Template question

[Jeroen]

Hi all,

I have a question which is illustrated by the following piece of code:

template <class T>
class A {
T my_value;
};

In a list, I'd like to store pointers to objects of class A. But I don't
know at forehand if these will be A<intor A<doubleor whatever (all
these flavours of A must be stored in the same list). And, lateron if I
retrieve the objects from the list I may have to know what flavour the
object was in order to call the proper routines for processing them.

My question: what does the list look like in C++? I'm lost, so if anyone
has some advice....

Thanks,

Jeroen

[Noah Roberts]

John Harrison wrote:

You need to store all different types in the same list? And later you
need to recover what was the type of object stored in the list?

Sorry but in C++ that can't be done.

Tell that to the author of boost::any.

[John Harrison]

Noah Roberts wrote:

John Harrison wrote:

>You need to store all different types in the same list? And later you
need to recover what was the type of object stored in the list?

Sorry but in C++ that can't be done.

Tell that to the author of boost::any.

Boost::any does not allow you to 'recover what was the type of object
stored'. Sure you can say, was it an int? was it a double? etc. But in
the absense of any clue (like a fixed list of possibilities) you cannot
tell what the type was.

john

[lordoffox]

On 3ÔÂ7ÈÕ, ÉÏÎç2ʱ43·Ö, Jeroen <no_m...@please .com>wrote:

Hi all,

I have a question which is illustrated by the following piece of code:

template <class T>
class A {
T my_value;

};

In a list, I'd like to store pointers to objects of class A. But I don't
know at forehand if these will be A<intor A<doubleor whatever (all
these flavours of A must be stored in the same list). And, lateron if I
retrieve the objects from the list I may have to know what flavour the
object was in order to call the proper routines for processing them.

My question: what does the list look like in C++? I'm lost, so if anyone
has some advice....

Thanks,

Jeroen

I suggest use the boost::variant.
On other way , you can do like this:
Plan A..
struct Type_Base
{
virtual std::string type_name() = 0;
virtual void * adress() = 0;
};

template <typename T>
class MyType : Type_Base
{
public:
std::string type_name() { return type_id(T).name (); }
void * address(){ return (void *)(&value);}
private:
T value;
}

Anyway boost::variant is better then paln A,but it only support
arithmetic type. You can read the document for detail.

[Jeroen]

Jeroen schreef:

John Harrison wrote:

OK guys, you all gave me a lot of pointers to think about. It will keep
me busy for some time to sort everything out. Thanks again!

[Grizlyk]

Jeroen wrote:

>
I have a question which is illustrated by the following piece of code:

template <class T>
class A {
T my_value;
};

In a list, I'd like to store pointers to objects of class A. But I don't
know at forehand if these will be A<intor A<doubleor whatever (all
these flavours of A must be stored in the same list). And, lateron if I
retrieve the objects from the list I may have to know what flavour the
object was in order to call the proper routines for processing them.

My question: what does the list look like in C++? I'm lost, so if anyone
has some advice....

Design pattern "adapter" can be used to adapt templated class into virtual
base interface.

class Base
{
public:
virtual void method()=0;
virtual ~Base(){}
};

template<class T>
class A_Base: public Base
{
A<T data;

public:
void method(){ data.method(); }
};

Multiple ingeritance also can be used

template<class T>
class A_Base: public Base, public A<T>
{
public:
void method(){ A<T>::method() ; }
};

--
Maksim A. Polyanin
http://grizlyk1.narod.ru/cpp_new

"In thi world of fairy tales rolls are liked olso"
/Gnume/

[Sarath]

On Mar 7, 3:43 am, Jeroen <no_m...@please .comwrote:

Hi all,

I have a question which is illustrated by the following piece of code:

template <class T>
class A {
T my_value;

};

In a list, I'd like to store pointers to objects of class A. But I don't
know at forehand if these will be A<intor A<doubleor whatever (all
these flavours of A must be stored in the same list). And, lateron if I
retrieve the objects from the list I may have to know what flavour the
object was in order to call the proper routines for processing them.

My question: what does the list look like in C++? I'm lost, so if anyone
has some advice....

Thanks,

Jeroen

As a basic technique, you just overload the functions with different
possible types.Compiler will do call the appropriate one.
You can also use typeid operator to identify the type. but it will
make some overhead to your program as it enables Runtime Type
Identification. (RTTI). typeid operator returns a type_info class
which can be use for comparison, type string etc.
You can also approach boost type traits library

[tragomaskhalos]

On Mar 6, 8:43 pm, Jeroen <no_m...@please .comwrote:

Some more explanation: it was about my matrix class with some extra
facilities. Let's say you can initialize such a matrix like (OK, a
vector in this case to keep it simple...):

{
matrix m = "1 2 3 7 8";
}

But I also want to to use variables in my string expression:

{
matrix m = "1 2 3 7 8";
matrix n;

register_user_v ar(n);
n = "m 3 4 5 8"; // concatenate vectors 'm' and [3 4 5 8]

}

Hopefully I am not wildly off base here, but it looks like you're
getting yourself in a lot of trouble trying to initialise a matrix via
a string.
Better is something like this:
Matrix<intim(3, 3); // 3x3 matrix of ints
im = 1,2,3,
4,5,6,
0,9,8;

This can be achieved by having Matrix<T>::oper ator=(T) return an
initialiser object - call it Minit<T>.
Then you define Minit<T>::opera tor,(T) to put T into the Matrix at
the next "slot", and return itself.
I have working code if you want me to post it, this technique is
nabbed from "Generative Programming" by Eisenecker and Czarnecki.