Constructor Initializer List

A constructor initializer list in C++ is a way to directly initialize member variables when an object is created. It sets variable values before the constructor body runs. This helps make the program more efficient and is required for const variables, references, and base class objects.

Syntax

A constructor initializer list comes after the constructor’s name and parameter list. It starts with a colon (:) and is followed by a list of variables being initialized:

class ClassName {
  private:
    int x;
    int y;
  public:
    ClassName(int a, int b) : x(a), y(b) {
      // Constructor body (optional)
    }
};

Examples

Using Constructor Initializer List

This example shows how to use a constructor initializer list to set values when an object is created:

#include <iostream>

class Point {
  private:
    int x;
    int y;
  public:
    // Constructor using initializer list
    Point(int a, int b) : x(a), y(b) {
      std::cout << "Point initialized with x = " << x << " and y = " << y << "\n";
    }
};

int main() {
  Point p1(3, 4); // Constructor is called with values 3 and 4
  return 0;
}

The result of the above code is the following:

Point initialized with x = 3 and y = 4

Initializing Constant and Reference Members

If a class has const or reference variables, they must be initialized using a constructor initializer list since they cannot be assigned values later:

#include <iostream>

class Example {
  private:
    const int value;
    int& ref;
  public:
    Example(int v, int& r) : value(v), ref(r) {
      std::cout << "Const value: " << value << " | Reference value: " << ref << "\n";
    }
};

int main() {
  int num = 10;
  Example ex(5, num);
  return 0;
}

The result of the above code is the following:

Const value: 5 | Reference value: 10

Codebyte Example

This example demonstrates how a constructor initializer list can be used to initialize multiple members, including a base class and constant members:

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Explanation

• The Base class has a constructor that initializes baseValue using an initializer list.
• The Derived class inherits from Base and initializes its own members using an initializer list.
• The Derived class constructor first calls the Base class constructor and then initializes derivedValue (a const member) and normalValue.
• The initializer list ensures all members are set before the constructor body executes.