C++ Constructor Initializer List Example

6 min read Jul 01, 2024

C++ Constructor Initializer List Examples

In C++, constructor initializer lists are used to initialize member variables of a class. They offer several advantages over directly assigning values within the constructor body, leading to cleaner, more efficient, and safer code.

Why Use Constructor Initializer Lists?

Initialization Order: Member variables are initialized in the order they are declared in the class, not in the order they appear in the constructor body. This can lead to unexpected behavior if you're relying on one member variable being initialized before another. Initializer lists ensure the correct initialization order.
Efficiency: Initializer lists allow member variables to be initialized directly, rather than through assignment statements within the constructor body. This can be more efficient, particularly when dealing with complex initialization processes.
Error Prevention: Using initializer lists can help prevent potential errors that can arise from uninitialized member variables.

Examples:

Here are some examples demonstrating how to use constructor initializer lists in C++:

1. Basic Initialization:

#include 

class MyClass {
public:
    int num;
    std::string name;

    MyClass(int n, const std::string& s) : num(n), name(s) {}

    void display() {
        std::cout << "Number: " << num << std::endl;
        std::cout << "Name: " << name << std::endl;
    }
};

int main() {
    MyClass obj(10, "John");
    obj.display();
    return 0;
}

Explanation:

The constructor MyClass(int n, const std::string& s) uses an initializer list : num(n), name(s) to initialize the num and name member variables.
The values passed to the constructor are used to initialize the corresponding member variables.

2. Initializing Constant Members:

#include 

class MyClass {
public:
    const int num;
    std::string name;

    MyClass(int n, const std::string& s) : num(n), name(s) {}

    void display() {
        std::cout << "Number: " << num << std::endl;
        std::cout << "Name: " << name << std::endl;
    }
};

int main() {
    MyClass obj(10, "John");
    obj.display();
    return 0;
}

Explanation:

The num member variable is declared as const, meaning its value cannot be changed after initialization.
It's crucial to initialize const members using the initializer list since assignment within the constructor body is not allowed.

3. Initializing Base Class Members:

#include 

class BaseClass {
public:
    int baseNum;

    BaseClass(int n) : baseNum(n) {}
};

class DerivedClass : public BaseClass {
public:
    int derivedNum;

    DerivedClass(int n, int d) : BaseClass(n), derivedNum(d) {}

    void display() {
        std::cout << "Base Number: " << baseNum << std::endl;
        std::cout << "Derived Number: " << derivedNum << std::endl;
    }
};

int main() {
    DerivedClass obj(5, 10);
    obj.display();
    return 0;
}

Explanation:

The DerivedClass constructor initializes its own member derivedNum and also initializes the member baseNum of the base class BaseClass using the initializer list.

4. Initializing Members with Complex Objects:

#include 

class MyClass {
public:
    int num;
    std::string name;

    MyClass(int n, const std::string& s) : num(n), name(s) {}

    void display() {
        std::cout << "Number: " << num << std::endl;
        std::cout << "Name: " << name << std::endl;
    }
};

int main() {
    MyClass obj(10, "John");
    obj.display();
    return 0;
}

Explanation:

The MyClass constructor uses the initializer list to initialize the num and name member variables.
You can also use initializer lists for complex objects like vectors or custom classes, ensuring they are initialized correctly and efficiently.

Summary:

Constructor initializer lists are a fundamental part of C++ that provide a clean and efficient way to initialize member variables. They ensure proper initialization order, prevent potential errors, and enhance code readability. It's recommended to use initializer lists whenever possible to maintain best practices and improve the quality of your C++ code.

C++ Constructor Initializer List Example