Question

I’m trying to figure out how I can switch between boost functions and c++11 functions depending on what the platform I’m compiling it on has available. I know that c++ doesn’t have template aliasing (pre-c++11) so I wrote the following, but I can’t understand the error message or why it’s not working:

#define FUNCTION_Boost

#if defined(FUNCTION_Boost)
   #include <boost/function.hpp>
#elif defined(FUNCTION_STL)
   #include <functional>
#endif

template<typename Res, typename... ArgTypes>
struct function {
   #if defined(FUNCTION_Boost)
   typedef boost::function<Res(ArgTypes...)> type;
   #elif defined(FUNCTION_STL)
   typedef std::function<Res(ArgTypes...)> type;
   #endif
};

// In instantiation of ‘function<void()>’:
// error: function returning a function
void foo(function<void ()>::type f) {
   f();
}

// this works fine
void bar(boost::function<void ()> f) {
   f();
}

Answer 1

no need to define one more function… everything could be done using using 😉

#define USE_BOOST_FUNCTION

#ifdef USE_BOOST_FUNCTION
# include <boost/function.hpp>
# define FUNCTION_NS boost
# else
#  include <functional>
# define FUNCTION_NS std
# endif

#include <iostream>

namespace test {
using FUNCTION_NS::function;
}

int main()
{
    test::function<void()> f = [](){ std::cout << __PRETTY_FUNCTION__ << std::endl; };
    f();
    return 0;
}