CODEWITHSUNDEEP
c++templatesaliastypedef
user149100
user149100

Reputation: 1089

STL function / Boost function typedef errors

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();
}

Upvotes: 1

Views: 666

Answers (2)

zaufi
zaufi

Reputation: 7119

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;
}

Upvotes: 1

dsign
dsign

Reputation: 12700

I have the impression that this code does not do what you think it does:

 typename function<void ()>::type

binds "void ()" as Res just creating a function that returns a function.

what about:

...

void foo(typename function<void >::type f) {
   f();
}

...

?

You can get something similar to the aliasing you are trying to get with boost::type_traits and boost::type_traits::function_traits specially. That said, I suspect that if you want a simple and portable solution, then the easiest to do is to use boost and wait for better times with C++ compilers and STL support for C++11.

Upvotes: 1

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