Re: How to overcome overloading ambiguity

Michael Doubez <>
Tue, 16 Jun 2009 06:49:46 -0700 (PDT)
On 16 juin, 14:22, "Fred Zwarts" <> wrote:

"Michael Doubez" <> wrote in messagenews:93a4aefc-d=

On 16 juin, 13:56, "Fred Zwarts" <> wrote:

"Michael Doubez" <> wrote in messagenews:8429d814=

On 16 juin, 11:36, "Fred Zwarts" <> 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 {
bool Flag;


// 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=


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


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

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


FlexeLint 9.0b to be exact. (
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.


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