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From: Hyman Rosen <hymie@prolifics.com>
Subject: Re: STL, Ada, C++ (Was Re: The Next Microsoft?)
Date: 2000/05/09
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Robert A Duff <bobduff@world.std.com> writes:
> Hyman Rosen <hymie@prolifics.com> writes:
> > The C++ Standard requires support for a minimum of 17 levels of templates.
> > The GNU compiler has a command-line argument to specify what depth you want
> > if you need more.
> 
> Why?  I mean, why is there a need for a limit?  (And, less importantly,
> why 17?)

The depth of template nesting is potentially unlimited, and erroneous
code can require infinite depth, so the standard sets a minimum depth
that must be supported, and strictly conforming code shouldn't require
anything deeper. Here's a trivial example -

template <int N>
struct factorial
{
	enum { v = N * factorial<N - 1>::v };
};

template <>
struct factorial<0>
{
	enum { v = 1 };
};

If I use factorial<5>::v, I'm fine. If I use factorial<20>::v,
I've gone over 17 levels, and a strictly conforming compiler
doesn't have to accept my code. If I use factorial<-1>::v, I'm
very happy that my compiler has some limit :-)