Function call failing with bind2nd

Hi all,

I have a templated function which, when used with bind2nd, receives an error message of "no matching function for call to de_tolower()".

Here's the relevant code:

Code:

...

template <typename T>
void de_tolower(char& ch, const ctype<T>& ct)
{
  ch = ct.tolower(ch);
}

int main(int argc, char* argv[])
{
  locale loc_de("de_DE.utf8");
  const ctype<char>& ct_de = use_facet<ctype<char> >(loc_de);

  string str_de, str_en;

......

            //Error occurs in next line.
	    for_each(str_de.begin(), str_de.end(), bind2nd(de_tolower(), ct_de));
	    f_de_out << str_de << endl;

.....

Why isn't the compiler properly calling de_tolower?

the compiler can't automatically deduce what type to instantiate the template on, so you need to be explicit, ie: de_tolower<wchar_t>...

No, that was actually my previous version, and it didn't work either. And I always thought the compiler could guess the type based on the template argument that was being passed to a function.
Please, correct me if I'm wrong.

ah, well, actually you're calling the function:
de_tolower()
...should be:
bind2nd(de_tolower<the_type>, ct_de)

Um, that gives me this:

Code:

/usr/bin/g++ -Wall -DCULL final_align.cpp -o final_align
/usr/lib/gcc/i486-linux-gnu/4.0.3/../../../../include/c++/4.0.3/bits/stl_function.h: In instantiation of ‚:
final_align.cpp:77:   instantiated from here
/usr/lib/gcc/i486-linux-gnu/4.0.3/../../../../include/c++/4.0.3/bits/stl_function.h:429: error: ‚ is not a class, struct, or union type
/usr/lib/gcc/i486-linux-gnu/4.0.3/../../../../include/c++/4.0.3/bits/stl_function.h:432: error: ‚ is not a class, struct, or union type
/usr/lib/gcc/i486-linux-gnu/4.0.3/../../../../include/c++/4.0.3/bits/stl_function.h:435: error: ‚ is not a class, struct, or union type
/usr/lib/gcc/i486-linux-gnu/4.0.3/../../../../include/c++/4.0.3/bits/stl_function.h:439: error: ‚ is not a class, struct, or union type
/usr/lib/gcc/i486-linux-gnu/4.0.3/../../../../include/c++/4.0.3/bits/stl_function.h:445: error: ‚ is not a class, struct, or union type
/usr/lib/gcc/i486-linux-gnu/4.0.3/../../../../include/c++/4.0.3/bits/stl_function.h:431: error: field ‚ invalidly declared function type
/usr/lib/gcc/i486-linux-gnu/4.0.3/../../../../include/c++/4.0.3/bits/stl_function.h: In function ‚:
final_align.cpp:77:   instantiated from here
/usr/lib/gcc/i486-linux-gnu/4.0.3/../../../../include/c++/4.0.3/bits/stl_function.h:454: error: ‚ is not a class, struct, or union type
/usr/lib/gcc/i486-linux-gnu/4.0.3/../../../../include/c++/4.0.3/bits/stl_function.h:455: error: ‚ is not a class, struct, or union type
/usr/lib/gcc/i486-linux-gnu/4.0.3/../../../../include/c++/4.0.3/bits/stl_algo.h: In function ‚:
final_align.cpp:77:   instantiated from here
/usr/lib/gcc/i486-linux-gnu/4.0.3/../../../../include/c++/4.0.3/bits/stl_algo.h:158: error: no match for call to ‚
make: *** [cull] Error 1

Which, unfortunately, I can't make heads or tails of.

I haven't used bind2nd before, but it looks as if the 'function' actually needs to be an object:

Code:

class de_tolower
{
    public:
    
    template <typename T>
    void operator()(char& ch, const ctype<T>& ct)
    {
      ch = ct.tolower(ch);
    }
};

that may work, but you may need to define some typedef's in the structure for the STL, as well...

