Initializer list in C++

Initializer List is used in initializing the 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.

Why using initializer list:

1. For initialization of non-static const data members
2. For initialization of reference members
3. For initialization of member objects which do not have default constructor
4. For initialization of base class members
5. When constructor's parameter name is same as data member: can be solved with “this->”

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Code Example

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Explanation

In this example, we have:

• A ParentClass with an integer member x.
• A SubClass with an integer member x.
• A ChildClass that inherits from ParentClass and contains several members, including constant and reference members, a member object, and its own integer members x and y.
• The ChildClass constructor uses an initializer list to initialize its members and the base class.

  ChildClass(int constMember, int& referenceMember, int subClassX, int parentClassX, int x, int y) :
    constMember(constMember), // 1. non-static constant member
    referenceMember(referenceMember), // 2. reference memeber
    subObj(subClassX), // 3. member objects that do no have default constructor
    ParentClass(parentClassX), // 4. base class that does not have default constructor
    x(x) // 5. [optional] input parameter name == memember name; avoid using "->"
  {
    this->y = y; // input parameter name == memember name, use "->" for member variable
  }
  Copy

  1. constMember(constMember): needed to initialize a non-static constant member
  2. referenceMember(referenceMember): needed to initialize a reference memeber
  3. subObj(subClassX): needed to initialize a member object that does no have default constructor
  4. ParentClass(parentClassX): needed to initialize the base class that does not have default constructor
  5. x(x): [optional] because input parameter name is same as memember variable name, "this->" can be avoided in initializer list
  6. this->y = y: in contrast, y needs to use "this->" to distinguish between input and member variable.

Conclusion

Initializer list in class constructor is powerful, efficient, and sometimes a must. Recommendation is to always use it whenever possible.