I can not compile code from <<modern c++ design>>

I can not compile code from <<modern c++ design>>

[Di Yang]

Hi,

Following code is listed in chapter 2.7 Detecting Convertibility and 
Inheritance at Compile Time:

#include <iostream>

template <class T, class U>
class Conversion
{
typedef char Small;
class Big { char dummy[2]; };

static Small Test(U);
static Big Test(...);
static T MakeT();

public:
enum { exists = sizeof(Test(MakeT())) == sizeof(Small) };
};

int main()
{
using namespace std;

cout<< Conversion<double, int>::exists << ' '<< Conversion<char, 
char*>::exists;

return 0;
}

and I got following errors:

C:\ttt>gcc -o template template.cpp
template.cpp:5: warning: all member functions in class `Conversion<T, U>' are
    private
template.cpp: In instantiation of `Conversion<double, int>':
template.cpp:21:   instantiated from here
template.cpp:21: invalid use of undefined type `class Conversion<double, int>'
template.cpp:5: declaration of `class Conversion<double, int>'
template.cpp:21: invalid use of undefined type `class Conversion<double, int>'
template.cpp:5: declaration of `class Conversion<double, int>'
template.cpp:21: enumerator value for `exists' not integer constant
template.cpp: In instantiation of `Conversion<char, char*>':
template.cpp:21:   instantiated from here
template.cpp:21: invalid use of undefined type `class Conversion<char, char*>'
template.cpp:5: declaration of `class Conversion<char, char*>'
template.cpp:21: invalid use of undefined type `class Conversion<char, char*>'
template.cpp:5: declaration of `class Conversion<char, char*>'
template.cpp:21: enumerator value for `exists' not integer constant

Anyone can help me?

Thanks!
Di Yang
Best Regards
Di Yang
Phone: 86-10-85296529 ext.130      QUALCOMM CDMA Technologies
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Email: diy@qualcomm.com               Beijing 100020, China

Re: I can not compile code from <<modern c++ design>>

[Matthias Oltmanns]

Am Mon, 2003-04-07 um 11.24 schrieb Di Yang:
> Hi,
> 
> public:
> enum { exists = sizeof(Test(MakeT())) == sizeof(Small) };

Hi,

this seems to be not valid C++ code because the calls to the
static members 'Test' and 'MakeT' are not const expressions at compile
time.
For assignments to enum literals there are only integer expressions
allowed which are evaluated at compile time.

cu
Matthias

-- 
Matthias Oltmanns

Tel: 04421-1543-274
mail: Mathias.Oltmanns.Oltmanns@sysde.eads.net

Re: I can not compile code from <<modern c++ design>>

[Di Yang]

At 13:02 4/7/2003 +0200, Matthias Oltmanns wrote:
>Am Mon, 2003-04-07 um 11.24 schrieb Di Yang:
> > Hi,
> >
> > public:
> > enum { exists = sizeof(Test(MakeT())) == sizeof(Small) };
>
>Hi,
>
>this seems to be not valid C++ code because the calls to the
>static members 'Test' and 'MakeT' are not const expressions at compile
>time.
>For assignments to enum literals there are only integer expressions
>allowed which are evaluated at compile time.
>
>cu
>Matthias

Thank you, I changed the code as following and passed GCC 3.2:  -Di

         template <class T, class U>
         struct ConversionHelper
         {
             typedef char Small;
             struct Big { char dummy[2]; };
             static Big   Test(...);
             static Small Test(U);
             static T MakeT();
         };

     template <class T, class U>
     struct Conversion
     {
         typedef ConversionHelper<T, U> H;
         enum { exists = sizeof(typename H::Small) == 
sizeof(H::Test(H::MakeT())) };

     };
......


