Variable declaration or function prototype

[Andrew Ward]

Hi All,
Could someone please tell me why this code does not compile.

struct A {};

struct B {
B(A &) {}
};

struct C {
C(B &) {}
void f() {}
};

void foo() {
A a;
C c(B(a));
c.f();
}

If I change foo() to the following then it does compile:

void foo() {
A a;
B b(a);
C c(b);
c.f();
}

I would have thought that you could pass the B temporary into C's
constructor like that?

Andy

[White Wolf]

Re: Variable declaration or function prototype

Andrew Ward wrote:[color=blue]
> Hi All,
> Could someone please tell me why this code does not compile.
>
> struct A {};
>
> struct B {
> B(A &) {}
> };
>
> struct C {
> C(B &) {}
> void f() {}
> };
>
> void foo() {
> A a;
> C c(B(a));
> c.f();
> }
>
> If I change foo() to the following then it does compile:
>
> void foo() {
> A a;
> B b(a);
> C c(b);
> c.f();
> }
>
> I would have thought that you could pass the B temporary into C's
> constructor like that?[/color]

You could if you did write the constructors properly:

B(A const &);

C(B const &);

Temporaries cannot be bound to non-const references.

--
WW aka Attila

[Jim Fischer]

Re: Variable declaration or function prototype

Andrew Ward wrote:[color=blue]
> Hi All,
> Could someone please tell me why this code does not compile.[/color]

Unless I'm mistaken, the statement

C c(B(a));

is a function declaration (prototype). It declares a function named 'c'
that is passed (by value) a class B object, and returns (by value) a
class C object. This statement does not instantiate a class C object
named 'c'.

--
Jim

To reply by email, remove "link" and change "now.here" to "yahoo"
jfischer_link58 09{at}now.here. com

[Andrew Ward]

Re: Variable declaration or function prototype


"White Wolf" <wolof@freemail .hu> wrote in message
news:bjodhd$70o $1@phys-news1.kolumbus. fi...[color=blue]
> Andrew Ward wrote:[color=green]
> > Hi All,
> > Could someone please tell me why this code does not compile.
> >
> > struct A {};
> >
> > struct B {
> > B(A &) {}
> > };
> >
> > struct C {
> > C(B &) {}
> > void f() {}
> > };
> >
> > void foo() {
> > A a;
> > C c(B(a));
> > c.f();
> > }
> >
> > If I change foo() to the following then it does compile:
> >
> > void foo() {
> > A a;
> > B b(a);
> > C c(b);
> > c.f();
> > }
> >
> > I would have thought that you could pass the B temporary into C's
> > constructor like that?[/color]
>
> You could if you did write the constructors properly:
>
> B(A const &);
>
> C(B const &);
>
> Temporaries cannot be bound to non-const references.
>
> --
> WW aka Attila
>
>[/color]

Even when I try passing by const & like you suggested it still does not
work, the error message is:

error C2228: left of '.f' must have class/struct/union type
type is 'overloaded-function'

Maybe Jim Fischer's suggestion is correct?

Andy

[Kevin Goodsell]

Re: Variable declaration or function prototype

Andrew Ward wrote:
[color=blue]
> "White Wolf" <wolof@freemail .hu> wrote in message
> news:bjodhd$70o $1@phys-news1.kolumbus. fi...
>[color=green]
>>Andrew Ward wrote:
>>[color=darkred]
>>>Hi All,
>>>Could someone please tell me why this code does not compile.
>>>
>>>struct A {};
>>>
>>>struct B {
>>> B(A &) {}
>>>};
>>>
>>>struct C {
>>> C(B &) {}
>>> void f() {}
>>>};
>>>
>>>void foo() {
>>> A a;
>>> C c(B(a));
>>> c.f();
>>>}
>>>
>>>If I change foo() to the following then it does compile:
>>>
>>>void foo() {
>>> A a;
>>> B b(a);
>>> C c(b);
>>> c.f();
>>>}
>>>
>>>I would have thought that you could pass the B temporary into C's
>>>constructo r like that?[/color]
>>
>>You could if you did write the constructors properly:
>>
>>B(A const &);
>>
>>C(B const &);
>>
>>Temporaries cannot be bound to non-const references.
>>
>>--
>>WW aka Attila
>>
>>[/color]
>
>
> Even when I try passing by const & like you suggested it still does not
> work, the error message is:
>
> error C2228: left of '.f' must have class/struct/union type
> type is 'overloaded-function'
>
> Maybe Jim Fischer's suggestion is correct?
>[/color]

In fact, they are both correct (I think). Attila's answer is what I
would have said. The problem Jim pointed out is a bit more insidious:

C c(B(a));

I don't know the exact rule, but basically because this *could* be
either a function declaration or an object instantiation, the former
wins. It's equivalent to

C c(B a);

With a pair of superfluous parenthesis.

-Kevin
--
My email address is valid, but changes periodically.
To contact me please use the address from a recent posting.

[Jim Fischer]

Re: Variable declaration or function prototype

Kevin Goodsell wrote:[color=blue][color=green]
>> Maybe Jim Fischer's suggestion is correct?
>>[/color]
>
> In fact, they are both correct (I think). Attila's answer is what I
> would have said. The problem Jim pointed out is a bit more insidious:
>
> C c(B(a));
>
> I don't know the exact rule, but basically because this *could* be
> either a function declaration or an object instantiation, the former
> wins. It's equivalent to
>
> C c(B a);
>
> With a pair of superfluous parenthesis.[/color]

I'm 99.999% sure the statement 'C c(B(a));' is a function declaration
(see: 8.2/1 in ISO 14882:1998).

FWIW, you can disambiguate the interior subexpression 'B(a)' by wrapping
it in parens, e.g.,

C c( (B(a)) );

Now the compiler treats 'B(a)' as an object declaration that uses a
function-style type cast as the initializer (and not a function
declaration with a redundant set of parens around a parameter name).
IOW, 'B(a)' is now considered an invocation of B::B(A&). Consequently,
the surrounding expression - i.e., the declaration of 'c' - is also
treated as an object declaration with a function-style type cast as the
initializer.

Caveat: Making this change to your code introduces another problem.
Specifically, the unnamed, temporary class B object that results from
the invocation of B::B(A&) cannot be bound to a non-const reference. So
the construction of 'c' cannot occur because class C does not have a
suitable constructor - e.g., C::C(B&) cannot be used because B& is a
non-const reference. There are (at least) two fixes for this problem:

1) Modify the existing class C ctor

C(B &) { }

so its reference is const-valued

C(const B &) { }

or,

2) Add a second ctor to class C; one with a suitable const reference
argument:

class C {
C (B &) { }
C (const B &) { }
void f () { }
}

--
Jim

To reply by email, remove "link" and change "now.here" to "yahoo"
jfischer_link58 09{at}now.here. com

[White Wolf]

Re: Variable declaration or function prototype

Andrew Ward wrote:
[SNIP][color=blue]
> Even when I try passing by const & like you suggested it still does
> not work, the error message is:
>
> error C2228: left of '.f' must have class/struct/union type
> type is 'overloaded-function'
>
> Maybe Jim Fischer's suggestion is correct?[/color]

Not maybe. :-) Definitely. I am ashamed not seeing it - especially because
I have got my copy - of the book mentioning this "gotcha" - signed by the
author. Well, well well. Time for me to look for a job where no brains are
required. :-)

--
WW aka Attila