为什么我不能在 C++ sort() 中使用成员函数 cmp？答案

【问题标题】：Why can't I use a member function cmp in C++ sort()？为什么我不能在 C++ sort() 中使用成员函数 cmp？
【发布时间】：2021-07-24 09:43:01
【问题描述】：

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，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，或者将它移到课堂之外，一切都会好起来的。为什么会这样？这些cmp 函数有什么区别？

【问题讨论】：

因为非静态成员函数需要调用对象。
因为（非静态）成员函数需要调用一个对象，而sort 函数没有MyClass 对象，所以它根本无法正确调用该函数。要么制作比较功能static，要么学习如何使用lambdas。或者因为默认比较是使用小于不传递任何函数（即仅sort(begin(v), end(v))）。
sort 需要一个可调用对象。在 C++ 中，引用成员函数（即非静态）会删除任何可能已使用的 this 指针，从而产生一个普通的成员函数指针。使用std::bind 使该实例可通过特定实例（即this）调用。
成员函数必须使用 MyClass 类型的对象调用，但 std::sort 仅适用于可以在没有对象的情况下调用的函数。
还有：为什么void mySort(vector<int>& v) 是myClass 的成员？

标签： c++ gcc

【解决方案1】：

一个非静态成员函数需要一个对象来调用。所以其实MyClass::cmp函数接受了三个参数，第三个（其实就是第一个）就是this。

您的问题的一个可能解决方案是使用 lambda 表达式。可以这样写。

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));

【讨论】：

解决办法是把static加到cmp()