Re: deriving a class with a subset of base class methods?
On 10 Apr., 23:19, Ulrich Eckhardt <eckha...@satorlaser.com> wrote:
Gerhard Menzl wrote:
Ulrich Eckhardt wrote:
Well, private inheritance is not much different than composition, so
yes, it works just as well.
I beg to differ. For a start, inheritance ties you to a 1:1
relationship, which composition does not.
Yes, obviously.
Composition has a much lower degree of coupling than inheritance.
It does, but is that also the case for /private/ inheritance? If I add or
remove private baseclasses is not externally visible, just as it isn't
visible with private members. However, in both cases it obviously requires
that you recompile client code. No, I don't think that private inheritance
is that much different to aggregation.
From a first point of view, there is not much difference,
and if I'm laxly speaking, I often compare them as nearly
equivalent. In fact they are not, because of a at least
one reason: A derived class will override a virtual member
function of it's private (or not) base class.
This subtle difference is relevant as both pro and con, if
you combine templates with dynamic polymorphism, e.g. with
the Curiously Recurring Template Pattern (CRTP), because
the provider of the derived class can not always foresee the
consequences.
One of the advantages of private inheritance versus composition
is to take advantage of the empty-base-class-optimization
(EBCO). If you want EBCO, but if you don't want to introduce
the silent danger of *direct* private inheritance, it is often
useful to apply the private (or non-private) inheritance to
a private data member. This technique can often be found in
C++ library implementations, where the allocator of a container
class is combined with some data member, e.g. the naive
construction:
template <typename T, typename Allocator>
class Container {
....
private:
...
Allocator a;
Whatever b;
size_t s;
};
is transformed to:
template <typename T, typename Allocator>
class Container {
....
private:
...
struct Data : Allocator {
Whatever b;
size_t s;
} d;
};
HTH & Greetings from Bremen,
Daniel Kr?gler
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