Re: How to overcome overloading ambiguity

From:
Michael Doubez <michael.doubez@free.fr>
Newsgroups:
comp.lang.c++
Date:
Tue, 16 Jun 2009 06:49:46 -0700 (PDT)
Message-ID:
<c742bc02-3c26-40f9-904d-c10275e33e00@b9g2000yqm.googlegroups.com>
On 16 juin, 14:22, "Fred Zwarts" <F.Zwa...@KVI.nl> wrote:

"Michael Doubez" <michael.dou...@free.fr> wrote in messagenews:93a4aefc-d=

7bc-4952-a4ab-d338c18c9717@t16g2000yqi.googlegroups.com...

On 16 juin, 13:56, "Fred Zwarts" <F.Zwa...@KVI.nl> wrote:

"Michael Doubez" <michael.dou...@free.fr> wrote in messagenews:8429d814=

-ba46-441d-900c-3750a9412310@x3g2000yqa.googlegroups.com...

On 16 juin, 11:36, "Fred Zwarts" <F.Zwa...@KVI.nl> wrote:

In my software I need a vector of boolean values.
My first attempt was to use vector<bool>, but vector has a specializ=

ation for the bool type

with unwanted implications. For example, I need to pass elements of =

the vector as

reference to bool (bool&) to certain functions which may modify the =

value of such elements.

This is not possible with vector<bool>.

My next idea was to create a class Boolean_t which can be used inste=

ad of the bool type,

with transparent functionality. So I attempted the following:

class Boolean_t {
private:
bool Flag;

public:

// Normal constructor:

Boolean_t (const bool InitialValue = false) : Flag (InitialValue) =

{}

// Define conversion to bool and reference to bool.

operator bool () const {
return Flag;
}


Which can be replaced by:
operator const bool& () const {return Flag; }

operator bool & () {
return Flag;
}

};

My problem is with two operator definitions. I need both.
The first one is needed to convert a const object to a bool value,
which cannot be done with the second one.
The second one is needed to pass objects of type Boolean_t to functi=

ons that need a reference to bool,

which cannot be done with the first one.

However, if an object of type Boolean_t is used in a boolean express=

ion, the compiler complains

about an ambiguity in the reference to a conversion function.


Can you post an example ?

Indeed, I understand that both
of these conversion functions could be used in this case.
Is there a solution to overcome this ambiguity and to keep the funct=

ionality?

I followed your suggestion, because it even allows references to const =

objects.

Doesn't make a lot of difference here. const reference bind to lvalue.

It does not make a difference for ambiguity according to the compiler.
However, I now see that another compiler does not complain.

When I then try something like:

Boolean_t Value;

....

if (Value) {
...

}

Then one compiler complains about the expression in the if statement th=

at there is an ambiguity

in choosing between the two operators.
Another compiler, however, does not complain.
Which one is right?


Which compilers ?
This seems to compile fine with MSVC9 and g++ and I don't see the
problem.

Well, in fact the program that complains is not a compiler, but a lint pr=

ogram.

FlexeLint 9.0b to be exact. (Seewww.gimpel.com.)
If it can be explained that this is a bug in FlexeLint, I could file a bu=

g report.

It is a bug then. I guess the overload resolution is not performed.
The cv-qualifier of the non-static member function should be enough
and the tol=ols says it is not.

I note that =A712.3/2 doesn't specify overload resolution should be
performed: only unambiguous and access control are required. I assume
unambiguous means name lookup and overload resolution.

--
Michael

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