Constructor Filter Through... ?

[JKop]

You know the whole "Linked List" concept, where each object knows where the
the next one is. Well, in one of its simplest forms I have:

template<class T>
struct LinkedObject : virtual public T
{
LinkedObject* p_next;
};


The only problem here is in calling the constructor:


#include <string>

int main()
{
LinkedObject<std::string> blah("Monkey!");

blah.p_next = new LinkedObject<std::string>("Ape!");


delete blah.p_next;
}


Is there any way to achieve something akin to the above, whereby the call to
the derived class's constructor "filters through" to the base class's
constructor... ?

What I'm trying to achieve is that an object of type "LinkedObject
<std::string>" will behave exactly as would an object of type
"std::string" - the only difference being that you can do:

blah.p_next = 0;


As such, I want the call to the constructor of "LinkedObject<T>" to fall
through to the constructor of the base class.


-JKop

 

[John Harrison]

You know the whole "Linked List" concept, where each object knows where the
the next one is. Well, in one of its simplest forms I have:

template<class T>
struct LinkedObject : virtual public T
{
LinkedObject* p_next;
};


The only problem here is in calling the constructor:


#include <string>

int main()
{
LinkedObject<std::string> blah("Monkey!");

blah.p_next = new LinkedObject<std::string>("Ape!");


delete blah.p_next;
}


Is there any way to achieve something akin to the above, whereby the call to
the derived class's constructor "filters through" to the base class's
constructor... ?

What I'm trying to achieve is that an object of type "LinkedObject
<std::string>" will behave exactly as would an object of type
"std::string" - the only difference being that you can do:

blah.p_next = 0;


As such, I want the call to the constructor of "LinkedObject<T>" to fall
through to the constructor of the base class.


-JKop

You can add generic templated constructors to LinkedObject, like this

template<class T>
struct LinkedObject : virtual public T
{
LinkedObject() {}
template <class A> LinkedObject(const A& a) : T(a) {}
template <class A, class B> LinkedObject(const A& a, const B& b) : T(a,
b) {}
template <class A, class B, class C> LinkedObject(const A& a, const B&
b, const C& c) : T(a, b, c) {}
// etc etc, go as high as you like
LinkedObject* p_next;
};

This works because the constructor will only be instantiated if it is used.
Therefore its not a problem if T doesn't have all the constructors that
LinkedObject is assuming. Does need a default constructor however.

john

 

[JKop]

John Harrison posted:

You can add generic templated constructors to LinkedObject, like this

template<class T>
struct LinkedObject : virtual public T
{
LinkedObject() {}
template <class A> LinkedObject(const A& a) : T(a) {}
template <class A, class B> LinkedObject(const A& a, const B& b) :
T(a,
b) {}
template <class A, class B, class C> LinkedObject(const A& a,
const B&
b, const C& c) : T(a, b, c) {}
// etc etc, go as high as you like
LinkedObject* p_next;
};

This works because the constructor will only be instantiated if it is
used. Therefore its not a problem if T doesn't have all the
constructors that LinkedObject is assuming. Does need a default
constructor however.

john

Thanks.

But shouldn't the function arguments be *non*-const?, as in:


template <class A> LinkedObject(A& a) : T(a) {}

template <class A, class B> LinkedObject(A& a, B& b) : T(a,b) {}

template <class A, class B, class C> LinkedObject(A& a, B& b, C& c) : T(a,
b, c) {}


template <class A, class B, class C, class D> LinkedObject(A& a, B& b, C& c,
D& d) : T(a,b,c,d){}



This way, if the expressions supplied as arguments to the constructor are
actually const, then the argument types will automagically become const too.


Is there any potential pitfalls in this... I don't see any at the moment.


-JKop

 

[John Harrison]

John Harrison posted:



Thanks.

But shouldn't the function arguments be *non*-const?, as in:


template <class A> LinkedObject(A& a) : T(a) {}

template <class A, class B> LinkedObject(A& a, B& b) : T(a,b) {}

template <class A, class B, class C> LinkedObject(A& a, B& b, C& c) : T(a,
b, c) {}


template <class A, class B, class C, class D> LinkedObject(A& a, B& b, C& c,
D& d) : T(a,b,c,d){}



This way, if the expressions supplied as arguments to the constructor are
actually const, then the argument types will automagically become const too.


Is there any potential pitfalls in this... I don't see any at the moment.

But then you won't be able to call any of your constructors with a
temporary.

I was looking at some code that did this just the other day. I don't have
access to it right now but I think they used const references, but said
something like 'if you want non-const references use something like the
boost ref library'.

John