Hello,
Recently I made some tests with lambda (mingw build) I found some
problems. Here they are:
1. Lambda and template type deduction.
Try this sample:
#include <iostream>
#include <algorithm>
#include <vector>
#include <functional>
typedef std::function<double(double)> FnType;
template<typename T>
FnType average_damp(T fn)
{
return [fn](double x) {return (x + fn(x)) / 2;};
}
int main(int argc, char* argv[])
{
auto fn = average_damp([](double x){return x * 2;});
std::cout << fn(10.0) << std::endl;
return 0;
}
This sample successfully built with VS 2010, but under gcc there are
some compilation errors:
P:\projects\Tests\Sandbox\C++0x\gcc43>g++ -std=c++0x -Os
-IP:\Projects\common\boost_1.36.0 lambda.cpp
In file included from
s:/programming/mingw/lib/gcc/../../include/c++/4.5.0/functional:70:0,
from lambda.cpp:4:
s:/programming/mingw/lib/gcc/../../include/c++/4.5.0/tr1_impl/functional: In
static member function 'static void std::_F
unction_base::_Base_manager<_Functor>::_M_clone(std::_Any_data&, const
std::_Any_data&, std::false_type) [with _Functor
= average_damp(T) [with T = main(int, char**)::<lambda>(double), FnType =
std::function<double(double)>]::<lambda>(doubl
e), std::false_type = std::integral_constant<bool, false>]':
s:/programming/mingw/lib/gcc/../../include/c++/4.5.0/tr1_impl/functional:1543:8:
instantiated from 'static bool std::_
Function_base::_Base_manager<_Functor>::_M_manager(std::_Any_data&, const
std::_Any_data&, std::_Manager_operation) [wit
h _Functor = average_damp(T) [with T = main(int, char**)::<lambda>(double),
FnType = std::function<double(double)>]::<la
mbda>(double)]'
s:/programming/mingw/lib/gcc/../../include/c++/4.5.0/tr1_impl/functional:2013:6:
instantiated from 'std::function<_Res
(_ArgTypes ...)>::function(_Functor, typename __gnu_cxx::__enable_if<(!
std::is_integral::value), std::function<_Res(_Ar
gTypes ...)>::_Useless>::__type) [with _Functor = average_damp(T) [with T =
main(int, char**)::<lambda>(double), FnType
= std::function<double(double)>]::<lambda>(double), _Res = double, _ArgTypes =
double, typename __gnu_cxx::__enable_if<(
! std::is_integral::value), std::function<_Res(_ArgTypes
...)>::_Useless>::__type = std::function<double(double)>::_Usel
ess]'
lambda.cpp:11:51: instantiated from 'FnType average_damp(T) [with T =
main(int, char**)::<lambda>(double), FnType = st
d::function<double(double)>]'
lambda.cpp:16:55: instantiated from here
s:/programming/mingw/lib/gcc/../../include/c++/4.5.0/tr1_impl/functional:1507:4:
error: no matching function for call to
'average_damp(T) [with T = main(int, char**)::<lambda>(double), FnType =
std::function<double(double)>]::<lambda>(doubl
e)::__lambda0(average_damp(T) [with T = main(int, char**)::<lambda>(double),
FnType = std::function<double(double)>]::<l
ambda>(double)&)'
lambda.cpp:11:25: note: candidates are: average_damp(T) [with T = main(int,
char**)::<lambda>(double), FnType = std::fun
ction<double(double)>]::<lambda>(double)
s:/programming/mingw/lib/gcc/../../include/c++/4.5.0/tr1_impl/functional: In
static member function 'static void std::_F
unction_base::_Base_manager<_Functor>::_M_init_functor(std::_Any_data&, const
_Functor&, std::false_type) [with _Functor
= average_damp(T) [with T = main(int, char**)::<lambda>(double), FnType =
std::function<double(double)>]::<lambda>(doub
le), std::false_type = std::integral_constant<bool, false>]':
s:/programming/mingw/lib/gcc/../../include/c++/4.5.0/tr1_impl/functional:1555:4:
instantiated from 'static void std::_
Function_base::_Base_manager<_Functor>::_M_init_functor(std::_Any_data&, const
_Functor&) [with _Functor = average_damp(
T) [with T = main(int, char**)::<lambda>(double), FnType =
std::function<double(double)>]::<lambda>(double)]'
s:/programming/mingw/lib/gcc/../../include/c++/4.5.0/tr1_impl/functional:2014:6:
instantiated from 'std::function<_Res
(_ArgTypes ...)>::function(_Functor, typename __gnu_cxx::__enable_if<(!
std::is_integral::value), std::function<_Res(_Ar
gTypes ...)>::_Useless>::__type) [with _Functor = average_damp(T) [with T =
main(int, char**)::<lambda>(double), FnType
= std::function<double(double)>]::<lambda>(double), _Res = double, _ArgTypes =
double, typename __gnu_cxx::__enable_if<(
! std::is_integral::value), std::function<_Res(_ArgTypes
...)>::_Useless>::__type = std::function<double(double)>::_Usel
ess]'
lambda.cpp:11:51: instantiated from 'FnType average_damp(T) [with T =
main(int, char**)::<lambda>(double), FnType = st
d::function<double(double)>]'
lambda.cpp:16:55: instantiated from here
s:/programming/mingw/lib/gcc/../../include/c++/4.5.0/tr1_impl/functional:1584:4:
error: no matching function for call to
'average_damp(T) [with T = main(int, char**)::<lambda>(double), FnType =
std::function<double(double)>]::<lambda>(doubl
e)::__lambda0(const average_damp(T) [with T = main(int,
char**)::<lambda>(double), FnType = std::function<double(double)
>]::<lambda>(double)&)'
lambda.cpp:11:25: note: candidates are: average_damp(T) [with T = main(int,
char**)::<lambda>(double), FnType = std::fun
ction<double(double)>]::<lambda>(double)
What's wrong?
2. Lambdas and result_of or something similar.
How I can determine return type of lambda in compile time? As far as I
could understood there is no way to do so. Is it right?
--
Best Regards,
Sergey mailto:flex_fer...@artberg.ru
