If you want to declare and initialize an array of five integers, you could do something like:

int a[] = { 10, 20, 5, 45, 22 };

But in C++03, what would happen if you would want to do the same thing using some STL container, like a std::vector? Something like:

std::vector<int> a = { 10, 20, 5, 45, 22 };

You guessed it!!! It would not get compiled. Initializer lists were just available for built-in data types…. Until now! :)

With C++0x, the code shown above is perfectly legal and works as expected; that is because C++0x introduces support for initializer lists for every data type.

So, the next question would be… How can I make my own container to support these initializer lists?

The answer: The STL library that will ship with C++0x includes a new template class called: std::initializer_list; when the compiler finds a declaration with an initialization similar to the one shown above { 1, 2, 3, 4, 5 };, it looks for a constructor that receives an std::initializer_list as argument. If it is found, the compiler will create an instance of the initializer_list class with the arguments inside the curly brackets and will invoke the constructor found.

The initializer list object can be iterated using a standard STL iterator.

So, let’s consider this wrapper to vector that implements the std::initializer list:

#include <vector>
#include <initializer_list>

using namespace std;

template <typename T>
class MyList
{
  private:
    vector<T> _items;

  public:
    MyList() { }

    MyList(const initializer_list& x)
    {
      for (auto i = x.begin(); i != end(); i++)
        _items.push_back(*i);
    }
};

With this implementation we will be able to create an instance of MyList in this way:

MyList<int> x = { 10, 20, 1, 4, 8, 16, 9 };
MyList<string> y = { "hello", "world", "of", "c++", "programmers" };