on Fri Apr 06 2012, Mathias Gaunard <mathias.gaunard-AT-ens-lyon.org> wrote:
On 06/04/12 12:26, Andrzej Krzemienski wrote:
But looking at it from the fresh programmer's perspective, the fact that the language requires "to specify whenever dependent names in template contexts are anything else than values" means that the language is not friendly to the fresh programmers (I understand that there are good reasons for requiring this, but still...).
Based on the Wkipedia's definition of "gotcha" I interpret this typename as one. "In programming, a gotcha is a feature of a system, a program or a programming language that works in the way it is documented but is counter-intuitive and almost invites mistakes because it is both enticingly easy to invoke and completely unexpected and/or unreasonable in its outcome."
When a compiler encounters a function or class template, it builds a parametric AST. Without knowing what the parameters are, it cannot know what the names that depend on the parameters are, and thus what kind of AST to generate; indeed C++, unlike other programming languages, makes it ambiguous what kind of entity a name refers to.
Some compilers do not require this because they do not build a real AST when parsing a template. As a result they have a lot of bugs, especially in name lookup.
I think Andrzej understands very well the reasons for "typename" and the consequences of failing to implement 2-phase name lookup. IIUC he's just saying (correctly) that such understanding does not come easily---even many very experienced C++ programmers don't have it---and the current situation is hard for many programmers to deal with.
An alternative would be to deduce whether it's a type or a value depending on the context it is used in.
...which is in some cases impossible. My book has examples of code that could be valid either way. -- Dave Abrahams BoostPro Computing http://www.boostpro.com