如何对UDT Vector降序排序？重载operator>报错解决方案

问题描述 #

我需要对自定义类型Point的vector进行降序排序，于是在Point类中重载了operator>运算符，并用sort搭配greater<Point>()执行排序，但编译报错，错误提示传递const Point*作为this参数丢弃了限定符。

原代码 #

#include <bits/stdc++.h>

using namespace std;

class Point
{
public:
    int x;
    int y;

    Point(int x = 0, int y = 0)
    {
        this->x = x;
        this->y = y;
    }

    bool operator>(const Point &p1)
    {
        return (x + y) > (p1.x + p1.y);
    }
};

int main()
{
    vector<Point> v = {{1, 2}, {3, 1}, {0, 1}};

    sort(v.begin(), v.end(), greater<Point>());

    for (auto i : v)
        cout << i.x << " " << i.y << endl;

    return 0;
}

错误信息 #

In file included from /opt/compiler-explorer/gcc-trunk-20221128/include/c++/13.0.0/string:49,
from /opt/compiler-explorer/gcc-trunk-20221128/include/c++/13.0.0/bitset:52,
from /opt/compiler-explorer/gcc-trunk-20221128/include/c++/13.0.0/x86_64-linux-gnu/bits/stdc++.h:52,
from :1:
/opt/compiler-explorer/gcc-trunk-20221128/include/c++/13.0.0/bits/stl_function.h: In instantiation of 'constexpr bool std::greater<_Tp>::operator()(const _Tp&, const _Tp&) const [with _Tp = Point]':
/opt/compiler-explorer/gcc-trunk-20221128/include/c++/13.0.0/bits/predefined_ops.h:158:30: required from 'constexpr bool __gnu_cxx::__ops::_Iter_comp_iter<_Compare>::operator()(_Iterator1, _Iterator2) [with _Iterator1 = __gnu_cxx::__normal_iterator<Point*, std::vector >; _Iterator2 = __gnu_cxx::__normal_iterator<Point*, std::vector >; _Compare = std::greater]'
/opt/compiler-explorer/gcc-trunk-20221128/include/c++/13.0.0/bits/stl_algo.h:1819:14: required from 'void std::__insertion_sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<Point*, vector >; _Compare = __gnu_cxx::__ops::_Iter_comp_iter<greater >]'
/opt/compiler-explorer/gcc-trunk-20221128/include/c++/13.0.0/bits/stl_algo.h:1859:25: required from 'void std::__final_insertion_sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<Point*, vector >; _Compare = __gnu_cxx::__ops::_Iter_comp_iter<greater >]'
/opt/compiler-explorer/gcc-trunk-20221128/include/c++/13.0.0/bits/stl_algo.h:1950:31: required from 'void std::__sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<Point*, vector >; _Compare = __gnu_cxx::__ops::_Iter_comp_iter<greater >]'
/opt/compiler-explorer/gcc-trunk-20221128/include/c++/13.0.0/bits/stl_algo.h:4893:18: required from 'void std::sort(_RAIter, _RAIter, _Compare) [with _RAIter = __gnu_cxx::__normal_iterator<Point*, vector >; _Compare = greater]'

:27:9: required from here /opt/compiler-explorer/gcc-trunk-20221128/include/c++/13.0.0/bits/stl_function.h:398:20: error: no match for 'operator>' (operand types are 'const Point' and 'const Point') 398 | { return __x > __y; } | ~~~~^~~~~ :17:10: note: candidate: 'bool Point::operator>(const Point&)' (near match) 17 | bool operator>(const Point &p1) | ^~~~~~~~ :17:10: note: passing 'const Point*' as 'this' argument discards qualifiers

错误原因 #

std::greater<Point>的operator()会接收两个const引用的Point对象，调用__x > __y时，__x是const对象。但你的operator>成员函数没有被标记为const，意味着编译器认为它可能会修改调用对象的状态，因此不允许在const对象上调用这个非const成员函数，最终抛出"丢弃const限定符"的错误。

解决方案 #

方案1：修改operator>为const成员函数 #

给operator>添加const修饰符，明确告知编译器该函数不会修改调用对象的状态，允许在const对象上调用：

bool operator>(const Point &p1) const
{
    return (x + y) > (p1.x + p1.y);
}

完整修改后的代码：

#include <bits/stdc++.h>

using namespace std;

class Point
{
public:
    int x;
    int y;

    Point(int x = 0, int y = 0)
    {
        this->x = x;
        this->y = y;
    }

    // 添加const修饰符
    bool operator>(const Point &p1) const
    {
        return (x + y) > (p1.x + p1.y);
    }
};

int main()
{
    vector<Point> v = {{1, 2}, {3, 1}, {0, 1}};

    sort(v.begin(), v.end(), greater<Point>());

    for (auto i : v)
        cout << i.x << " " << i.y << endl;

    return 0;
}

方案2：使用自定义lambda比较器 #

如果不想修改Point类的成员函数，可以直接给sort传递lambda表达式作为比较器，这种方式更灵活，适合仅在特定场景需要自定义排序的情况：

#include <bits/stdc++.h>

using namespace std;

class Point
{
public:
    int x;
    int y;

    Point(int x = 0, int y = 0)
    {
        this->x = x;
        this->y = y;
    }
};

int main()
{
    vector<Point> v = {{1, 2}, {3, 1}, {0, 1}};

    // 使用lambda作为自定义比较器
    sort(v.begin(), v.end(), [](const Point& a, const Point& b) {
        return (a.x + a.y) > (b.x + b.y);
    });

    for (auto i : v)
        cout << i.x << " " << i.y << endl;

    return 0;
}

内容的提问来源于stack exchange，提问作者Rushabh Laddha