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From: Jens Maurer (Jens.Maurer_at_[hidden])
Date: 2001-01-12 14:15:42

"Moore, Paul" wrote:
> From: "Stephen Silver" <boost_at_[hidden]>
> > template <class T> rational(T n) : num(), den(1) { num = n; }
> Good catch - I'll add this.

Do make sure to document it right in the code. Any time I see such
code as "default initialize + assign" in foreign code, I assume
the code writer has not understood C++ at all.

> Hmm. Overall, I agree with this, although I wonder whether there could be
> cases where default-construct and assign is significantly slower than just
> constructing. Seems far less likely than even the explicit constructor case,
> though, and it's "only" performance, so I'd go with it.

What about a fixed-size bigint based on a C array which needs to be
default-initialized to 0 all over?

> Thinking some more, I suspect that we have to use this version as we must
> avoid constructing a rational<T> from a type U when the only constructor
> from U to T is explicit. So yes, I go with your code.

It's ugly. It's a template implicit conversion. :-(
For the problem at hand, it seems sufficient to have a rational
constructor which takes an "int" (or long) only. The template
approach is only really needed if there is some non-builtin
type U which can be converted to T, for example, another bigint
class. And in this case, we arguably invoke two user-defined
conversions (though technically speaking, we do it in two steps),
i.e. from U to T and from T to rational<T>. Not my favourite
perspective regarding complexity. Sorry, but listening to
Steve Adamczyk at the Toronto ISO C++ meeting explaining auto_ptr
implicit conversions built up a rather strong opinion against
complex conversion chaining.

I'd vote for the conservative "int only" approach.

Jens Maurer

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