Jun 242012

In my previous article about the latest C++11 standard I talked about automatic type detection and keyword decltype

Today I will take another major enhancement in C++11 called the Initializer List or Uniform Initialization Syntax. 

Let us first see the user pain points this feature is trying to resolve :

Till now C++ has multiple ways to initialize objects, for example:

1. The conventional way

    int x = 10;
    std::string str = "mcn";

2. The constructor way

    int x(10);
    std::string str("mcn");
    int x = int();   // default constructor

3. The C language way of initializer list

    int arr[] = {1,2,3,4,5};

    struct Student
        std::string name;
        int rollno;

    Student a = {"Kamal", 5};

4. The Member Initialization list of constructors

    // Keping struct (and not class) for convenience
    // default members of struct are public.
    struct Student
        std::string name;
        int rollno;

        Student():name(""), rollno(-1){} // default values

        Student(_name, _rollno) : name(_name), rollno(_rollno){}

The multiple ways of initialization possible is a source of confusion. Plus there is no way to initialize the POD (Plain Old Data) arrays allocated memory on heap using new[].

    int *ptr = new int[10]; // Cannot initialize the array in such cases

The root cause of problem was because C++ inherit its initialization list from C language, which does not have the concept of classes and constructors.

But, C++11 has merged the concept of initializer list with templates and defined a new class in STL, std::initializer_list which allows constructors (and other functions) to take initializer list as parameters (in addition to the usual comma separated list of parameters.

For example: If my class and constructor are defined as below:

    struct MyClass

Than I can pass the initializer list of int which initializing the object of MyClass, as shown below:

    MyClass obj  = {1,2,3,4};
    MyClass obj2 = {1,2,3};
    MyClass obj3 = {1};

This initializer_list has made the initialization easy and uniform for developer. consider the below examples:

    class MCN
        int a;
        int b;
        public: MCN(int i, int j); // constructor

The objects of MCN can be created like below:

    MCN obj {0 , 1}; // Same as MCN obj(0,1);

or, on heap

    int * ptr = new int[5] {1,2,3,4,5};

or, in the member initialization list like

    struct MCN
        int arr[3];
        MCN() : arr{1,2,3} {}

vector class in C++11 Template Library will have a constructor which will take initializer_list for its template type as below:

    std:vector<int> v {1, 2, 3, 4};

Please let me know your feedbacks / comments / suggestions


Previous: Automatic type detection and decltype
Next: Range based for loop similar to for-each

  4 Responses to “Uniform Initializer lists in C++11”

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