c++ - 通过引用传递2D vector

我已经有一段时间没有接触过c++了，在通过引用传递2D vector 时，我刚遇到一个非常奇怪的问题，它在main函数中进行了描述:

class Solution {
public:
    vector<vector<int> > permute(vector<int> &num) {


    vector<vector<int> > result;
    //I intentionally omit the codes for permutation,
    //only pushing the original one to the result
    result.push_back(num);

    return result;


    }

    //print the permutation, I want to pass by referrence
    void printPerm(vector<vector<int> >  &result) {

    for(int i=0; i<result.size(); i++) {

        for(int j=0; j<result[i].size(); j++) {

            cout << result[i][j]<<" ";
            }
        }

    }

};

int main() {

    vector<int> old;
    old.push_back(1);
    old.push_back(2);
    old.push_back(3);

    Solution sol;

    //this gives me error for no matching function
    //call, candidate argument doesn't match
    sol.printPerm((sol.permute(old)));

    //but the one below works!

    vector<vector<int> > result = sol.permute(old);

    sol.printPerm(result);


}

最佳答案

您需要添加const作为结果以允许绑定(bind)到右值

 void printPerm(const vector<vector<int> >  &result)
 //             ^^^^^^