Problems with ....hmm, linker perhaps

[Patrick Kowalzick]

Dear all,

IMO the following program is legal and should compile.

//***** code start *********
// Definition of class A
template <typename T> struct A
{
typedef T value_type;
};

// Definition of class B
template <typename> struct B {};

// Function foo
// returntype important !
template <typename T>
B<typename T::value_type> foo()
{
return B<typename T::value_type>();
};

int main()
{
typedef B< int >( * ptr )(); // Function pointer

ptr foo_ptr = foo< A<int> >; // Initialization of function pointer

return 0;
}
//***** code end *********

It builds fine with g++ and MSVC7.1, but the latter one fails linking:

unresolved external symbol "struct B<int> __cdecl foo<struct A<int> >(void)"
(??$foo@U?$A@H@@@@YA?AU?$B@H@@XZ) referenced in function _main

what I do not really understand. It is getting a lit more weird now. If I
change

ptr foo_ptr = foo< A<int> >;

to

ptr foo_ptr ;
foo_ptr = foo< A<int> >;

it compiles and links fine as well with MSVC 7.1.

Before I blame anyone, I want to ask if I overlooked something? IMO the code
is fine.

Regards,
Patrick

[JKop]

If I change

ptr foo_ptr = foo< A<int> >;

to

ptr foo_ptr ;
foo_ptr = foo< A<int> >;

it compiles and links fine as well with MSVC 7.1.

Well, without scrutanizing your code too much, the only difference in the
above is that the former calls a constructor, while the latter calls the
default constructor and then calls operator=.
-JKop

[Thomas Maier-Komor]

Patrick Kowalzick wrote:

Dear all,

IMO the following program is legal and should compile.

//***** code start *********
// Definition of class A
template <typename T> struct A
{
typedef T value_type;
};

// Definition of class B
template <typename> struct B {};

// Function foo
// returntype important !
template <typename T>
B<typename T::value_type> foo()
{
return B<typename T::value_type>();
};

int main()
{
typedef B< int >( * ptr )(); // Function pointer

ptr foo_ptr = foo< A<int> >; // Initialization of function pointer

return 0;
}
//***** code end *********

It builds fine with g++ and MSVC7.1, but the latter one fails linking:

unresolved external symbol "struct B<int> __cdecl foo<struct A<int> >(void)"
(??$foo@U?$A@H@@@@YA?AU?$B@H@@XZ) referenced in function _main

what I do not really understand. It is getting a lit more weird now. If I
change

ptr foo_ptr = foo< A<int> >;

to

ptr foo_ptr ;
foo_ptr = foo< A<int> >;

it compiles and links fine as well with MSVC 7.1.

Before I blame anyone, I want to ask if I overlooked something? IMO the code
is fine.

Regards,
Patrick

compiles and links fine with Sun Studio 9...

[Unforgiven]

"Patrick Kowalzick" <Pa***************@cern.ch> wrote in message
news:cl**********@sunnews.cern.ch...

Dear all,

IMO the following program is legal and should compile.

I concur with your opinion. I tested and verified that VC7.1 does not. I
think it's not a problem with the linker though, it looks as if the compiler
isn't instantiating the foo<A<int> > template, so when the linker looks for
it, it finds nothing.

I also tested this under Visual C++ 2005 (VC8) beta 1
(http://lab.msdn.microsoft.com/vs2005/), and it compiles and links fine,
which seems to suggest this is indeed a bug in VC7.1 which has been fixed
for VC8.

--
Unforgiven

[Patrick Kowalzick]

Dear Unforgiven,

IMO the following program is legal and should compile.
I concur with your opinion. I tested and verified that VC7.1 does not. I
think it's not a problem with the linker though, it looks as if the

compiler isn't instantiating the foo<A<int> > template, so when the linker looks for it, it finds nothing.

I also tested this under Visual C++ 2005 (VC8) beta 1
(http://lab.msdn.microsoft.com/vs2005/), and it compiles and links fine,
which seems to suggest this is indeed a bug in VC7.1 which has been fixed
for VC8.

Thanks a lot.

So it seems that I have to search a workaround for my application :(, which
has a little bit different structure compared to the testcase.

Patrick

[Patrick Kowalzick]

Hi JKop,

If I change

ptr foo_ptr = foo< A<int> >;

to

ptr foo_ptr ;
foo_ptr = foo< A<int> >;

it compiles and links fine as well with MSVC 7.1.