C:\ttt>g++ -o template template.cpp
template.cpp: In instantiation of `Conversion<double, int>':
template.cpp:40:   instantiated from here
template.cpp:40: warning: passing `double' for argument 1 of `static char
    ConversionHelper<T, U>::Test(U) [with T = double, U = int]'

C:\ttt>template
1 0
C:\ttt>

>--
>Matthias Oltmanns
>
>Tel: 04421-1543-274
>mail: Mathias.Oltmanns.Oltmanns@sysde.eads.net

Re: I can not compile code from <<modern c++ design>>

[Matthias Oltmanns]

Am Mon, 2003-04-07 um 15.54 schrieb Di Yang:
> At 13:02 4/7/2003 +0200, Matthias Oltmanns wrote:
> >Am Mon, 2003-04-07 um 11.24 schrieb Di Yang:
> > > Hi,
> > >
> > > public:
> > > enum { exists = sizeof(Test(MakeT())) == sizeof(Small) };
> >
> >Hi,
> >
> >this seems to be not valid C++ code because the calls to the
> >static members 'Test' and 'MakeT' are not const expressions at compile
> >time.
> >For assignments to enum literals there are only integer expressions
> >allowed which are evaluated at compile time.
> >
> >cu
> >Matthias
> 
> Thank you, I changed the code as following and passed GCC 3.2:  -Di
> 
>          template <class T, class U>
>          struct ConversionHelper
>          {
>              typedef char Small;
>              struct Big { char dummy[2]; };
>              static Big   Test(...);
>              static Small Test(U);
>              static T MakeT();
>          };
> 
>      template <class T, class U>
>      struct Conversion
>      {
>          typedef ConversionHelper<T, U> H;
>          enum { exists = sizeof(typename H::Small) == 
> sizeof(H::Test(H::MakeT())) };
> 
>      };
> ......
> 
> 
> C:\ttt>g++ -o template template.cpp
> template.cpp: In instantiation of `Conversion<double, int>':
> template.cpp:40:   instantiated from here
> template.cpp:40: warning: passing `double' for argument 1 of `static char
>     ConversionHelper<T, U>::Test(U) [with T = double, U = int]'
> 
> C:\ttt>template
> 1 0
> C:\ttt>

Hi,

i'am confused. Your changes shows that I'm wrong. But i' cant see why.
You could also change your first version making all members public:

class Conversion
{
public:
  typedef char Small;
  class Big { char dummy[2]; };
  
  static Small Test(U);
  static Big Test(...);
  static T MakeT();
  
  enum { exists = sizeof(Test(MakeT())) == sizeof(Small) };
};

which also works.

So, if the initialization of the enum is legal it should be legal in
your previous version having only the enum public.

cu
Matthias

Re: I can not compile code from <<modern c++ design>>

[Oscar Fuentes]

Matthias Oltmanns <Mathias.Oltmanns.Oltmanns@sysde.eads.net> writes:

> Am Mon, 2003-04-07 um 11.24 schrieb Di Yang:
>> Hi,
>> 
>> public:
>> enum { exists = sizeof(Test(MakeT())) == sizeof(Small) };
>
> Hi,
>
> this seems to be not valid C++ code because the calls to the
> static members 'Test' and 'MakeT' are not const expressions at compile
> time.

It is valid C++. Note that we are applying 'sizeof' to the result
*type* of the function.

> For assignments to enum literals there are only integer expressions
> allowed which are evaluated at compile time.

Right. And 'sizeof' is always a compile-time thing.

To the OP: There is a GCC compatible implementation of Loki on

www.moderncppdesign.com

-- 
Oscar

Re: I can not compile code from <<modern c++ design>>

[Di Yang]

>Hi,
>
>i'am confused. Your changes shows that I'm wrong. But i' cant see why.
>You could also change your first version making all members public:
>
>class Conversion
>{
>public:
>   typedef char Small;
>   class Big { char dummy[2]; };
>
>   static Small Test(U);
>   static Big Test(...);
>   static T MakeT();
>
>   enum { exists = sizeof(Test(MakeT())) == sizeof(Small) };
>};
>
>which also works.

