I'm trying to write a function that will take a functor as an argument, invoke the functor and then return its return value wrapped in a boost::shared_ptr.
The following refuses to compile and I'm all out of ideas. I get "std::vector< std::string > does not provide a call operator" (roughly). I'm using Clang 3.1 on Mac OS X.
template< typename T >
boost::shared_ptr< T > ReturnValueAsShared(
boost::function< T() > func )
{
return boost::make_shared< T >( func() );
}
This is the context in which I'm trying to use it:
make_shared< packaged_task< boost::shared_ptr< std::vector< std::string > > > >(
bind( ReturnValueAsShared< std::vector< std::string > >,
bind( [a function that returns a std::vector< std::string >] ) ) );
EDIT: Here's a complete self-contained test case. This code fails to compile with the same error, and for the life of me I can't see what's wrong:
#include <boost/make_shared.hpp>
#include <boost/shared_ptr.hpp>
#include <boost/function.hpp>
#include <boost/bind.hpp>
#include <string>
#include <vector>
std::vector< std::string > foo( std::string a )
{
std::vector< std::string > vec;
vec.push_back( a );
return vec;
}
template< typename T >
boost::shared_ptr< T > ReturnValueAsShared(
boost::function< T() > func )
{
return boost::make_shared< T >( func() );
}
int main()
{
auto f = boost::bind( ReturnValueAsShared< std::vector< std::string > >,
boost::bind( foo, std::string("a") ) );
f();
} // main
And here's the error output:
In file included from testcase.cpp:3:
In file included from /usr/local/include/boost/function.hpp:64:
In file included from /usr/local/include/boost/preprocessor/iteration/detail/iter/forward1.hpp:47:
In file included from /usr/local/include/boost/function/detail/function_iterate.hpp:14:
In file included from /usr/local/include/boost/function/detail/maybe_include.hpp:13:
/usr/local/include/boost/function/function_template.hpp:132:18: error: type 'std::vector<std::basic_string<char>, std::allocator<std::basic_string<char> > >' does not provide a call operator
return (*f)(BOOST_FUNCTION_ARGS);
^~~~
/usr/local/include/boost/function/function_template.hpp:907:53: note: in instantiation of member function 'boost::detail::function::function_obj_invoker0<std::vector<std::basic_string<char>, std::allocator<std::basic_string<char> > >, std::vector<std::basic_string<char>, std::allocator<std::basic_string<char> > > >::invoke' requested here
{ { &manager_type::manage }, &invoker_type::invoke };
^
/usr/local/include/boost/function/function_template.hpp:722:13: note: in instantiation of function template specialization 'boost::function0<std::vector<std::basic_string<char>, std::allocator<std::basic_string<char> > > >::assign_to<std::vector<std::basic_string<char>, std::allocator<std::basic_string<char> > > >' requested here
this->assign_to(f);
^
/usr/local/include/boost/function/function_template.hpp:1042:5: note: in instantiation of function template specialization 'boost::function0<std::vector<std::basic_string<char>, std::allocator<std::basic_string<char> > > >::function0<std::vector<std::basic_string<char>, std::allocator<std::basic_string<char> > > >' requested here
base_type(f)
^
/usr/local/include/boost/bind/bind.hpp:243:43: note: in instantiation of function template specialization 'boost::function<std::vector<std::basic_string<char>, std::allocator<std::basic_string<char> > > ()>::function<std::vector<std::basic_string<char>, std::allocator<std::basic_string<char> > > >' requested here
return unwrapper<F>::unwrap(f, 0)(a[base_type::a1_]);
^
/usr/local/include/boost/bind/bind_template.hpp:20:27: note: in instantiation of function template specialization 'boost::_bi::list1<boost::_bi::bind_t<std::vector<std::basic_string<char>, std::allocator<std::basic_string<char> > >, std::vector<std::basic_string<char>, std::allocator<std::basic_string<char> > > (*)(std::basic_string<char>), boost::_bi::list1<boost::_bi::value<std::basic_string<char> > > > >::operator()<boost::shared_ptr<std::vector<std::basic_string<char>, std::allocator<std::basic_string<char> > > >, boost::shared_ptr<std::vector<std::basic_string<char>, std::allocator<std::basic_string<char> > > > (*)(boost::function<std::vector<std::basic_string<char>, std::allocator<std::basic_string<char> > > ()>), boost::_bi::list0>' requested here
BOOST_BIND_RETURN l_(type<result_type>(), f_, a, 0);
^
testcase.cpp:27:4: note: in instantiation of member function 'boost::_bi::bind_t<boost::shared_ptr<std::vector<std::basic_string<char>, std::allocator<std::basic_string<char> > > >, boost::shared_ptr<std::vector<std::basic_string<char>, std::allocator<std::basic_string<char> > > > (*)(boost::function<std::vector<std::basic_string<char>, std::allocator<std::basic_string<char> > > ()>), boost::_bi::list1<boost::_bi::bind_t<std::vector<std::basic_string<char>, std::allocator<std::basic_string<char> > >, std::vector<std::basic_string<char>, std::allocator<std::basic_string<char> > > (*)(std::basic_string<char>), boost::_bi::list1<boost::_bi::value<std::basic_string<char> > > > > >::operator()' requested here
f();
^
1 error generated.
Here are some more clues. The following code compiles just fine, but that doesn't help me since this is not the code that I want :)
#include <boost/make_shared.hpp>
#include <boost/shared_ptr.hpp>
#include <boost/function.hpp>
#include <boost/bind.hpp>
#include <string>
#include <vector>
std::vector< std::string > foo()
{
std::vector< std::string > vec;
return vec;
}
template< typename T >
boost::shared_ptr< T > ReturnValueAsShared(
boost::function< T() > func )
{
return boost::make_shared< T >( func() );
}
int main()
{
auto f = boost::bind( ReturnValueAsShared< std::vector< std::string > >,
foo );
f();
} // main
Some constructions (such as bind) return intermediate "expression" types which you don't actually want to capture on the nose. In that case, you mustn't capture the type via auto, and you may need to specify explicit conversions, since otherwise there isn't a unique, single user-defined conversion chain. In your case, add the explicit conversion from the bind expression to function:
typedef std::vector<std::string> G;
auto f = boost::bind(ReturnValueAsShared<G>,
static_cast<boost::function<G()>(boost::bind(foo, std::string("a")))
);
(This itself doesn't actually work for me, but it does work if you use the corresponding std constructions.)