Re: great c++ question
On Jun 21, 8:37 am, John Harrison <john_androni...@hotmail.com> wrote:
Gianni Mariani wrote:
Amar Kumar Dubedy wrote:
implement a c++ class such that it allows us
to add data members at runtime.
This is usually implemented as a map like so:
#include <string>
#include <map>
#include <at_any.h> // or boost any
struct Extensible
{
std::map< std::string, at::Any<> > m_members;
};
Extensible a;
int main()
{
a.m_members[ "new_member" ] = at::ToAny( 5 );
}
If you want to enforce that every Extensible object has the same members
it gets a little more complex but nothing too hard.
Well this last sentence is the point.
One of your points, anyway:-).
And it still remains the case that Extensible has only one data member
'm_members', so this approach is only ever going to be a simulation. But
the original question didn't say anything about simulation.
I still think the correct answer is 'impossible in C++'.
I think that the problem is understanding at what level the
question was asked. I don't think that there's any doubt that
you cannot change the topology of a C++ class at runtime, at the
C++ level. And the question *did* ask about a "C++ class". But
I'd hesitate to respond "impossible" myself, if asked the
question during an interview, because in practice, I suspect
that what the person asking really means is "implement a class
[conceptual type] in C++ such that...". And that can be done:
how, and how difficult it is, depends on what the questionner
really means---I suspect that in most cases, a solution like
Gianni's is more or less what they are really looking for. Even
though it "fails" on two grounds: you are adding elements to
individual objects, not to the class (but that can be handled by
some sort of a static "set" with the names of the elements), and
that the elements aren't associated with a type---no problem if
they are only present in each separate object (because
boost::any, and I suppose Gianni's at::Any, manage type), but
you'd need some sort of shared typemap as well if you wanted to
manage type at the "class" level.
I think it's an often overlooked point that we often use the
same, or very similar, vocabulary for the concept, and the way
we implement it in the language. Thus, for example, when I
"inherit" in C++, I may be doing so to implement the concept of
inheritance in OO design, but I may be doing so for some
entirely different reason; there's not necessarily a one to one
mapping. In this case, given the way the question is
formulated, I suspect that---despite the presicion "C++
class"---what is really meant is a conceptual class, or a user
defined type, if you prefer. I suspect this because it is
really very rare for people to make the distinction properly,
and a question of the form "implement X in C++", or even
"implement a C++ X", usually means "implement the concept X in
the programming language C++". Maybe it shouldn't, but in my
experience, it usually does.
And of course, I don't want to get turned down for a job just
because the questionner doesn't formulate the questions as
precisely as I would like.
--
James Kanze (GABI Software, from CAI) email:james.kanze@gmail.com
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