Hidden activities
Automatic type conversion can introduce
more underlying activities than you may expect. As a little brain teaser, look
at this modification of CopyingVsInitialization.cpp:
//: C12:CopyingVsInitialization2.cpp
class Fi {};
class Fee {
public:
Fee(int) {}
Fee(const Fi&) {}
};
class Fo {
int i;
public:
Fo(int x = 0) : i(x) {}
operator Fee() const { return Fee(i); }
};
int main() {
Fo fo;
Fee fee = fo;
} ///:~
There is no constructor to create the
Fee fee from a Fo object. However, Fo has an automatic type
conversion to a Fee. There’s no copy-constructor to create a
Fee from a Fee, but this is one of the special functions the
compiler can create for you. (The default constructor, copy-constructor,
operator=, and destructor can be synthesized automatically by the
compiler.) So for the relatively innocuous statement
Fee fee = fo;
the automatic type conversion operator is
called, and a copy-constructor is created.
Use automatic type conversion carefully.
As with all operator overloading, it’s excellent when it significantly
reduces a coding task, but it’s usually not worth using
gratuitously.
