为什么我不能像这样插入对值？ v.push_back（矢量<pair<int,string> &gt;({1,"A"}));? </pair<int,string>

Question

I have written this code and want to insert value in the vector of vector like this: v.push_back(vector<pair<int,string> >({1,"A"})); ??
But it is showing error....

Inserting an empty value is working in this code. But I want to know why this is not possible??

#include<bits/stdc++.h>
using namespace std;

void printVec(vector<pair<int,string> > &v){
    cout << "Size: " << v.size() << endl;
    for(int i=0; i<v.size(); i++)
    {
        cout << v[i].first << " " << v[i].second << endl;
    }
}

int main()
{
    vector<vector<pair<int,string> > > v;

    int N;
    cin >> N;
    for(int i=0; i<N; i++)
    {
        int n;
        cin >> n;
        v.push_back(vector<pair<int,string> >({}));
        for(int j=0; j<n; j++)
        {
            int x;
            string s;
            cin >> x >>s;
            v[i].push_back({x,s});
        }
    }

    v.push_back(vector<pair<int,string> >({})); // UNABLE TO STORE A PAIR AT THE END. ONLY CAN ADD EMPTY PAIR. DON'T KNOW WHY??
    // v.push_back({}); // USING ANY OF THEM

    for(int i=0; i<v.size(); i++)
    {
        printVec(v[i]);
    }
}

I want to insert a value at the end of a vector of vector of pair<int, string> type. Using this:- v.push_back(vector<pair<int,string> >({1,"A"}));

But it showing error.

When using this:-

v.push_back(vector<pair<int,string> >({}));

it working fine. Please explain why??

Answer 1

Here you are pushing a pair instead of a vector of pairs.

v.push_back(vector<pair<int,string> >({1,"A"}));

This works because it considers ({}) as an empty vector of pairs. `

v.push_back(vector<pair<int,string> >({}));`

Although the following will work:

vector<pair<int,string> > temp;
temp.push_back({1, "Hi"});
v.push_back(vector<pair<int,string> >({temp}));

Answer 2

If you want to list-initialize your vector with the {1,"A"} as the initializer for one element of the std::initializer_list constructor you need to use braces, not parentheses:

vector<pair<int,string>>{{1,"A"}}

Or alternatively, if you aren't sure how to initialize directly with an element in the vector or the above is not something you learned about yet, simply define a variable to temporarily hold the inner vector you want to push into v , push the pair element into it and then use that variable. Then you don't have to guess anything:

vector<pair<int,string>> inner;
inner.push_back({1,"A"});
 
v.push_back(inner); // alternatively `std::move(inner)` if `inner` is not used later

---

vector<pair<int,string> >({}) works because the {} argument can initialize the std::initializer_list of the std::initializer_list -constructor for std::vector directly to be empty. An additional set of braces in your original attempt would have worked the same way as well, although then the parentheses would be redundant.