Default object values in functions with default argument values

Question

So far I know that if you want to pass a default value for an argument to a function that is an object, you do it like this:

void function(MyObject obj = MyObject()){
    ...
}

However, I've come around to some interesting syntax lately, which confuses me. What happens when we call the function like this?

void function(MyObject obj = 0){
    ...
}

Note, we are passing an object, not a pointer. The above code compiles just fine, no errors or warnings. And this always calls the constructor with one argument - MyObject is defined like this:

class MyObject{
public:
    MyObject(double n){std::cout << "Argumented\n";}
    MyObject(){std::cout << "Default\n";}
};

Also, where is this behavior documented (because I searched and couldn't find it)?

Answer 1

The pararameter defaults to a MyObject implicitly constructed from 0, by calling the MyObject(double) constructor. This constructor allows you to implicitly instantiate MyObjects like this:

MyObject o1 = 0;
MyObject o2 = 420/10.;

If this behaviour is not intended, then make the constructor explicit. This will also require a change to function's default parameter:

explicit MyObject(double n);

and

void function(MyObject obj = MyObject(0));