vector string push_back is not working in c++

Question

This code snippet receives string, delimiter(space) and vector as argument and splits the string according to delimiter and stores it in vector. It is not storing anything into vector if i use push_back but works if i use [] operator. Can someone explain why push_back is not working?

void split(const string & input,char delim,vector<string> & elems){
    stringstream  ss;
    ss.str(input);
    string item;
    int i = 0;
    while(getline(ss,item,delim)){
        //elems.push_back(item);
        elems[i] = item;
        i++;
    }
}

int main(){
   char delim = ' ';
   vector<string> item(2);
   string input;
   getline(cin,input);
   split(input,delim,item);
}

Answer 1

If you've pre-allocated the vector with some size (n), then pushback(item) puts item at index n and resizes the vector to an even larger size. If you know the string count due in, then you should use elems[i] = item; anyway after an allocation of size n.

If you don't know the count coming in, but know it's going to be larger than some n, do not pre-allocate. Instead, RESERVE some memory with elems.reserve(n);

Then use elems.push_back(item);