Re: Global Dependencies Made Easy
John Moeller schrieb:
John Moeller schrieb:
I have to admit that I don't know precisely what you're saying here,
but I view this as more of an advantage. With private inheritance,
delegation to a method is as simple as "using Base::method;", whereas
with full delegation, you have to create a silly wrapper function.
You mention here an advantage of private inheritance and I
never said that there are *no* advantages. I wanted to point
out the differences, which can be dangerous sometimes or
at least have unexpected side-effects. Concerning unwanted
side-effects of virtual function override, see my answer to
Gerhard.
Concerning unwanted side-effects due to overload selection
consider this, slightly artifical situation (just to demonstrate
what I mean):
// Std Library:
namspace std {
template <typename KeyType, typename Compare>
struct __set_impl {
typedef .. iterator;
typedef .. const_iterator;
iterator find(const KeyType&);
const_iterator find(const KeyType&) const;
..
};
template <typename KeyType, typename Compare, ..>
// Only, if is_class<Compare>::value == true:
class set : private __set_impl<KeyType, Compare>, private Compare {
// Note: *No* using __set_impl<KeyType, Compare>::find in sight!
};
} // ns std
// User code:
class Obj{..};
class MyComp {
private:
bool find(const Obj*) const; // It's private - no harm...
public:
bool operator()(const Obj*, const Obj*) const; // uses find somehow
};
int main() {
MySet<Obj*, MyComp> s = ..;
MySet<Obj*, MyComp>::iterator it = s.find(0); // Ooops, ambiguity!
}
Of course, we could argue that the library was not
well written and it could have simply enforced the
proper find selection by the correct using-declaration.
But what are the odds that the compiler writer will
find this possible scenario even under careful unit
test conditions?
This contribution is not written from hindering anyone
using private inheritance where it's advantages are
larger than its disadvantages - I only wanted to point
out some subtle differences.
Greetings from Bremen,
Daniel Kr?gler
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