c++/5/complex:45,
from /home/alaya/.softwares/linear_algebra/eigen3.3/include/eigen3/Eigen/Core:80,
from /home/alaya/.softwares/linear_algebra/eigen3.3/include/eigen3/Eigen/Dense:1,
from test.cpp:1:
/usr/include/c++/5/bits/basic_string.h:5011:5: note: candidate: template<class _CharT, class _Traits, class _Alloc> bool std::operator<(const _CharT*, const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&)
operator<(const _CharT* __lhs,
^
/usr/include/c++/5/bits/basic_string.h:5011:5: note: template argument deduction/substitution failed:
In file included from /usr/include/c++/5/string:48:0,
from /usr/include/c++/5/bits/locale_classes.h:40,
from /usr/include/c++/5/bits/ios_base.h:41,
from /usr/include/c++/5/ios:42,
from /usr/include/c++/5/istream:38,
from /usr/include/c++/5/sstream:38,
from /usr/include/c++/5/complex:45,
from /home/alaya/.softwares/linear_algebra/eigen3.3/include/eigen3/Eigen/Core:80,
from /home/alaya/.softwares/linear_algebra/eigen3.3/include/eigen3/Eigen/Dense:1,
from test.cpp:1:
/usr/include/c++/5/bits/stl_function.h:387:20: note: mismatched types ‘const _CharT*’ and ‘Eigen::Matrix<double, -1, 1>’
{ return __x < __y; }
^
In file included from /usr/include/c++/5/bits/ios_base.h:46:0,
from /usr/include/c++/5/ios:42,
from /usr/include/c++/5/istream:38,
from /usr/include/c++/5/sstream:38,
from /usr/include/c++/5/complex:45,
from /home/alaya/.softwares/linear_algebra/eigen3.3/include/eigen3/Eigen/Core:80,
from /home/alaya/.softwares/linear_algebra/eigen3.3/include/eigen3/Eigen/Dense:1,
from test.cpp:1:
/usr/include/c++/5/system_error:200:3: note: candidate: bool std::operator<(const std::error_code&, const std::error_code&)
operator<(const error_code& __lhs, const error_code& __rhs) noexcept
^
/usr/include/c++/5/system_error:200:3: note: no known conversion for argument 1 from ‘const Eigen::Matrix<double, -1, 1>’ to ‘const std::error_code&’
/usr/include/c++/5/system_error:274:3: note: candidate: bool std::operator<(const std::error_condition&, const std::error_condition&)
operator<(const error_condition& __lhs,
^
/usr/include/c++/5/system_error:274:3: note: no known conversion for argument 1 from ‘const Eigen::Matrix<double, -1, 1>’ to ‘const std::error_condition&’
In file included from /usr/include/c++/5/tuple:39:0,
from /usr/include/c++/5/functional:55,
from /home/alaya/.softwares/linear_algebra/eigen3.3/include/eigen3/Eigen/Core:262,
from /home/alaya/.softwares/linear_algebra/eigen3.3/include/eigen3/Eigen/Dense:1,
from test.cpp:1:
/usr/include/c++/5/array:250:5: note: candidate: template<class _Tp, long unsigned int _Nm> bool std::operator<(const std::array<_Tp, _Nm>&, const std::array<_Tp, _Nm>&)
operator<(const array<_Tp, _Nm>& __a, const array<_Tp, _Nm>& __b)
^
/usr/include/c++/5/array:250:5: note: template argument deduction/substitution failed:
In file included from /usr/include/c++/5/string:48:0,
from /usr/include/c++/5/bits/locale_classes.h:40,
from /usr/include/c++/5/bits/ios_base.h:41,
from /usr/include/c++/5/ios:42,
from /usr/include/c++/5/istream:38,
from /usr/include/c++/5/sstream:38,
from /usr/include/c++/5/complex:45,
from /home/alaya/.softwares/linear_algebra/eigen3.3/include/eigen3/Eigen/Core:80,
from /home/alaya/.softwares/linear_algebra/eigen3.3/include/eigen3/Eigen/Dense:1,
from test.cpp:1:
/usr/include/c++/5/bits/stl_function.h:387:20: note: ‘const Eigen::Matrix<double, -1, 1>’ is not derived from ‘const std::array<_Tp, _Nm>’
{ return __x < __y; }
^
In file included from /usr/include/c++/5/functional:55:0,
from /home/alaya/.softwares/linear_algebra/eigen3.3/include/eigen3/Eigen/Core:262,
from /home/alaya/.softwares/linear_algebra/eigen3.3/include/eigen3/Eigen/Dense:1,
from test.cpp:1:
/usr/include/c++/5/tuple:928:5: note: candidate: template<class ... _TElements, class ... _UElements> constexpr bool std::operator<(const std::tuple<_Elements ...>&, const std::tuple<_Elements ...>&)
operator<(const tuple<_TElements...>& __t,
^
/usr/include/c++/5/tuple:928:5: note: template argument deduction/substitution failed:
In file included from /usr/include/c++/5/string:48:0,
from /usr/include/c++/5/bits/locale_classes.h:40,
