push_back an element into a vector of an another structure vector's back()-CodePudding

struct Thing
{
    int id;
    std::vector<int> v2;
};

std::vector<Thing> v1;
int main()
{
    int n;
    cin>>n;

    for(int i=0;i<n;i  )
    {
        Thing pic;
        cin>>pic.id;
        v1.push_back(pic);
        int x;
        cin>>x;
        v1.back().v2.push_back(x);
    }
}

v1 is not an empty vector. I can't understand the line v1.back().v2.push_back(x); what's the actual meaning of this line

CodePudding user response：

v1.back() is the last element of v1 with the type of Thing

v1.back().v2 is the v2 member of that element with the type of vector<int>

So you are calling the push_back() on a vector<int> with an int what is fine

Note that if v1 is empty then v1.back() is UB

Edit: v1.back().v2.push_back(x); is kind of like

{
Thing& t=v1.back();
vector<int>& v=t.v2;
v.push_back(x);
}