Re: [Boost-users] Boost-users Digest, Vol 1918, Issue 3

26 Feb 2009


      I'm not sure how that could work, since I want different access rights for the
very same object?

Stefan
...
Though I have not used it myself, there is enable_if or disable_if in BOOST. That might help.
-dhruva
----- Original Message ----
...
From: Stefan Funck 
To: boost-users@lists.boost.org
Sent: Thursday, 26 February, 2009 7:34:01 PM
Subject: [Boost-users] make member functions (in)accessible
Dear all,
I'm trying to find some magic that makes a member function of some object x
accessible if and only if it were called from within a class that has object x
as a member. Is there a (boost)-way of doing this?
As this may seem a bit obscure, I'm trying to give an example:
template 
class Property
{
public:
    T get() const;
    void set(const T& val);
private:
    T  m_val;
};
is a simple class holding a value of T.
I have some classes holding potentially many Properties. I'd like to have these
properties publically available, some as only readable, others writable as well,
so that others (e.g. gui-classes) can access them:
class A
{
public:
    Property  propA;
    Property  propB;
};
so I may do:
A  a;
a.propA.get();
a.propA.set(0.5);
a.propB.get();
but NOT:
a.propB.set(1);
because propB was defined with Writable=false. BUT, from within class A, I would
like to do
propB.set(1);
because otherwise this property is useless. (How) can this be achieved (with 
boost)?
I could make set() private in the Writable=false - specialisation of Property
and declare A as a friend, but that's prohibitive since Property shouldn't know
about A and it won't work for derived classes of A. Another option would be
class A
{
public:
    A() : propA(propA_priv), propB(propB_priv) {}
PropertyWrapper  propA;
    PropertyWrapper  propB;
private:
    Property  propA_priv;
    Property  propB_priv;
};
so the actual properties are private and the wrapper provides the actual
interface to the outside world. But as there may be many properties I'd like to
get away without the wrapper objects ...
Thanks for any ideas!
Stefan