[type_traits] superfluous add_pointer in function_traits or something else?
Hello!
I'm trying to use function_traits but it fails to compile. It seems
like the add_pointer is somehow related to the error, as using the
helper struct directly (and thus avoiding add_pointer on supplied
type) compiles ok. I'm using boost 1.30.2 with g++ 3.2.2 on linux.
The minimal example is:
#include
Pavel Antokolsky aka Zigmar wrote:
Hello! I'm trying to use function_traits but it fails to compile. It seems like the add_pointer is somehow related to the error, as using the helper struct directly (and thus avoiding add_pointer on supplied type) compiles ok. I'm using boost 1.30.2 with g++ 3.2.2 on linux.
The minimal example is: #include
int foo() {}
template<typename T> void bar(T f) { typedef typename boost::function_traits<T>::result_type X; //fails to compile typedef typename boost::detail::function_traits_helper<T>::result_type Y; //OK
}
int main(int argc, char** argv) { bar(foo); }
First of all, you really shouldn't be using anything in 'detail' namespaces -- what's in there are implementation details which a) might be removed at any time b) might be changed to behave differently at any time c) might even be defined differently or not at all for some compilers. 'T' is deduced to a function /pointer/. You have to use the plain function type (without the pointer on top) for 'function_traits' to work: typedef typename boost::function_traits< typename boost::remove_pointer<T>::type >::result_type X; Note that 'function_traits' won't work with function objects (that is clases with an overloaded operator()). Guessing from the code you posted you might be better off using Boost.ResultOf: http://www.boost.org/libs/utility/utility.htm#result_of Further, a library that provides a more complete set of traits classes entirely dedicated to built-in functional types (and pointers/references thereof) has been accepted into Boost, recently. It's not in the CVS yet and can be downloaded from the Boost file vault: http://tinyurl.com/qaf5f - zip archive Regards, Tobias
Hi, Tobias. Thanks for your answer
On 1/24/07, Tobias Schwinger
First of all, you really shouldn't be using anything in 'detail' namespaces -- what's in there are implementation details which a) might be removed at any time b) might be changed to behave differently at any time c) might even be defined differently or not at all for some compilers. Sure, I know. I didn't use it in production of course, but I played with it trying to pin down the problem.
'T' is deduced to a function /pointer/. You have to use the plain function type (without the pointer on top) for 'function_traits' to work:
typedef typename boost::function_traits< typename boost::remove_pointer<T>::type >::result_type X; Ah, thanks. That what I've been missing. I can of course "remove_pointer", but what I'm wandering, is why the function_traits tries to add pointer in a first place? Besides, it seems relatively easy to detect if type is pointer or not, and make function_traits to work with both...
Note that 'function_traits' won't work with function objects (that is clases with an overloaded operator()). Guessing from the code you posted you might be better off using Boost.ResultOf:
http://www.boost.org/libs/utility/utility.htm#result_of Thanks. It looks like nice solution, but unfortunately it is not available in the boost version I'm using (1.30).
Further, a library that provides a more complete set of traits classes entirely dedicated to built-in functional types (and pointers/references thereof) has been accepted into Boost, recently. Thats a cool library, I agree. But again, it is not currently available in my version, and it would be overkill in terms of management to introduce a library that is not currently part of the boost for relatively simple functionality I need.
-- Best regards, Zigmar
Pavel Antokolsky aka Zigmar wrote:
'T' is deduced to a function /pointer/. You have to use the plain function type (without the pointer on top) for 'function_traits' to work:
typedef typename boost::function_traits< typename boost::remove_pointer<T>::type >::result_type X; Ah, thanks. That what I've been missing. I can of course "remove_pointer", but what I'm wandering, is why the function_traits tries to add pointer in a first place?
It's done to please broken compilers, I guess.
Further, usually you want to accept both function objects and function
pointers:
template
participants (2)
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Pavel Antokolsky aka Zigmar -
Tobias Schwinger