C++ Cookbook

15.6. Initializing a Sequence with Comma-Separated Values

Problem

You want to initialize a sequence with a comma-delimited set of values, like you can with a built-in array.

Solution

You can use a comma-initialization syntax on standard sequences (such as vector and list) by defining a helper class and overloading the comma operator for it as demonstrated in Example 15-6.

Example 15-6. Utilities for comma initialization of standard sequences

#include <vector>
#include <iostream>
#include <iterator>
#include <algorithm>

using namespace std; 

template<class Seq_T>
struct comma_helper
{
  typedef typename Seq_T::value_type value_type;
  explicit comma_helper(Seq_T& x) : m(x) { }
  comma_helper& operator=(const value_type& x) {
    m.clear();
    return operator+=(x);
  }
  comma_helper& operator+=(const value_type& x) {
    m.push_back(x);
    return *this;
  }
  Seq_T& m;
};

template<typename Seq_T>
comma_helper<Seq_T>
initialize(Seq_T& x) {
  return comma_helper<Seq_T>(x);
}

template<class Seq_T, class Scalar_T>
comma_helper<Seq_T>&
operator,(comma_helper<Seq_T>& h, Scalar_T x) {
  h += x;
  return h;
}

int main() {
  vector v;
  int a = 2;
  int b = 5;
  initialize(v) = 0, 1, 1, a, 3, b, 8, 13;
  cout << v[3] << endl; // outputs 2
  system("pause");
  return EXIT_SUCCESS;
}

Discussion

Often time standard sequences are initialized by calling a push_back member function several times. Since this is somewhat repetitive, I wrote a function, initialize, which helps eliminate the tedium, by enabling comma initialization à la built-in arrays. ...