Re: [Boost-users] boost-users: boost enable_if

3 Oct 2007

      hmm doesn't work.
g++.exe -Wall -D__DEBUG__ -c  ./cs512/c++/cppdef/BitOperation.cpp -o
./lib/BitOp
eration.o -I"C:/Boost/include" -I"./bigint/c++"    -fexceptions
-fverbose-asm -f
expensive-optimizations -O3 -pg -g3
./cs512/c++/cppdef/../MyOperators.hpp:22: error: expected initializer before
'te
mplate'
./cs512/c++/cppdef/BitOperation.cpp:44: error: 'return_type' was not
declared in
 this scope
./cs512/c++/cppdef/BitOperation.cpp:44: error: template argument 2 is
invalid
./cs512/c++/cppdef/BitOperation.cpp:45: error: expected initializer before
'Log2
'
./cs512/c++/cppdef/BitOperation.cpp:151: error: 'return_type' was not
declared i
n this scope
./cs512/c++/cppdef/BitOperation.cpp:151: error: template argument 2 is
invalid
./cs512/c++/cppdef/BitOperation.cpp:152: error: expected initializer before
'boo
st'

heres the code ..
typename boost::enable_if, return_type>::type // BitOperation.cpp line 44.

namespace boost
{
        // line 18 of MyOperators.hpp
        template 
        char (& is_op_test(boost::operators const&) )[2]

        template <class T>
        char is_op_test(T const&);
        // line 24 of MyOperators.hpp
}

I pass T as an int which should pass the first test.

On 10/1/07, David Abrahams  wrote:
...
on Mon Oct 01 2007, "chun ping wang"  wrote:
...
hmm that is what i want for now... (i might change it later). I want to
know to keept it in back
of my head. Thanks.
On 10/1/07, David Abrahams < dave@boost-consulting.com> wrote:
on Wed Sep 26 2007, "chun ping wang" < cablepuff-AT-gmail.com>
wrote:
> what i want to do is if type "T" happens to be a integeral type or
a class type that
    inherits
    > from boost::operators than let the function compile otherwise a
    > compile time error appears.
I could tell you how, but it seems ill-advised.  I really doubt you
    want to fail compilation if someone passes you a class type that
acts
    just like int, but that isn't written using boost::operators.  Are
you
    sure that's what you want?
// untested
template 
char (& is_op_test(operators const&) )[2]
template <class T>
char is_op_test(T const&);
template<class T>
T const& make();
template <class T>
struct is_operators
  : mpl::bool_<(sizeof(is_op_test(make<T>())) == 2)>
{};
template <class T>
typename enable_if,
return_type>::type
f(T x)
{
   ...
}
--
Dave Abrahams
Boost Consulting
http://www.boost-consulting.com
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