Re: C++ Frequently Questioned Answers

David Abrahams <>
Thu, 15 Nov 2007 19:46:43 CST
on Fri Nov 09 2007, Yossi Kreinin <> wrote:

On Nov 6, 9:39 pm, David Abrahams <>

This seems to imply that the sequence size is bound by a constant,

Yes. But that's okay; you can change the constant.

Well, I can also change any other fixed size limit like a buffer size
constant, but I'd rather have it dynamically allocated as required by
the context where it's used...

And I still don't see how you can set that constant to an arbitrary
value. You can only generate templates with arbitrary number of
parameters using macros, and macros don't really support compile time
loops, so the maximal number of iterations in macro meta-programming
is /also/ limited by a constant,

Not really; Paul Mensonides developed a preprocessor library called
"Chaos" that uses a kind of low-level recursion in the preprocessor.

Boost.Preprocessor (which is more portable to broken preprocessors ---
of which there are surprisingly many) does rely on fixed limits, but
those limits are so large in practice that you'll never run into them.

and *this* constant *can't* be a preprocessor parameter since C
macros can't generate C macros.

So isn't the compile time constant bound by another compile time

I don't follow you.

No, you also have

struct vector0 {};
template <class A0> struct vector1 {};
template <class A0, class A1> struct vector2 {};
template <class A0, class A1, class A3> struct vector3 {};

So by "type array", you mean a template with N arguments?


Doesn't it have the problem mentioned above (the fixed size limit
that can't be configured by a preprocessor flag)?

Nope; in fact that's exactly what you do.

I was thinking about nesting these templates, so you'd get a tree,
which doesn't have the fixed size limit problem, but has a
complexity problem, which is probably much worse in practice.

Tree structures are good for limiting recursion depth, which can be
costly in compile time.

Sure you can. Anything you can do in other pure functional
languages, you can do with templates. It's just a lot uglier ;-)

I rest my case :)

You won't ever hear me arguing that TMP is the most elegant approach
possible. However, your case was, in this instance,

I think that implementing type lists to be "compile time type hashes
or sets" (that is, represent hash tables using TMP) is really really
hard (both for a human to write or read and for a compiler to

and my point was that once you have the library written that does
these things, it gets pretty easy to read the code that uses them.

In practice, I think that TMP proponents will claim that compile time
is cheap,

I would never claim that. It's a real concern, which is why in my
book with Aleksey Gurtovoy we do extensive efficiency analysis.

not that TMP is easy to implement efficiently in terms of compiler

Nor that.

Dave Abrahams
Boost Consulting

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