索引
一、错误报告
二、报错代码
三、修正后的代码
四、原因和总结
一、错误报告
g++ 4.cpp -o 4.exe
In file included from /usr/include/c++/8/string:48,
from /usr/include/c++/8/bits/locale_classes.h:40,
from /usr/include/c++/8/bits/ios_base.h:41,
from /usr/include/c++/8/ios:42,
from /usr/include/c++/8/ostream:38,
from /usr/include/c++/8/iostream:39,
from 4.cpp:1:
/usr/include/c++/8/bits/stl_function.h: In instantiation of ‘constexpr bool std::greater<_Tp>::operator()(const _Tp&, const _Tp&) const [with _Tp = Test]’:
/usr/include/c++/8/bits/predefined_ops.h:143:18: required from ‘constexpr bool __gnu_cxx::__ops::_Iter_comp_iter<_Compare>::operator()(_Iterator1, _Iterator2) [with _Iterator1 = Test*; _Iterator2 = Test*; _Compare = std::greater<Test>]’
/usr/include/c++/8/bits/stl_algo.h:1847:14: required from ‘void std::__insertion_sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = Test*; _Compare = __gnu_cxx::__ops::_Iter_comp_iter<std::greater<Test> >]’
/usr/include/c++/8/bits/stl_algo.h:1885:25: required from ‘void std::__final_insertion_sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = Test*; _Compare = __gnu_cxx::__ops::_Iter_comp_iter<std::greater<Test> >]’
/usr/include/c++/8/bits/stl_algo.h:1971:31: required from ‘void std::__sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = Test*; _Compare = __gnu_cxx::__ops::_Iter_comp_iter<std::greater<Test> >]’
/usr/include/c++/8/bits/stl_algo.h:4866:18: required from ‘void std::sort(_RAIter, _RAIter, _Compare) [with _RAIter = Test*; _Compare = std::greater<Test>]’
4.cpp:15:32: required from here
/usr/include/c++/8/bits/stl_function.h:376:20: error: no match for ‘operator>’ (operand types are ‘const Test’ and ‘const Test’)r
{ return __x > __y; }
~~~~^~~~~
4.cpp:7:7: note: candidate: ‘bool Test::operator>(const Test&)’ <near match>
bool operator>(const Test &a){
^~~~~~~~
4.cpp:7:7: note: passing ‘const Test*’ as ‘this’ argument discards qualifiers
In file included from /usr/include/c++/8/bits/stl_algobase.h:64,
from /usr/include/c++/8/bits/char_traits.h:39,
from /usr/include/c++/8/ios:40,
from /usr/include/c++/8/ostream:38,
from /usr/include/c++/8/iostream:39,
from 4.cpp:1:
/usr/include/c++/8/bits/stl_pair.h:467:5: 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++/8/bits/stl_pair.h:467:5: note: template argument deduction/substitution failed:
In file included from /usr/include/c++/8/string:48,
from /usr/include/c++/8/bits/locale_classes.h:40,
from /usr/include/c++/8/bits/ios_base.h:41,
from /usr/include/c++/8/ios:42,
from /usr/include/c++/8/ostream:38,
from /usr/include/c++/8/iostream:39,
from 4.cpp:1:
/usr/include/c++/8/bits/stl_function.h:376:20: note: ‘const Test’ is not derived from ‘const std::pair<_T1, _T2>’
{ return __x > __y; }
~~~~^~~~~
In file included from /usr/include/c++/8/bits/stl_algobase.h:67,
from /usr/include/c++/8/bits/char_traits.h:39,
from /usr/include/c++/8/ios:40,
from /usr/include/c++/8/ostream:38,
from /usr/include/c++/8/iostream:39,
from 4.cpp:1:
/usr/include/c++/8/bits/stl_iterator.h:320:5: 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++/8/bits/stl_iterator.h:320:5: note: template argument deduction/substitution failed:
In file included from /usr/include/c++/8/string:48,
from /usr/include/c++/8/bits/locale_classes.h:40,
from /usr/include/c++/8/bits/ios_base.h:41,
from /usr/include/c++/8/ios:42,
from /usr/include/c++/8/ostream:38,
