On Sep 25, 3:58 pm, Barry <dhb2...@gmail.comwrote:
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Zeppe wrote: Quote:
Barry wrote: Quote:
Zeppe wrote:
>I have the following problem, that I'll try to explain with a very
>minimal example:
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>class B
>{
>public:
> B() { }
> B(const A&) { }
> B(const B&) { }
>};
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>class C
>{
>public:
> operator A() const { return A(); }
> operator B() const { return B(); }
>};
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>int main(){
> const C& c = C();
> B b = static_cast<B>(c);
> return 0;
>}
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>Basically, I would expect that casting a const C& to B
>would call the operator B() of C and that the construction
>C -B (through the copy
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what makes "operator B()" have higher priority?
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because I specified a method for the conversion of C into B,
in C. I would expect it to be called in the static cast. The
other way (C->A->B) implies one level of implicit
conversion, doesn't it?
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According to §5.2.9/3 (static_cast):
Otherwise, an expression e can be explicitly converted
to a type T using a static_cast of the form
static_cast<T>(e) if the declaration T t(e); is
well-formed, for some invented temporary variable t
(8.5). The effect of such an explicit conversion is the
same as performing the declaration and initialization
and then using the temporary variable as the result of
the conversion. [...]
So overload resolution treats the expression as if it were:
B untamedTemp( c ) ;
(followed by B b( untamedTemp ) ;). B has two constructors, one
which takes a const A&, and one which takes a const B&. C can
convert (equally well) to one or the other.
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>constructor) would be preferred to C -A -B. In Visual C++ 9 it
>actually does so, but gcc complains that there is an ambiguity. Who's
>right (I guess Visual c++) and, in case, how could I nicely work
>around the problem?
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gcc is right,
You can compile using VC with /Za option, which disables the
extension, and produces the compile error.
I know ya gonna complain. :-)
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No, I'm not, just a little bit disappointed :) And what about a
workaround? Is the only way to solve this to clean up one of the two
conversion ways?
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I think it is, since it violates the standard.
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The "work-around" is to not define so many implicit conversions.
Generally, too many implicit conversions will lead to
ambiguities. And even if it doesn't, it leads to code which is
hard to understand and to maintain.
--
James Kanze (GABI Software) email:james.kanze@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientierter Datenverarbeitung
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ambiguous constructor? Is it right? 