Sorry, but I still failed at this point:

#include <iostream>

template <class T, class U>
class Conversion
{
public:
typedef char Small;
class Big { char dummy[2]; };

static Small Test(U);
static Big Test(...);
static T MakeT();

enum { exists = sizeof(Test(MakeT())) == sizeof(Small) };
};

int main()
{
using namespace std;

cout<< Conversion<double, int>::exists << ' '<< Conversion<char, 
char*>::exists;

return 0;
}

Same errror as before:

C:\ttt>g++ -o template template.cpp
template.cpp: In instantiation of `Conversion<double, int>':
template.cpp:42:   instantiated from here
template.cpp:42: invalid use of undefined type `class Conversion<double, int>'
template.cpp:5: declaration of `class Conversion<double, int>'
template.cpp:42: invalid use of undefined type `class Conversion<double, int>'
template.cpp:5: declaration of `class Conversion<double, int>'
template.cpp:42: enumerator value for `exists' not integer constant
template.cpp: In instantiation of `Conversion<char, char*>':
template.cpp:42:   instantiated from here
template.cpp:42: invalid use of undefined type `class Conversion<char, char*>'
template.cpp:5: declaration of `class Conversion<char, char*>'
template.cpp:42: invalid use of undefined type `class Conversion<char, char*>'
template.cpp:5: declaration of `class Conversion<char, char*>'
template.cpp:42: enumerator value for `exists' not integer constant

C:\ttt>g++ -dumpversion
3.2

C:\ttt>

Only with the Conversionhelper version works for me.

Thanks.
Di
>So, if the initialization of the enum is legal it should be legal in
>your previous version having only the enum public.
>
>cu
>Matthias

Re: I can not compile code from <<modern c++ design>>

[Matthias Oltmanns]

Am Die, 2003-04-08 um 10.48 schrieb Di Yang:
> 
> Sorry, but I still failed at this point:
> 

Hmmm ... me too :-) I've tested yesterday the wrong file.

I've learned, that using sizeof with a function as parameter is a
constant integer expression and therefore valid in using 
enumeration initializers.

The question remaining is, why does the following not compile:

#include <iostream>

template<typename T>
class Foo
{
public:
  static T bar();

  enum { test = sizeof(bar()) };
};

int main()
{
  std::cout << Foo<char>::test << std::endl;
}

> g++ -o conv conv.cc
conv.cc: In instantiation of `Foo<char>':
conv.cc:14:   instantiated from here
conv.cc:14: invalid use of undefined type `class Foo<char>'
conv.cc:5: declaration of `class Foo<char>'
conv.cc:14: enumerator value for `test' not integer constant

> g++ --version
g++ (GCC) 3.2 20020903 (Red Hat Linux 8.0 3.2-7)
Copyright (C) 2002 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is
NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR
PURPOSE.

Is it a bug or does i still miss some point.

cu
Matthias

Re: I can not compile code from <<modern c++ design>>

[Di Yang]

At 12:00 4/8/2003 +0200, Matthias Oltmanns wrote:
>Am Die, 2003-04-08 um 10.48 schrieb Di Yang:
> >
> > Sorry, but I still failed at this point:
> >
>
>Hmmm ... me too :-) I've tested yesterday the wrong file.
>
>I've learned, that using sizeof with a function as parameter is a
>constant integer expression and therefore valid in using
>enumeration initializers.
>
>The question remaining is, why does the following not compile:
>
>#include <iostream>
>
>template<typename T>
>class Foo
>{
>public:
>   static T bar();
>
>   enum { test = sizeof(bar()) };
>};
>
>int main()
>{
>   std::cout << Foo<char>::test << std::endl;
>}
>
> > g++ -o conv conv.cc
>conv.cc: In instantiation of `Foo<char>':
>conv.cc:14:   instantiated from here
>conv.cc:14: invalid use of undefined type `class Foo<char>'
>conv.cc:5: declaration of `class Foo<char>'
>conv.cc:14: enumerator value for `test' not integer constant
>
> > g++ --version
>g++ (GCC) 3.2 20020903 (Red Hat Linux 8.0 3.2-7)
>Copyright (C) 2002 Free Software Foundation, Inc.
>This is free software; see the source for copying conditions.  There is
>NO
>warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR
>PURPOSE.
>
>Is it a bug or does i still miss some point.

I don't know either, can someone familiar c++ standard explain this?

And with a FooHelper, it works:

// code start
#include <iostream>

template <typename T>
class FooHelper
{
public:
   static T bar();
};

template<typename T>
class Foo
{
public:
   enum { test = sizeof(FooHelper<T>::bar()) };
};


int main()
{
   std::cout << Foo<char>::test << std::endl;
}

// code end

Thanks.
Di

>cu
>Matthias