Why "std::vector<std::string> &" instead of "void"?

Question

I would like to use the string splitting function for the highest rated answer to this question:

Split a string in C++?

Copying the answer for convenience:

#include <string>
#include <sstream>
#include <vector>

std::vector<std::string> &split(const std::string &s, char delim, std::vector<std::string> &elems) {
    std::stringstream ss(s);
    std::string item;
    while (std::getline(ss, item, delim)) {
        elems.push_back(item);
    }
    return elems;
}


std::vector<std::string> split(const std::string &s, char delim) {
    std::vector<std::string> elems;
    split(s, delim, elems);
    return elems;
}

I have a question here. Why the declaration is std::vector<std::string> &split(...) rather than void split(...)? As an argument we receive std::vector<std::string> &elems and this is what we want to end up having. Why return anything if we receive a reference?

I have:

int &multA (int &n)
{
    n = 5*n;
    return n;
}

void multB (int &n)
{
    n = 5*n;
}

and:

int n1, n2;
n1 = 1;
n2 = 1;
cout << "n1: " << n1 << endl;
cout << "n2: " << n2 << endl;
n1 = multA(n1);
multB(n2);
cout << "n1: " << n1 << endl;
cout << "n2: " << n2 << endl;

And as a result I have:

n1: 1
n2: 1
n1: 5
n2: 5

What's the difference then?

Answer 1

Returning a reference allows a call to split() to be used as an argument to another function. For example, assume we have some other function that accepts a vector of strings by reference

  void func(std::vector<std::string> &elements);

If split() returns a reference, it is possible to pass the reference it returns directly to func().

  func(split(s, delim, elems));

whereas, if split() did not return a reference, it would be necessary to do

  split(s,delim, elems);
  func(elems);

The second usage approach still works if split() returns a reference.

The difference between these two is purely programmer convenience, but a lot of programmers prefer to use the first calling style. Returning a reference is not a particularly expensive operation so allowing the programmer a choice in how to use it does not lose much.

Answer 2

It's syntactic sugar that make the functions more flexible.

By example: you can write

std::cout << "there are " << split(str, delim, vect).size() << " elemements\n";

With the void hypothesis case, you need 2 statements.