为什么我不能在 C++ sort() 中使用成员函数 cmp？

Question

我正在创建一个名为MyClass的类，其中一个成员函数使用 C++ sort()函数，并且我将另一个成员函数cmp作为参数传递给它。

#include <algorithm>
using namespace std;

/* If I use this cmp for sort(), there is no error.
bool cmp(int& x, int& y) {
    return x < y;
}
*/

class MyClass {
public:
    /* Or I use this cmp, there is no error too.
    static bool cmp(int& x, int& y) {
        return x < y;
    }
    */

    // compile failed.
    bool cmp (int& x, int& y) {
        return x < y;
    }

    void mySort(vector<int>& v) {
        sort(v.begin(), v.end(), cmp);
    }
};

（我知道cmp函数实际上是多余的，但显示错误并不重要）。 然后编译器给出以下错误信息：

In file included from C:/MinGW/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/bits/stl_algobase.h:71,
                 from C:/MinGW/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/bits/char_traits.h:39,
                 from C:/MinGW/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/ios:40,
                 from C:/MinGW/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/ostream:38,
                 from C:/MinGW/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/iostream:39,
                 from Untitled.cpp:1:
C:/MinGW/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/bits/predefined_ops.h: In instantiation of 'constexpr bool __gnu_cxx::__ops::_Iter_comp_iter<_Compare>::operator()(_Iterator1, _Iterator2) [with _Iterator1 = __gnu_cxx::__normal_iterator<int*, std::vector<int> >; _Iterator2 = __gnu_cxx::__normal_iterator<int*, std::vector<int> >; _Compare = bool (MyClass::*)(const int&, const int&)]':
C:/MinGW/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/bits/stl_algo.h:81:17:   required from 'void std::__move_median_to_first(_Iterator, _Iterator, _Iterator, _Iterator, _Compare) [with _Iterator = __gnu_cxx::__normal_iterator<int*, std::vector<int> >; _Compare = __gnu_cxx::__ops::_Iter_comp_iter<bool (MyClass::*)(const int&, const int&)>]'
C:/MinGW/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/bits/stl_algo.h:1921:34:   required from '_RandomAccessIterator std::__unguarded_partition_pivot(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<int*, std::vector<int> >; _Compare = __gnu_cxx::__ops::_Iter_comp_iter<bool (MyClass::*)(const int&, const int&)>]'
C:/MinGW/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/bits/stl_algo.h:1953:38:   required from 'void std::__introsort_loop(_RandomAccessIterator, _RandomAccessIterator, _Size, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<int*, std::vector<int> >; _Size = long 
long int; _Compare = __gnu_cxx::__ops::_Iter_comp_iter<bool (MyClass::*)(const int&, const int&)>]'
C:/MinGW/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/bits/stl_algo.h:1968:25:   required from 'void std::__sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<int*, std::vector<int> >; _Compare = __gnu_cxx::__ops::_Iter_comp_iter<bool (MyClass::*)(const int&, const int&)>]'
C:/MinGW/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/bits/stl_algo.h:4866:18:   required from 'void std::sort(_RAIter, _RAIter, _Compare) [with _RAIter = __gnu_cxx::__normal_iterator<int*, std::vector<int> >; _Compare = bool (MyClass::*)(const int&, const int&)]'
Untitled.cpp:28:37:   required from here
C:/MinGW/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/bits/predefined_ops.h:143:18: error: must use '.*' or '->*' to call pointer-to-member function in '((__gnu_cxx::__ops::_Iter_comp_iter<bool (MyClass::*)(const int&, const int&)>*)this)->__gnu_cxx::__ops::_Iter_comp_iter<bool (MyClass::*)(const int&, const int&)>::_M_comp (...)', e.g. '(... ->* ((__gnu_cxx::__ops::_Iter_comp_iter<bool (MyClass::*)(const int&, const int&)>*)this)->__gnu_cxx::__ops::_Iter_comp_iter<bool (MyClass::*)(const int&, const int&)>::_M_comp) (...)'
         { return bool(_M_comp(*__it1, *__it2)); }
                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
C:/MinGW/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/bits/predefined_ops.h: In instantiation of 'bool __gnu_cxx::__ops::_Val_comp_iter<_Compare>::operator()(_Value&, _Iterator) [with _Value = int; _Iterator = __gnu_cxx::__normal_iterator<int*, std::vector<int> >; _Compare = bool (MyClass::*)(const int&, const int&)]':
C:/MinGW/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/bits/stl_algo.h:1828:20:   required from 'void std::__unguarded_linear_insert(_RandomAccessIterator, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<int*, std::vector<int> >; _Compare = __gnu_cxx::__ops::_Val_comp_iter<bool (MyClass::*)(const int&, const int&)>]'
C:/MinGW/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/bits/stl_algo.h:1855:36:   required from 'void std::__insertion_sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<int*, std::vector<int> >; _Compare = __gnu_cxx::__ops::_Iter_comp_iter<bool (MyClass::*)(const int&, const int&)>]'
C:/MinGW/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/bits/stl_algo.h:1885:25:   required from 'void std::__final_insertion_sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<int*, std::vector<int> >; _Compare = __gnu_cxx::__ops::_Iter_comp_iter<bool (MyClass::*)(const int&, const int&)>]'
