When do we use Initializer List in C++?

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Initializer List is used to initialize data members of a class. The list of members to be initialized is indicated with constructor as a comma separated list followed by a colon. Following is an example that uses initializer list to initialize x and y of Point class.

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#include using namespace std; class Point { private: int x; int y; public: Point(int i = 0, int j = 0):x(i), y(j) {} /* The above use of Initializer list is optional as the constructor can also be written as: Point(int i = 0, int j = 0) { x = i; y = j; } */ int getX() const {return x;} int getY() const {return y;} }; int main() { Point t1(10, 15); cout<<"x = "<", "; cout<<"y = "< return 0; } /* OUTPUT: x = 10, y = 15 */

The above code is just an example for syntax of Initializer list. In the above code, x and y can also be easily initialed inside the constructor. But there are situations where initialization of data members inside constructor doesn’t work and Initializer List must be used. Following are such cases:

1) For initialization of non-static const data members:
const data members must be initialized using Initializer List. In the following example, “t” is a const data member of Test class and is initialized using Initializer List.

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#include using namespace std; class Test { const int t; public: Test(int t):t(t) {} //Initializer list must be used int getT() { return t; } }; int main() { Test t1(10); cout< return 0; } /* OUTPUT: 10 */

2) For initialization of reference members:
Reference members must be initialized using Initializer List. In the following example, “t” is a reference member of Test class and is initialized using Initializer List.

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// Initialization of reference data members #include using namespace std; class Test { int &t; public: Test(int &t):t(t) {} //Initializer list must be used int getT() { return t; } }; int main() { int x = 20; Test t1(x); cout< x = 30; cout< return 0; } /* OUTPUT: 20 30 */

3) For initialization of member objects which do not have default constructor:
In the following example, an object “a” of class “A” is data member of class “B”, and “A” doesn’t have default constructor. Initializer List must be used to initialize “a”.

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#include using namespace std; class A { int i; public: A(int ); }; A::A(int arg) { i = arg; cout << "A's Constructor called: Value of i: " << i << endl; } // Class B contains object of A class B { A a; public: B(int ); }; B::B(int x):a(x) { //Initializer list must be used cout << "B's Constructor called"; } int main() { B obj(10); return 0; } /* OUTPUT: A's Constructor called: Value of i: 10 B's Constructor called */

If class A had both default and parameterized constructors, then Initializer List is not must if we want to initialize “a” using default constructor, but it is must to initialize “a” using parameterized constructor.

4) For initialization of base class members : Like point 3, parameterized constructor of base class can only be called using Initializer List.

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#include using namespace std; class A { int i; public: A(int ); }; A::A(int arg) { i = arg; cout << "A's Constructor called: Value of i: " << i << endl; } // Class B is derived from A class B: A { public: B(int ); }; B::B(int x):A(x) { //Initializer list must be used cout << "B's Constructor called"; } int main() { B obj(10); return 0; }

5) When constructor’s parameter name is same as data member
If constructor’s parameter name is same as data member name then the data member must be initialized either using this pointer or Initializer List. In the following example, both member name and parameter name for A() is “i”.

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