C++ Programming: Implicit and Explicit Constructors

By Syed Wahaj

Constructors are a fundamental concept in C++ programming. They are special member functions that initialize objects of a class. Constructors can be implicitly generated by the compiler or explicitly defined by the programmer. In this article, we will explore the differences between implicit and explicit constructors in C++ and provide examples to illustrate their usage.

Implicit Constructors

Implicit constructors, also known as default constructors, are generated by the compiler when a class does not define any constructor explicitly. These constructors have no parameters and perform basic initialization of class members. Here’s an example:

#include <iostream>

class MyClass {
public:
    int value;
    MyClass() {
        value = 0;
    }
};

int main() {
    MyClass obj;
    std::cout << "Default value: " << obj.value << std::endl;
    return 0;
}

In the above code, MyClass has an implicit default constructor that sets the value member to 0 when an object of the class is created. The obj object is created using this default constructor, and we can access its value member, which is initialized to 0.

Implicit Copy Constructor

In addition to the default constructor, the C++ compiler also generates an implicit copy constructor when you don’t define one explicitly. The copy constructor is used to create a new object as a copy of an existing object. Here’s an example:

#include <iostream>

class MyString {
public:
    char* str;
    MyString(const char* s) {
        str = new char[strlen(s) + 1];
        strcpy(str, s);
    }
    // No explicit copy constructor defined
    ~MyString() {
        delete[] str;
    }
};

int main() {
    MyString s1("Hello");
    MyString s2 = s1;  // Implicit copy constructor is called
    std::cout << "s2: " << s2.str << std::endl;
    return 0;
}

In this example, the MyString class defines a constructor to initialize its str member with a C-style string. Since we didn’t explicitly define a copy constructor, the compiler generates one for us. When we create s2 as a copy of s1, the implicit copy constructor is invoked, and a deep copy of the string is made.

Explicit Constructors

Explicit constructors are user-defined constructors that you write in your code. They allow you to control how objects of your class are initialized. You can have multiple constructors with different parameter lists to provide flexibility in object creation.

Parameterized Constructors

Here’s an example of a class with explicit parameterized constructors:

#include <iostream>

class Point {
public:
    int x, y;

    // Parameterized constructor
    Point(int _x, int _y) {
        x = _x;
        y = _y;
    }
};

int main() {
    Point p1(1, 2);  // Explicit constructor call
    std::cout << "Point p1: (" << p1.x << ", " << p1.y << ")" << std::endl;

    Point p2(3, 4);  // Explicit constructor call
    std::cout << "Point p2: (" << p2.x << ", " << p2.y << ")" << std::endl;

    return 0;
}

In this example, we define a Point class with a parameterized constructor that allows us to specify the x and y coordinates when creating a Point object. We explicitly call the constructor with different values to create two Point objects, p1 and p2.

Copy Constructor

You can also define your own copy constructor explicitly to control how objects of your class are copied. Here’s an example:

#include <iostream>

class MyString {
public:
    char* str;

    // Parameterized constructor
    MyString(const char* s) {
        str = new char[strlen(s) + 1];
        strcpy(str, s);
    }

    // Explicit copy constructor
    MyString(const MyString& other) {
        str = new char[strlen(other.str) + 1];
        strcpy(str, other.str);
    }

    ~MyString() {
        delete[] str;
    }
};

int main() {
    MyString s1("Hello");
    MyString s2 = s1;  // Explicit copy constructor is called
    std::cout << "s2: " << s2.str << std::endl;
    return 0;
}

In this example, we define an explicit copy constructor for the MyString class. This constructor performs a deep copy of the str member to ensure that each object has its own dynamically allocated memory. When we create s2 as a copy of s1, the explicit copy constructor is invoked.

Conclusion

Constructors are a crucial aspect of C++ programming as they allow you to initialize objects of your class properly. Implicit constructors are generated by the compiler when you don’t define any constructors explicitly, while explicit constructors are user-defined constructors that you write to control object initialization. Understanding the difference between these two types of constructors is essential for effective C++ programming, as it allows you to design classes that behave as expected and provide meaningful initialization and copy behavior for your objects.