C:/MinGW/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/bits/stl_algo.h:1971:31:   required from 'void std::__sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<int*, std::vector<int> >; _Compare = __gnu_cxx::__ops::_Iter_comp_iter<bool (MyClass::*)(const int&, const int&)>]'
C:/MinGW/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/bits/stl_algo.h:4866:18:   required from 'void std::sort(_RAIter, _RAIter, _Compare) [with _RAIter = __gnu_cxx::__normal_iterator<int*, std::vector<int> >; _Compare = bool (MyClass::*)(const int&, const int&)]'
Untitled.cpp:28:37:   required from here
C:/MinGW/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/bits/predefined_ops.h:215:11: error: must use '.*' or '->*' to call pointer-to-member function in '((__gnu_cxx::__ops::_Val_comp_iter<bool (MyClass::*)(const int&, const int&)>*)this)->__gnu_cxx::__ops::_Val_comp_iter<bool (MyClass::*)(const int&, const int&)>::_M_comp (...)', e.g. '(... ->* ((__gnu_cxx::__ops::_Val_comp_iter<bool (MyClass::*)(const int&, const int&)>*)this)->__gnu_cxx::__ops::_Val_comp_iter<bool (MyClass::*)(const int&, const int&)>::_M_comp) (...)'
  { return bool(_M_comp(__val, *__it)); }
           ^~~~~~~~~~~~~~~~~~~~~~~~~~~
C:/MinGW/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/bits/predefined_ops.h: In instantiation of 'bool __gnu_cxx::__ops::_Iter_comp_val<_Compare>::operator()(_Iterator, _Value&) [with _Iterator = __gnu_cxx::__normal_iterator<int*, std::vector<int> >; _Value = int; _Compare = bool (MyClass::*)(const int&, const int&)]':
C:/MinGW/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/bits/stl_heap.h:133:48:   required from 'void std::__push_heap(_RandomAccessIterator, _Distance, _Distance, _Tp, _Compare&) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<int*, std::vector<int> >; _Distance = long long int; _Tp = int; _Compare = __gnu_cxx::__ops::_Iter_comp_val<bool (MyClass::*)(const int&, const int&)>]'
C:/MinGW/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/bits/stl_heap.h:237:23:   required from 'void std::__adjust_heap(_RandomAccessIterator, 
_Distance, _Distance, _Tp, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<int*, std::vector<int> >; _Distance = long long int; _Tp = int; _Compare = __gnu_cxx::__ops::_Iter_comp_iter<bool (MyClass::*)(const int&, const int&)>]'
C:/MinGW/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/bits/stl_heap.h:342:22:   required from 'void std::__make_heap(_RandomAccessIterator, _RandomAccessIterator, _Compare&) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<int*, std::vector<int> >; _Compare = __gnu_cxx::__ops::_Iter_comp_iter<bool (MyClass::*)(const int&, const int&)>]'
C:/MinGW/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/bits/stl_algo.h:1672:23:   required from 'void std::__heap_select(_RandomAccessIterator, _RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<int*, std::vector<int> >; 
_Compare = __gnu_cxx::__ops::_Iter_comp_iter<bool (MyClass::*)(const int&, const int&)>]'
C:/MinGW/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/bits/stl_algo.h:1933:25:   required from 'void std::__partial_sort(_RandomAccessIterator, _RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<int*, std::vector<int> >; _Compare = __gnu_cxx::__ops::_Iter_comp_iter<bool (MyClass::*)(const int&, const int&)>]'
C:/MinGW/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/bits/stl_algo.h:1948:27:   required from 'void std::__introsort_loop(_RandomAccessIterator, _RandomAccessIterator, _Size, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<int*, std::vector<int> >; _Size = long 
long int; _Compare = __gnu_cxx::__ops::_Iter_comp_iter<bool (MyClass::*)(const int&, const int&)>]'
C:/MinGW/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/bits/stl_algo.h:1968:25:   required from 'void std::__sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<int*, std::vector<int> >; _Compare = __gnu_cxx::__ops::_Iter_comp_iter<bool (MyClass::*)(const int&, const int&)>]'
C:/MinGW/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/bits/stl_algo.h:4866:18:   required from 'void std::sort(_RAIter, _RAIter, _Compare) [with _RAIter = __gnu_cxx::__normal_iterator<int*, std::vector<int> >; _Compare = bool (MyClass::*)(const int&, const int&)]'
Untitled.cpp:28:37:   required from here
C:/MinGW/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/bits/predefined_ops.h:177:11: error: must use '.*' or '->*' to call pointer-to-member function in '((__gnu_cxx::__ops::_Iter_comp_val<bool (MyClass::*)(const int&, const int&)>*)this)->__gnu_cxx::__ops::_Iter_comp_val<bool (MyClass::*)(const int&, const int&)>::_M_comp (...)', e.g. '(... ->* ((__gnu_cxx::__ops::_Iter_comp_val<bool (MyClass::*)(const int&, const int&)>*)this)->__gnu_cxx::__ops::_Iter_comp_val<bool (MyClass::*)(const int&, const int&)>::_M_comp) (...)'
  { return bool(_M_comp(*__it, __val)); }
           ^~~~~~~~~~~~~~~~~~~~~~~~~~~

我还发现，如果我将cmp函数设为静态，或者将其移到课堂外，一切都会好起来的。 为什么会发生这种情况？ 这些cmp函数之间有什么区别？

Answer 1

非静态成员函数需要一个对象来调用。 所以实际上， MyClass::cmp函数接受三个参数，第三个（实际上是第一个）是this 。

您的问题的一个可能解决方案是使用 lambda 表达式。 可以写成如下。

std::sort(v.begin(), v.end(), [this](auto a, auto b){ return this->cmp(a, b);});

或者你可以使用std::bind 。

using namespace std::placeholders;
std::sort(v.begin(), v.end(), std::bind(&MyClass::cmp, this, _1, _2));