Why is this not ambiguous?

[REH]

Can some tell me why the chooses the constructor in class B over operator B
in class A? Is this not ambiguous?

Thanks.
#include <iostream>

using namespace std;

struct A;

struct B {
B() {}
B(const A&) {} // this is choosen
};

struct A {
A() {}
operator B() const {return B();} // this is not
};

int main()
{
A a;
B b;

b = a; // ambiguous?
}

[Victor Bazarov]

REH wrote:

Can some tell me why the chooses the constructor in class B over operator B
in class A? Is this not ambiguous?
I am not sure I understand the first question. The program is ill-formed
because the "b = a" _is_ in fact ambiguous.

Thanks.
#include <iostream>

using namespace std;

struct A;

struct B {
B() {}
B(const A&) {} // this is choosen
};

struct A {
A() {}
operator B() const {return B();} // this is not
};

int main()
{
A a;
B b;

b = a; // ambiguous?
}

V

[Howard]

"REH" <bo***@nowhere. net> wrote in message
news:d2******** *@cui1.lmms.lmc o.com...

Can some tell me why the chooses the constructor in class B over operator
B
in class A? Is this not ambiguous?

Thanks.
#include <iostream>

using namespace std;

struct A;

struct B {
B() {}
B(const A&) {} // this is choosen
};

struct A {
A() {}
operator B() const {return B();} // this is not
};

int main()
{
A a;
B b;

b = a; // ambiguous?
}

I think the answer is given in Stroustrup's "The C++ Programming Language",
p277: "User-defined conversions are considered only if they are neccessary
to resolve a call." I don't have the Standard to back up that statement,
but if he's correct, then the fact that a copy-constructor exists that can
handle the job negates the need to bother looking at other alternatives.
Thus, no ambiguity is encountered.

-Howard

[Alf P. Steinbach]

* REH:

Can some tell me why the chooses the constructor in class B over operator B
in class A? Is this not ambiguous?

Thanks.
#include <iostream>

using namespace std;

struct A;

struct B {
B() {}
B(const A&) {} // this is choosen
};

struct A {
A() {}
operator B() const {return B();} // this is not
};

int main()
{
A a;
B b;

b = a; // ambiguous?
}

Comeau online compilation:

Comeau C/C++ 4.3.3 (Aug 6 2003 15:13:37) for ONLINE_EVALUATI ON_BETA1
Copyright 1988-2003 Comeau Computing. All rights reserved.
MODE:strict errors C++

"ComeauTest .c", line 18: error: more than one user-defined conversion from "A"
to
"const B" applies:
function "A::operato r B() const"
function "B::B(const A &)"
b = a; // ambiguous?
^

1 error detected in the compilation of "ComeauTest .c".

In strict mode, with -tused, Compile failed

--
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?

[Grant]

As far as the microsoft VS.NET 2003 compiler is concerned this is an
error.

Grant

[REH]

"Alf P. Steinbach" <al***@start.no > wrote in message
news:42******** ********@news.i ndividual.net.. .

* REH:

Can some tell me why the chooses the constructor in class B over operator B in class A? Is this not ambiguous?

Thanks.
#include <iostream>

using namespace std;

struct A;

struct B {
B() {}
B(const A&) {} // this is choosen
};

struct A {
A() {}
operator B() const {return B();} // this is not
};

int main()
{
A a;
B b;

b = a; // ambiguous?
}
Comeau online compilation:

Comeau C/C++ 4.3.3 (Aug 6 2003 15:13:37) for ONLINE_EVALUATI ON_BETA1
Copyright 1988-2003 Comeau Computing. All rights reserved.
MODE:strict errors C++

"ComeauTest .c", line 18: error: more than one user-defined conversion from

"A" to
"const B" applies:
function "A::operato r B() const"
function "B::B(const A &)"
b = a; // ambiguous?
^

1 error detected in the compilation of "ComeauTest .c".

In strict mode, with -tused, Compile failed

That's interesting. Even compiling with all warnings, GCC failed to
complain. I would file a bug report, but I'm not sure which is correct. I
just trying it with VC 2003 ToolKit, and says:

error C2679: binary '=' : no operator found which takes a right-hand operand
of type 'A' (or there is no acceptable conversion)

Which I think is wrong, or at least misleading. I guess I will have to look
in the standard, which is hard for me to comprehend.

[REH]

"Victor Bazarov" <v.********@com Acast.net> wrote in message
news:qO******** ***********@new sread1.mlpsca01 .us.to.verio.ne t...

REH wrote:

Can some tell me why the chooses the constructor in class B over operator B in class A? Is this not ambiguous?

I am not sure I understand the first question. The program is ill-formed
because the "b = a" _is_ in fact ambiguous.

Sorry, that first question should have read, "Can some tell me why then
compiler chooses the constructor in class B over operator B in class A?"
Meaning, instead of complaining about the ambiguity, my compiler, given the
choice of:

1) B::B(const A&);
2) A::operator B() const;

It chose option #1.

[Ali Çehreli]

REH wrote:

"Alf P. Steinbach" <al***@start.no > wrote in message

"ComeauTest .c", line 18: error: more than one user-defined
conversion from "A" to
"const B" applies:
function "A::operato r B() const"
function "B::B(const A &)"
b = a; // ambiguous?

That's interesting. Even compiling with all warnings, GCC failed to
complain.

Get a newer gcc; version 3.4.2 spots the ambiguity.

Ali

[REH]

"Ali Çehreli" <ac******@yahoo .com> wrote in message
news:3b******** *****@individua l.net...

REH wrote:

"Alf P. Steinbach" <al***@start.no > wrote in message

"ComeauTest .c", line 18: error: more than one user-defined
conversion from "A" to
"const B" applies:
function "A::operato r B() const"
function "B::B(const A &)"
b = a; // ambiguous?

That's interesting. Even compiling with all warnings, GCC failed to
complain.

Get a newer gcc; version 3.4.2 spots the ambiguity.

Ali

Um, I *am* using 3.4.2. What is your targetted for?

[REH]

"REH" <bo***@nowhere. net> wrote in message
news:d2******** *@cui1.lmms.lmc o.com...

"Ali Çehreli" <ac******@yahoo .com> wrote in message
news:3b******** *****@individua l.net...

Get a newer gcc; version 3.4.2 spots the ambiguity.

Ali

Um, I *am* using 3.4.2. What is your targetted for?

I just tried it with version 3.4.3, and it still accepts it.