How can the assignment from int to object be possible in C++?

Question

class phone {  
    public:  
        phone(int x) { num = x; }
        int number(void) { return num; }
        void number(int x) { num = x; }

    private:
        int num;
};

int main(void)
{
    phone p1(10);

    p1 = 20;    // here!

    return 0;
}

Hi, guys

Just I declared a simple class like above one.
After that I assigned int value to the object that class, then it worked!
(I printed its value. It was stored properly)

If there is not a construct with int parameter, a compile error occurred.
So, I think it's related with a constructor. Is that right?

Please give me a good explanation.
Thanks.

Answer 1

This is legal because C++ interprets any constructor that can be called with a single argument of type T as a means of implicitly converting from Ts to the custom object type. In your case, the code

p1 = 20;

is interpreted as

p1.operator= (20);

Which is, in turn, interpreted as

p1.operator= (phone(20));

This behavior is really weird, and it's almost certainly not what you wanted. To disable it, you can mark the constructor explicit to disable the implicit conversion:

class phone {  
    public:  
        explicit phone(int x) { num = x; }
        int number(void) { return num; }
        void number(int x) { num = x; }

    private:
        int num;
};

Now, the constructor won't be considered when doing implicit conversions, and the above code will cause an error.