from /usr/include/c++/8/iostream:39,
from 4.cpp:1:
/usr/include/c++/8/bits/stl_function.h:376:20: note: ‘const Test’ is not derived from ‘const std::reverse_iterator<_Iterator>’
{ return __x > __y; }
~~~~^~~~~
In file included from /usr/include/c++/8/bits/stl_algobase.h:67,
from /usr/include/c++/8/bits/char_traits.h:39,
from /usr/include/c++/8/ios:40,
from /usr/include/c++/8/ostream:38,
from /usr/include/c++/8/iostream:39,
from 4.cpp:1:
/usr/include/c++/8/bits/stl_iterator.h:358:5: 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++/8/bits/stl_iterator.h:358:5: note: template argument deduction/substitution failed:
In file included from /usr/include/c++/8/string:48,
from /usr/include/c++/8/bits/locale_classes.h:40,
from /usr/include/c++/8/bits/ios_base.h:41,
from /usr/include/c++/8/ios:42,
from /usr/include/c++/8/ostream:38,
from /usr/include/c++/8/iostream:39,
from 4.cpp:1:
/usr/include/c++/8/bits/stl_function.h:376:20: note: ‘const Test’ is not derived from ‘const std::reverse_iterator<_Iterator>’
{ return __x > __y; }
~~~~^~~~~
In file included from /usr/include/c++/8/bits/stl_algobase.h:67,
from /usr/include/c++/8/bits/char_traits.h:39,
from /usr/include/c++/8/ios:40,
from /usr/include/c++/8/ostream:38,
from /usr/include/c++/8/iostream:39,
from 4.cpp:1:
/usr/include/c++/8/bits/stl_iterator.h:1163:5: note: candidate: ‘template<class _IteratorL, class _IteratorR> bool std::operator>(const std::move_iterator<_IteratorL>&, const std::move_iterator<_IteratorR>&)’
operator>(const move_iterator<_IteratorL>& __x,
^~~~~~~~
/usr/include/c++/8/bits/stl_iterator.h:1163:5: note: template argument deduction/substitution failed:
In file included from /usr/include/c++/8/string:48,
from /usr/include/c++/8/bits/locale_classes.h:40,
from /usr/include/c++/8/bits/ios_base.h:41,
from /usr/include/c++/8/ios:42,
from /usr/include/c++/8/ostream:38,
from /usr/include/c++/8/iostream:39,
from 4.cpp:1:
/usr/include/c++/8/bits/stl_function.h:376:20: note: ‘const Test’ is not derived from ‘const std::move_iterator<_IteratorL>’
{ return __x > __y; }
~~~~^~~~~
In file included from /usr/include/c++/8/bits/stl_algobase.h:67,
from /usr/include/c++/8/bits/char_traits.h:39,
from /usr/include/c++/8/ios:40,
from /usr/include/c++/8/ostream:38,
from /usr/include/c++/8/iostream:39,
from 4.cpp:1:
/usr/include/c++/8/bits/stl_iterator.h:1169:5: note: candidate: ‘template<class _Iterator> bool std::operator>(const std::move_iterator<_IteratorL>&, const std::move_iterator<_IteratorL>&)’
operator>(const move_iterator<_Iterator>& __x,
^~~~~~~~
/usr/include/c++/8/bits/stl_iterator.h:1169:5: note: template argument deduction/substitution failed:
In file included from /usr/include/c++/8/string:48,
from /usr/include/c++/8/bits/locale_classes.h:40,
from /usr/include/c++/8/bits/ios_base.h:41,
from /usr/include/c++/8/ios:42,
from /usr/include/c++/8/ostream:38,
from /usr/include/c++/8/iostream:39,
from 4.cpp:1:
/usr/include/c++/8/bits/stl_function.h:376:20: note: ‘const Test’ is not derived from ‘const std::move_iterator<_IteratorL>’
{ return __x > __y; }
~~~~^~~~~
In file included from /usr/include/c++/8/string:52,
from /usr/include/c++/8/bits/locale_classes.h:40,
from /usr/include/c++/8/bits/ios_base.h:41,
from /usr/include/c++/8/ios:42,
from /usr/include/c++/8/ostream:38,
from /usr/include/c++/8/iostream:39,
from 4.cpp:1:
/usr/include/c++/8/bits/basic_string.h:6171:5: 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++/8/bits/basic_string.h:6171:5: note: template argument deduction/substitution failed:
In file included from /usr/include/c++/8/string:48,
from /usr/include/c++/8/bits/locale_classes.h:40,
from /usr/include/c++/8/bits/ios_base.h:41,
from /usr/include/c++/8/ios:42,
from /usr/include/c++/8/ostream:38,
from /usr/include/c++/8/iostream:39,
from 4.cpp:1:
/usr/include/c++/8/bits/stl_function.h:376:20: note: ‘const Test’ is not derived from ‘const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>’
{ return __x > __y; }
~~~~^~~~~
In file included from /usr/include/c++/8/string:52,
from /usr/include/c++/8/bits/locale_classes.h:40,
from /usr/include/c++/8/bits/ios_base.h:41,
from /usr/include/c++/8/ios:42,
from /usr/include/c++/8/ostream:38,
from /usr/include/c++/8/iostream:39,
from 4.cpp:1:
/usr/include/c++/8/bits/basic_string.h:6184:5: 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++/8/bits/basic_string.h:6184:5: note: template argument deduction/substitution failed:
In file included from /usr/include/c++/8/string:48,
from /usr/include/c++/8/bits/locale_classes.h:40,
from /usr/include/c++/8/bits/ios_base.h:41,
from /usr/include/c++/8/ios:42,
from /usr/include/c++/8/ostream:38,
from /usr/include/c++/8/iostream:39,
from 4.cpp:1:
/usr/include/c++/8/bits/stl_function.h:376:20: note: ‘const Test’ is not derived from ‘const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>’
{ return __x > __y; }
~~~~^~~~~
In file included from /usr/include/c++/8/string:52,
from /usr/include/c++/8/bits/locale_classes.h:40,
from /usr/include/c++/8/bits/ios_base.h:41,
from /usr/include/c++/8/ios:42,
from /usr/include/c++/8/ostream:38,
from /usr/include/c++/8/iostream:39,
from 4.cpp:1:
/usr/include/c++/8/bits/basic_string.h:6196:5: 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++/8/bits/basic_string.h:6196:5: note: template argument deduction/substitution failed:
In file included from /usr/include/c++/8/string:48,
from /usr/include/c++/8/bits/locale_classes.h:40,
from /usr/include/c++/8/bits/ios_base.h:41,
from /usr/include/c++/8/ios:42,
from /usr/include/c++/8/ostream:38,
from /usr/include/c++/8/iostream:39,
from 4.cpp:1:
/usr/include/c++/8/bits/stl_function.h:376:20: note: mismatched types ‘const _CharT*’ and ‘Test’
{ return __x > __y; }
~~~~^~~~~
二、报错代码
#include<iostream>
#include<algorithm>
using namespace std;
class Test{
public:
int k;
bool operator>(const Test &a){
return this->k > a.k;
}
};
Test A[100];
int main(){
for(int i = 0;i < 100;i++)
A[i].k = i;
sort(A,A + 100,greater<Test>());
return 0;
}
三、修正后的代码
#include<iostream>
#include<algorithm>
using namespace std;
class Test{
public:
int k;
bool operator>(const Test &a)const{
return this->k > a.k;
}
};
Test A[100];
int main(){
for(int i = 0;i < 100;i++)
A[i].k = i;
sort(A,A + 100,greater<Test>());
return 0;
}
四、原因和总结
原代码报错的地方是在greater<Test>()里.
greater的定义非常简单：
template <class T> struct greater {
bool operator() (const T& x, const T& y) const {return x>y;}
typedef T first_argument_type;
typedef T second_argument_type;
typedef bool result_type;
};
即简单第将x > y的值返回。
为什么这里会出错呢？
我们注意到greater类的小括号重载的参数为两个用const修饰的常对象。
因为是常对象，所以只能使用常成员，而我们原先的大于号重载是这样的：
bool operator>(const Test &a){
return this->k > a.k;
}
即不是常成员，所以不能被成功调用。
使用const修饰之后，问题解决。
原创不易，感谢支持。