🚀 go-pugleaf

Eyretel Ltd. (eyretel@fastnet.co.uk) wrote:
: neis@cs.uni-sb.de (Stefan Neis) says:
: >
: >        Hi there,
: >
: >Using g++-2.7.0 (which normally just works great...) I got a strange error
: >message today:
: >module_div2.C: In function `int main()':
: >module_div2.C:26: call of overloaded method `operator <<(order)' is ambiguous
: >alg_number.h:418: candidates are: operator <<(ostream &, const module &)
: >alg_number.h:174:                 operator <<(ostream &, order &)
: >
: >I defined classes "order" and "module" and there is a cast - operator which
: >makes modules out of orders. 
: >
: > (Stuff from my original posting deleted;
: 
: The compiler's confusion is justified; since it can convert an order to a
: module, there is no way of telling which << it should use.

Bjarne Stroustrup states (in "The C++ programming language", p.589):
"[1] Exact match: Sequences of zero or more trivial conversions are
	better than all other sequences...."

	(Note: Conversion from "order" to "order &" is such a so
	called trivial conversion)

So in my understanding, the compiler should detect, that
	"operator <<(ostream &, order &)"
matches better, than "operator <<(ostream &, const module &) which
involves a "user defined conversion", shouldn't it?

Note, that gcc-2.6.3 does not give this error. It produces perfectly
well working code.

: If my understanding of the error message is correct, the compiler is doing
: a conversion from order to order& anyway, since there is no operator
: <<(ostream &, order)

Well, I still assume, that such "trivial conversions" aren't really
counted -- unless that's been changed in the ANSI-draft-standard.
 
: If you can't see the difference, re-read!  One's a reference, the
: other isn't. Try a cast of your order object to order&.

Of course I tried that and it works, but I didn't understand, why this
should be necessary...


	Stefan Neis