Explicit Constructor in C++

In C++ it is possible to declare constructors for a class, taking a single parameter, and use those constructors for doing type conversion. 

class A 
{ 
       public: 
              A(int); 
}; 
void main()
{
       A a1=37;
}

A declaration like:       A a1 = 37; 
says to call the A(int) constructor to create an A object from the integer value. Such a constructor is called a "converting constructor".

However, this type of implicit conversion can be confusing, and there is a way of disabling it, using a new keyword "explicit" in the constructor declaration: 
class A 

{ 

        public: 

                explicit A(int); 

}; 

void f(A) { } 

void g() 

{ 

       A a1 = 37; // illegal 

       A a2 = A(47); // OK 

       A a3(57); // OK 

       a1 = 67; // illegal 

       f(77); // illegal 

}