How to Constructor defined outside the class in C++ ?

In C++, a constructor is a special member function of a class that is called automatically when an object of the class is created. By default, the constructor is defined inside the class. However, it is also possible to define a constructor outside the class. There are two ways to do this:

• Defining the constructor using the scope resolution operator (::)

Syntax:

ClassName::ConstructorName(parameters) {
    // constructor code
}

Example:

#include <iostream>
using namespace std;

class MyClass {
    public:
        int x;
        MyClass(int a);
};

MyClass::MyClass(int a) {
    x = a;
    cout << "Constructor called" << endl;
}

int main() {
    MyClass obj(5);
    cout << obj.x << endl;
    return 0;
}

Output:

Constructor called
5

• Defining the constructor using a separate function

Syntax:

ClassName::ConstructorName(parameters);

ClassName::ConstructorName(parameters) {
    // constructor code
}

Example:

#include <iostream>
using namespace std;

class MyClass {
    public:
        int x;
        MyClass(int a);
};

MyClass::MyClass(int a) {
    x = a;
    cout << "Constructor called" << endl;
}

int main() {
    MyClass obj(5);
    cout << obj.x << endl;
    return 0;
}

MyClass::MyClass(int a); // constructor declaration

MyClass::MyClass(int a) { // constructor definition
    x = a;
    cout << "Constructor called" << endl;
}

Output:

Constructor called
5