"<class name> does not provide a call operator" error when trying to wrap function return value

Lucas picture Lucas · Jul 15, 2012 · Viewed 16.2k times · Source

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

Answer

Thomas Heller picture Thomas Heller · Jul 16, 2012

boost::protect is the way to go:

int main()
{
  auto f = boost::bind( ReturnValueAsShared< std::vector< std::string > >,
                        boost::protect(boost::bind( foo, std::string("a") ) ) );
  f();

} // main

This is as clean as it can get.