[Boost-users] Functors and function_traits<>

From: Hossein Haeri (powerprogman_at_[hidden])
Date: 2004-12-27 09:08:00

Dear all,

Yep, again it's me, and it's agian
function_traits<>...

AFAIK, std::iterator_traits<> is one of the
counterparts of function_traits<>, not?

A sort of symmetry which iterator_traits<> has got is
that it can work for Iterators as well as plain
pointers. 

This seems to be not the case for function_traits<>.
That is, you can pass a function type to it, but you
can't do that for a functor. Is that right?

If it is, a big drawback turns out to happen for
variations of the following which are not that few
common:

template <
          typename ...,
          ...,
          typename Function
         >
struct Foo
{
  typedef 
    typename 
      function_traits<Function>
        ::result_type result_type;

  Foo (Function f) : f_(f) {}

  //...
  result_type operator () () {/*..*/}

  Function f_;
};

This well works for sth like this:

double f(double) {/*...*/}
Foo<blah, ..., double (&) (double)> foo(f);

But it does not work with the following:

struct F
{
  double operator () () {/*...*/}
};
Foo f;
Foo<blah, ..., F> foo(f);

Is this what it should be? Or am I missing anything? 

In fact, the following code:

struct F
{
  double operator () (double x) const {return f(x);}
};
  
typedef 
  boost::function_traits<F>::result_type result_type;

makes GCC 3.3.1 to give me an error like the previous
one. This is what it says:

Compiler: Default compiler
Building Makefile: "C:\Documents and
Settings\Hossein\My Documents\My
Programmes\Probability
I\Distributions\7.2.1\Makefile.win"
Executing  make...
make.exe -f "C:\Documents and Settings\Hossein\My
Documents\My Programmes\Probability
I\Distributions\7.2.1\Makefile.win" all
g++.exe -D__DEBUG__ -c 7_2_1Main.cpp -o 7_2_1Main.o
-I"C:/Dev-Cpp/include/c++/3.3.1" 
-I"C:/Dev-Cpp/include/c++/3.3.1/mingw32" 
-I"C:/Dev-Cpp/include/c++/3.3.1/backward" 
-I"C:/Dev-Cpp/lib/gcc-lib/mingw32/3.3.1/include" 
-I"C:/Dev-Cpp/include"    -pg -g3

7_2_1Main.cpp: In instantiation of
`boost::function_traits<F>':
7_2_1Main.cpp:70:   instantiated from
`inverse_variate_generator<main()::Engine,
main()::Distribution, F>'
7_2_1Main.cpp:106:   instantiated from here
7_2_1Main.cpp:70: error: base class
`boost::detail::function_traits_helper<F*>' 
   has incomplete type
7_2_1Main.cpp: In instantiation of
`inverse_variate_generator<main()::Engine,
main()::Distribution, F>':
7_2_1Main.cpp:106:   instantiated from here
7_2_1Main.cpp:70: error: no type named `result_type'
in `struct 
   boost::function_traits<F>'
7_2_1Main.cpp:76: error: no type named `result_type'
in `struct 
   boost::function_traits<F>'

7_2_1Main.cpp: In function `int main()':
7_2_1Main.cpp:113: error: no match for call to `(
   inverse_variate_generator<main()::Engine,
main()::Distribution, F>) ()'
7_2_1Main.cpp:117: error: no match for call to `(
   inverse_variate_generator<main()::Engine,
main()::Distribution, F>) ()'

make.exe: *** [7_2_1Main.o] Error 1

Execution terminated

What's wrong with it? Am I missing anything?

If I'm not missing anything, then what is the
work-around? I find myself right in expecting myself
to be able to write a functor (like Foo) which can
work equaly well with both function pointers and
functors, as like as how the Standard algorithms do.
Am I right? Or am I right? ;)

TIA,
--Hossein

                
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