What is the necessity of using an explicit constructor in a class?

An explicit constructor in C++ is used to prevent implicit type conversions that can lead to errors or unexpected behavior. When a constructor is marked with the explicit keyword, the compiler will not use it for implicit conversions of arguments to the class object.

For example, without explicit, you can accidentally write:

class MyClass {
public:
    MyClass(int x) { /*...*/ }
};

void func(MyClass obj) { /*...*/ }

func(10); // implicit conversion from int to MyClass

With explicit:

class MyClass {
public:
    explicit MyClass(int x) { /*...*/ }
};

func(10); // error
func(MyClass(10)); // correct

Thus, explicit helps make the code clearer and avoid errors related to implicit conversions.