...and keep this line as is:
for_each(str_de.begin(), str_de.end(), bind2nd(de_tolower(), ct_de));
...since you'd be instantiating a de_tolower object...

The issue seems to be that std::ctype has a protected destructor (though I don't quite understand why that gives compile errors).

However, I don't understand why you are trying to define a de_tolower function that accepts a ctype argument in the first place. Maybe it's just a naming issue.

Will this not work?

Code:

struct de_tolower
{
  typedef char first_argument_type;
  typedef std::locale second_argument_type;
  typedef void result_type;

  void operator()(char& ch, const std::locale& loc) const
  {
    ch = std::tolower(ch,loc);
  }

};

Still, it seems unnecessary...I'm not sure if there's a way using standard libraries, but with boost this seems to work:

Code:

  std::transform(str_de.begin(), str_de.end(),str_de.begin(), boost::bind(std::tolower<char>,_1,loc_de));

And no need to define a new function/functor

Well, I ended up with these convoluted (and nonfunctional) alternatives after the most obvious attempts didn't work.

Here's attempt 1:

Code:

transform(str_de.begin(), str_de.end(),str_de.begin(), ct_de.tolower());

which gave me this error message:

Code:

final_align.cpp: In function ‚:
final_align.cpp:77: error: no matching function for call to ‚
/usr/lib/gcc/i486-linux-gnu/4.0.3/../../../../include/c++/4.0.3/bits/locale_facets.h:829: note: candidates are: char std::ctype<char>::tolower(char) const
/usr/lib/gcc/i486-linux-gnu/4.0.3/../../../../include/c++/4.0.3/bits/locale_facets.h:846: note:                 const char* std::ctype<char>::tolower(char*, const char*) const

Then I tried:

Code:

transform(str_de.begin(), str_de.end(),str_de.begin(), bind2nd(tolower(), loc_de));

which gave me

Code:

final_align.cpp: In function ‚:
/usr/include/ctype.h:116: error: too few arguments to function ‚
final_align.cpp:77: error: at this point in file
final_align.cpp:19: warning: unused variable ‚

Then I tried

Code:

transform(str_de.begin(), str_de.end(),str_de.begin(), use_facet<ctype<char> >(loc_de).tolower());

which gave me

Code:

/usr/bin/g++ -Wall -DCULL final_align.cpp -o final_align
final_align.cpp: In function ‚:
/usr/include/ctype.h:116: error: too few arguments to function ‚
final_align.cpp:77: error: at this point in file
final_align.cpp:19: warning: unused variable ‚
make: *** [cull] Error 1
tsmoon@odyssey:~/BACKUPEXCLUDE/lex_ac/Data/final$ make -k cull
/usr/bin/g++ -Wall -DCULL final_align.cpp -o final_align
final_align.cpp: In function ‚:
final_align.cpp:77: error: no matching function for call to ‚
/usr/lib/gcc/i486-linux-gnu/4.0.3/../../../../include/c++/4.0.3/bits/locale_facets.h:829: note: candidates are: char std::ctype<char>::tolower(char) const
/usr/lib/gcc/i486-linux-gnu/4.0.3/../../../../include/c++/4.0.3/bits/locale_facets.h:846: note:                 const char* std::ctype<char>::tolower(char*, const char*) const
final_align.cpp:77: error: expected `;' before ‚ token
final_align.cpp:19: warning: unused variable ‚
make: *** [cull] Error 1

So my solutions got more and more complicated to the version you are looking at now.
Anyway, I'll try your class based approach.

Thanks, guys.

The class based approach worked, and now I know that you have to pass a class object to bind2nd.

If someone wouldn't mind answering though, why didn't the easiest approach below work in the first place?

Code:

transform(str_de.begin(), str_de.end(),str_de.begin(), ct_de.tolower());

[EDIT]
Ditch that. Josuttis says that to use user-defined function objects for function adapters, one must, "type members for the type of their arguments and the result," which I guess means that they have to be class wrapped.

Anyway, thanks again, folks.