from /usr/include/c++/5/bits/ios_base.h:41,
from /usr/include/c++/5/ios:42,
from /usr/include/c++/5/istream:38,
from /usr/include/c++/5/sstream:38,
from /usr/include/c++/5/complex:45,
from /home/alaya/.softwares/linear_algebra/eigen3.3/include/eigen3/Eigen/Core:80,
from /home/alaya/.softwares/linear_algebra/eigen3.3/include/eigen3/Eigen/Dense:1,
from test.cpp:1:
/usr/include/c++/5/bits/stl_function.h:387:20: note: ‘const Eigen::Matrix<double, -1, 1>’ is not derived from ‘const std::tuple<_Elements ...>’
{ return __x < __y; }
^
In file included from /usr/include/c++/5/vector:64:0,
from /usr/include/c++/5/bits/random.h:34,
from /usr/include/c++/5/random:49,
from /usr/include/c++/5/bits/stl_algo.h:66,
from /usr/include/c++/5/algorithm:62,
from /home/alaya/.softwares/linear_algebra/eigen3.3/include/eigen3/Eigen/Core:269,
from /home/alaya/.softwares/linear_algebra/eigen3.3/include/eigen3/Eigen/Dense:1,
from test.cpp:1:
/usr/include/c++/5/bits/stl_vector.h:1528:5: note: candidate: template<class _Tp, class _Alloc> bool std::operator<(const std::vector<_Tp, _Alloc>&, const std::vector<_Tp, _Alloc>&)
operator<(const vector<_Tp, _Alloc>& __x, const vector<_Tp, _Alloc>& __y)
^
/usr/include/c++/5/bits/stl_vector.h:1528:5: note: template argument deduction/substitution failed:
In file included from /usr/include/c++/5/string:48:0,
from /usr/include/c++/5/bits/locale_classes.h:40,
from /usr/include/c++/5/bits/ios_base.h:41,
from /usr/include/c++/5/ios:42,
from /usr/include/c++/5/istream:38,
from /usr/include/c++/5/sstream:38,
from /usr/include/c++/5/complex:45,
from /home/alaya/.softwares/linear_algebra/eigen3.3/include/eigen3/Eigen/Core:80,
from /home/alaya/.softwares/linear_algebra/eigen3.3/include/eigen3/Eigen/Dense:1,
from test.cpp:1:
/usr/include/c++/5/bits/stl_function.h:387:20: note: ‘const Eigen::Matrix<double, -1, 1>’ is not derived from ‘const std::vector<_Tp, _Alloc>’
{ return __x < __y; }
^
make: *** [try] Error 1
I正在使用g ++ 5.4.0和Eigen 3.3
I am using g++ 5.4.0 and Eigen 3.3
推荐答案
这就是问题。您的 T 不能可靠地再现情况。您省略了命名空间，这很重要。与 Eigen 的情况更接近的示例是：
Here's the thing. Your T doesn't recreate the situation reliably. You omitted the namespace, which is very important. A closer example to the case with Eigen would be this:
namespace Foo {
struct T
{
int x;
};
}
bool operator<(const Foo::T& t1, const Foo::T& t2)
{
return t1.x < t2.x;
}
它会产生完全相同的错误。这是因为模板仅考虑在模板定义（而不是实例化）或根据操作数类型进行参数依赖查找。
And it will produce the exact same error. That is because a template will only consider overloads of this operator that are found at the point of the template definition (and not instantiation), or by argument dependent lookup on the types of the operands.
并根据参数进行查找为了工作，运算符和类型必须在相同的命名空间中定义，而您显然不是。
And for argument dependent lookup to work, the operator and the type must be defined in the same namespace, which yours clearly isn't.
那么这是怎么回事，您要包括功能。这就定义了翻译单元中的模板 std :: less 。那时，没有Eigen :: VectorXd 的 operator< 。因此，模板定义在以后定义时将不考虑。
So what happens is this, you include functional. That defines the template std::less in your translation unit. At that time, there is no operator< for Eigen::VectorXd in sight. So the template definition will not take it into consideration when it's defined later.
实例化模板时，它会尝试在意识到的模板中寻找合适的重载。关闭，然后通过ADL。由于您的 operator< 不在 Eigen 名称空间中，因此ADL也找不到该名称空间。
When you instantiate the template, it attempts to look for a suitable overload among those it was made aware off, and then by ADL. Since your operator< is not inside the Eigen namespace, it isn't found by ADL either.
长话短说，对于库类型重载运算符是不切实际的。您应该做的是定义一个自定义比较器类型：
So long story short, it's impractical to overload operators for library types. What you should do, is to define a custom comparator type:
struct VectorXdCompare {
bool operator()(const VectorXd& v1, const VectorXd& v2) {
return (v1.array() <= v2.array()).all() and (v1 != v2);
}
};
并将 VectorXdCompare 作为类型传递，只要标准库需要它。
And pass that VectorXdCompare as the type, whenever the standard library needs it.
这篇关于为什么std :: less< Eigen :: VectorXd>无法编译？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！
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