vector string push_back is not working in c++

问题:

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);
}

回答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);

回答2:

In the main function, please change the declaration of item from

vector<string> item(2);

to

vector<string> item;

And change the split function to use push_back:

void split(const std::string & input,char delim, std::vector<std::string> & elems){
    std::stringstream  ss;
    ss.str(input);
    std::string item;
    int i = 0; // no more required unless for some other purpose
    while(getline(ss,item,delim)){
        elems.push_back(item);
        //elems[i] = item;
        i++; // no more required
    }
}