c++ - 使用 std::set 实现稀疏 3D 网格时出错

Question

我正在尝试使用 std::set 容器实现稀疏 3D 网格，但我无法理解编译器返回的错误，这是我尝试运行的最小示例:

#include <iostream>
#include <vector>
#include <limits>
#include <set>

#include <Eigen/Core>

using namespace std;

class Cell {
public:
    EIGEN_MAKE_ALIGNED_OPERATOR_NEW
    Cell(const Eigen::Vector3i idx=Eigen::Vector3i::Zero()):_idx(idx) {
        _center = Eigen::Vector3f::Zero();
        _parent = 0;
        _distance = std::numeric_limits<int>::max();
    }

    inline bool operator < (const Cell& c){
        for (int i=0; i<3; i++){
            if (_idx[i]<c._idx[i])
                return true;
            if (_idx[i]>c._idx[i])
                return false;
        }
        return false;
    }

    inline bool operator == (const Cell& c) { return c._idx == _idx;}

private:
    Eigen::Vector3i _idx;
    Eigen::Vector3f _center;
    vector<Eigen::Vector3f> _points;
    Cell* _parent;
    size_t _closest_point;
    float _distance;
    int _tag;
};


int main(int argc, char* argv[]) {

    set<Cell> grid;

    float max = 1, min = -1;
    int dim = 5;
    float delta = (max-min)/(dim-1);

    for(int k = 0; k < dim; k++)
        for(int j = 0; j < dim; j++)
            for(int i = 0; i < dim; i++)
                grid.insert(Cell(Eigen::Vector3i(i,j,k)));

    return 0;
}

这是编译器错误:

In file included from /usr/include/c++/4.8/string:48:0, from /usr/include/c++/4.8/bits/locale_classes.h:40, from /usr/include/c++/4.8/bits/ios_base.h:41, from /usr/include/c++/4.8/ios:42, from /usr/include/c++/4.8/ostream:38, from /usr/include/c++/4.8/iostream:39, from /home/dede/build/sparse_grid/main.cpp:1: /usr/include/c++/4.8/bits/stl_function.h: In instantiation of 'bool std::less<_Tp>::operator()(const _Tp&, const _Tp&) const [with _Tp = Cell]': /usr/include/c++/4.8/bits/stl_tree.h:1324:11: required from 'std::pair std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_M_get_insert_unique_pos(const key_type&) [with _Key = Cell; _Val = Cell; _KeyOfValue = std::_Identity; _Compare = std::less; _Alloc = std::allocator; std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::key_type = Cell]' /usr/include/c++/4.8/bits/stl_tree.h:1377:47: required from 'std::pair, bool> std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_M_insert_unique(_Arg&&) [with _Arg = Cell; _Key = Cell; _Val = Cell; _KeyOfValue = std::_Identity; _Compare = std::less; _Alloc = std::allocator]' /usr/include/c++/4.8/bits/stl_set.h:472:40:
required from 'std::pair, _Compare, typename _Alloc::rebind<_Key>::other>::const_iterator, bool> std::set<_Key, _Compare, _Alloc>::insert(std::set<_Key, _Compare, _Alloc>::value_type&&) [with _Key = Cell; _Compare = std::less; _Alloc = std::allocator; typename std::_Rb_tree<_Key, _Key, std::_Identity<_Key>, _Compare, typename _Alloc::rebind<_Key>::other>::const_iterator = std::_Rb_tree_const_iterator; std::set<_Key, _Compare, _Alloc>::value_type = Cell]' /home/dede/build/sparse_grid/main.cpp:53:57: required from here /usr/include/c++/4.8/bits/stl_function.h:235:20: error: passing 'const Cell' as 'this' argument of 'bool Cell::operator<(const Cell&)' discards qualifiers [-fpermissive] { return __x < __y; } ^ make[2]: * [CMakeFiles/sparse_grid.dir/main.cpp.o] Error 1 make[1]: * [CMakeFiles/sparse_grid.dir/all] Error 2 make: *** [all] Error 2

如果有人能告诉我我做错了什么，我将不胜感激。

谢谢，费德里科

Answer 1

您应该将 bool 运算符函数声明为 const 成员:

inline bool operator < (const Cell& c) const {
                                    // ^^^^^
    for (int i=0; i<3; i++){
        if (_idx[i]<c._idx[i])
            return true;
        if (_idx[i]>c._idx[i])
            return false;
    }
    return false;
}

inline bool operator == (const Cell& c) const { return c._idx == _idx;}
                                     // ^^^^^

否则这些不能与 Cell 的右值对象一起使用。