Re: Need a C++ compiler
kwikius wrote:
First you need to know an awful lot about the C++ language to get
things working. The obvious effect of this is that C++
mertaprogramming is seen as for experts only, which is a shame as
metaprogramming is a beautiful technique which many less experienced
programmers would find rewarding if it was less complicated.
A case in point: I attended a presentation by Scott Meyers that involved
doing some C++ template metaprogramming. A lot of the focus was on the
mechanics of what was, essentially, an array of types. It was pretty
clear that manipulating an array of types in C++ is hard, time
consuming, and when you make a mistake, how to fix it is often very
difficult to deduce.
But if it was an array of integers, rather than an array of types, the
presentation would have been over in 5 minutes. This seems to happen a
lot with C++ metaprogramming - once you get past the mechanics and
figure out what the programmer was trying to actually do, it's pretty
simple. And the reason it is simple is because it's *too hard* to do
anything else with C++ TMP.
So, a huge goal with D TMP is to be able to do simple things in a
straightforward, simple manner. One should be able to manipulate an
array of types as easily as an array of ints. Writing templates should
not be harder than writing a function.
The other problem is that, because the techniques of metaprogramming
in C++ are basically based on hacks, there is a large cost in compile-
time. Once you create simple types using metaprogramming you find that
these can be used in theory to construct more complex types, but there
is a practical limit both in compile time and in compiler resources
which puts a cap on what you can do in practise.
Right. Case in point: the factorial template which recursively
instantiates itself to compute a factorial at compile time. For every
iteration, the compiler must:
1) instantiate a template
2) encode the entire state of the template and all previous iterations
into the (mangled) name of the template
3) emit symbol table info (and possibly debug info) for each of these
templates, pretty much making an explosion of output
If you think about it, that's a fantastic amount of processing and
memory required to do something as trivial as a factorial. It is why
complex calculations cannot be done with C++ templates, even if they
theoretically can be. It's why C++ TMP compiles very, very slowly.
In D metaprogramming has been designed in as part of the language. It
is much simpler to use and also compiles fast. I believe that Daveed
Vandevoorde has been working on a separate metaprogramming language
for C++. It is a shame that the two sides cant get together and try to
look at getting a C++ compiler with these extensions together in a
similar way that the ConceptGCC compiler has been used to test and
prove the Concept ideas.
I'm not familiar with Daveed's work, nor did I know he was working on
such a language.
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Walter Bright
Digital Mars C, C++, D programming language compilers
http://www.astoriaseminar.com Extraordinary C++