On 7/1/2011 6:44 PM, Paul Mensonides wrote:
On Fri, 01 Jul 2011 08:34:35 -0700, Eric Niebler wrote:
On 6/30/2011 11:09 PM, Paul Mensonides wrote:
...but this is terrible. It doesn't take away from the point of the talk, but it should be something like count<typelist<A, B, C>>, not count<A, B, C>.
I'm not disagreeing, but I'd like to know why you believe this is true for templates as well as macros.
I'd go further and say that it would be terrible to use variadic argument lists (the function parameter kind, not the template parameter kind) to as runtime data structures also. Off the top of my head, here are some reasons:
1) It forces a particular argument order. By itself, this isn't the end of the world, but either tends to break symmetries or tends to require argument orders to change on a bunch of other things. 2) It does not play well with overloading (via number of arguments with macros--which you can do, via specialization of templates, or via plain overloading of functions). 3) It frequently destroys the ability to use default arguments (which could be accounted for if it weren't for #2). 4) It does not work when multiple data structures need to be passed (e.g. Cartesian products). 5) It removes the ability to use the *single* variadic clump for other stuff.
I don't have a problem with variadic data used to initialize regular data structures in the right contexts. For example, make_tuple(a, b, c). (I do believe, however, that std::initializer_list<T> is a misfeature. I personally think the { a, b, c } should have been mapped onto a constructor call taking a, b, and c--which may or may not use variadic templates).
I agree that using variadic argument lists as opposed to a data structure containing the data ( list, vector, etc. ) is a poor way to design in general. But sometimes one creates interfaces with which programmers are more comfortable just for the sake of end-user ease. This goes back discussions about variadic data in macros being a more natural way of passing data to macros than pp-lib data structures. When i created VMD, and subsequently worked with you to add variadic data to pp-lib, I was always quick to acknowledge that my main reason for doing so was almost entirely to satisfy end-user expectations of the way macro parameters are normally passed. Other than that variadic data in macros holds no advantage to pp-lib data structures and can generally be ignored. In the same way a designer may decide to create fairly simple functionality using variadic argument lists, just to satsfy end-user programmer expectations. While in the vast majority of cases it should be avoided for all the reasons you give above, and mostly because it is just a poor way of designing software of any complexity.