"Kevin Sopp" wrote in message > Hi Maarten,
On Sun, Mar 30, 2008 at 10:13 PM, Maarten Kronenburg wrote:
Just for your info I designed another interface, see document N2143 on:
I knew about your document after I searched the internet for ideas on how to design the interface of the mp_int class template.
http://open-std.org/jtc1/sc22/wg21/docs/papers/2007/#mailing2007-01 The root question here seems to be: do you use runtime polymorphism
functions) or compile time polymorphism (templates). My argument is
(virtual that as
this class is so close to the hardware, and performance is so important, that runtime polymorphism will in this case provide the runtime flexibility needed.
I never really understood why you chose this approach, it is really different from the rest of the standard library. I understand that you achieve interoperability with the other integer types but this could be hardcoded once all integer types to be standardized are known. Also I think that interoperability of the integer types is a minor concern and as such should not govern the major design decision.
In your design the algorithms will probably end up in headers (just
Interoperability is a major concern, because it is also available for the base type ints: int x = -5; unsigned int y = 10; x -= y; y -= x; etc. The STL is made with templates, and rightly so, because containers have template parameters (e.g. what type they should contain). But this does not mean that other designs should be "templatized". Sometimes programmers need runtime polymorphism to achieve runtime flexibility, and in my opinion the integer class is an example. like the
STL), while my algorithms will end up in DLLs. In other words: my design considers the allocator and the traits as implementation details (although in my design it is possible to change the allocator dynamically), while your design considers these as design parameters.
The traits parameter is totally implementation defined. I included it because I wanted to give the user a way to customize some of the internal workings of the class, usually in C libraries this is done via macro definitions at compile time. Most users don't need to bother but if you're a poweruser with very large calculations you have at least a few knobs to finetune the internals. There was almost never a question in my mind about the allocator parameter. After all it would be strange not to have one for a class that will potentially allocate a large amount of memory.
In my opinion implementation parameters should be kept out of the interface. I agree about the allocator, this is why in my design I have added an allocator, only it is set at runtime.
In other words: in my opinion an integer is not a container, so
compile-time
flexibility is not needed, while runtime flexibility is needed for combination in expressions of signed/unsigned/modular types and base tpye pointers to derived type objects.
It does not look like a container but internally it is one. From a Standard perspective it makes sense to give it an allocator parameter, as this is what a user will expect.
In this view everything is internally a container. In my view a container is something that can contain anything (the template type parameter), while an integer can only contain contiguous bits. Yes I agree that the STL uses allocator template parameters, but what if I want to concatenate two standard strings with different allocators? In my design adding two integers with different allocators is possible because they are derived. Whether signed, unsigned, modular, allocated etc., because they are derived from the same base class (in this case integer), they can be mixed in expressions, just like the base type ints. Perhaps you think this interoperability is not so important, but what if large user groups use this and suddenly they require this? In a way this is the same issue as with the impossibility of derivation from the STL string (no virtual destructor). In the case of this integer class I don't think we can ignore this. Regards, Maarten.