Well, without scrutanizing your code too much, the only difference in the
above is that the former calls a constructor, while the latter calls the
default constructor and then calls operator=.

[answer for JKop]
Bullshit.

[answer for the rest]
I doubt that for function pointers constructors must be called. Is this
compiler dependent? Whats the terminology for built-in types?

Reagrds,
Patrick

[Patrick Kowalzick]

> compiles and links fine with Sun Studio 9...

Thx.

[JKop]

[answer for JKop]
Bullshit.

Perhaps I should be more general:
A) AnyClass blah = some_expression;

This calls a contructor, supplying "some_expression" as an argument.
B) AnyClass blah; blah = some_expression;
This calls the default constructor. It then calls operator=.
Use your own intelligence to see what I'm getting at.
-JKop

[Richard Herring]

In message <5z*******************@news.indigo.ie>, JKop <NU**@NULL.NULL>
writes

[answer for JKop]
Bullshit.

Perhaps I should be more general:
A) AnyClass blah = some_expression;

This calls a contructor, supplying "some_expression" as an argument.
B) AnyClass blah; blah = some_expression;
This calls the default constructor. It then calls operator=.
Use your own intelligence to see what I'm getting at.

What is the type of foo_ptr?

--
Richard Herring

[Patrick Kowalzick]

> > [answer for JKop]

Bullshit.

Yep.

Perhaps I should be more general:
A) AnyClass blah = some_expression;

This calls a contructor, supplying "some_expression" as an argument.
B) AnyClass blah; blah = some_expression;
This calls the default constructor. It then calls operator=.

You are talking about classes, don't you?
See the OP, I was talking about function pointers.

Regards,
Patrick

[Rob Williscroft]

Patrick Kowalzick wrote in news:cl**********@sunnews.cern.ch in
comp.lang.c++:

int main()
{
typedef B< int >( * ptr )(); // Function pointer

ptr foo_ptr = foo< A<int> >; // Initialization of function pointer

return 0;
}
//***** code end *********

It builds fine with g++ and MSVC7.1, but the latter one fails linking:

unresolved external symbol "struct B<int> __cdecl foo<struct A<int>

(void)" (??$foo@U?$A@H@@@@YA?AU?$B@H@@XZ) referenced in function

_main

This is a bug in MSVC 7.1, Its "forgoten" to do the implicit
instantiation of foo< A< int > >.

I haven't seen it in precisly this context (taking a function pointer)
but in a few other's I have.

As a minor data point I compiled with:

cl -nologo -Za -Zc:forScope,wchar_t -EHsc -O1 -ML -Tptest.cpp

and msvc *didn't* exhibit the problem, but if I compiled:

cl test.cpp

I got the unresolved external, I don't know which combination of
option's achived this turnaround or why.

Rob.
--
http://www.victim-prime.dsl.pipex.com/

[Andrey Tarasevich]

Patrick Kowalzick wrote:

...
int main()
{
typedef B< int >( * ptr )(); // Function pointer

ptr foo_ptr = foo< A<int> >; // Initialization of function pointer

return 0;
}
//***** code end *********

It builds fine with g++ and MSVC7.1, but the latter one fails linking:

unresolved external symbol "struct B<int> __cdecl foo<struct A<int> >(void)"
(??$foo@U?$A@H@@@@YA?AU?$B@H@@XZ) referenced in function _main
...

Seems to be a compiler problem. Try using the address-of operator explicitly

ptr foo_ptr = &foo< A<int> >;

I remember that sometimes it helped to solve similar problems in MSVC++
6, but I don't know whether it will help in MSVC++ 7.

Explicit instantiation will probably also solve the problem, but that
would be a high-maintenance solution.

--
Best regards,
Andrey Tarasevich

[Ron Natalie]

There is a known bug in visual studio 6 when defining
function templates that don't have the templated variables
in their signature.

[Patrick Kowalzick]

Hi Andrey,

Seems to be a compiler problem. Try using the address-of operator explicitly
ptr foo_ptr = &foo< A<int> >;
same problem,...
I remember that sometimes it helped to solve similar problems in MSVC++
6, but I don't know whether it will help in MSVC++ 7.

Explicit instantiation will probably also solve the problem, but that
would be a high-maintenance solution.

but while I get around it like this:

ptr foo_ptr;
foo_ptr = &foo< A<int> >;

it is ok for now :). I documented in my code as a bug. Shall be enough....

Thanks,
Patrick