Laurent Laurent - 3 months ago 26
C++ Question

Limits of BOOST_FUSION_ADAPT_STRUCT

I have tried to play with the

BOOST_FUSION_ADAPT_STRUCT
macro and tried some naive things such as use Fusion to print any arbitrary structure.

Starting from this example code given in the documentation, I was unable to perform on my adapted structure some operations that are allowed with a fusion sequence.

#include <boost/fusion/adapted.hpp>
#include <boost/fusion/sequence/io/out.hpp>
#include <boost/fusion/sequence/intrinsic.hpp>
#include <boost/fusion/view.hpp>
#include <iostream>

namespace fuz = boost::fusion;

namespace demo
{
struct employee
{
std::string name;
int age;
};
}

// demo::employee is now a Fusion sequence
BOOST_FUSION_ADAPT_STRUCT(
demo::employee,
(std::string, name)
(int, age))

int main()
{
// tried to initialize an employee like a fusion sequence
// but it didnt work
// demo::employee e("bob", 42);

demo::employee e;
e.name = "bob";
e.age = 42;

// Access struct members with fusion random access functions
// ok
std::cout << fuz::at_c<0>(e) << std::endl;

// tried to print the struct like any othe fusion sequence
// didnt work
// std::cout << e << std::endl;

// I made it work by using a fusion view
// is it the right way?
std::cout << fuz::as_nview<0, 1>(e) << std::endl;
}


This leads me to the following questions :


  • Why the Fusion magik does not operate here?

  • Using a view is the correct way to print an adapted struct?

  • How far can an adapted struct be used as a Fusion sequence?


Answer

From the boost::fusion documentation:

The I/O operators are overloaded in namespace boost::fusion

Which means that if you want a implicit integration of these operator<<, you will need to inject the boost::fusion namespace in your current namespace (:: here), or use them explicitly.

To sum it all up, adding:

using namespace boost::fusion;

Should work in your case. Or for an explicit use, you will have to write:

boost::fusion::operator<<(std::cout, e) << std::endl;

--- EDIT ---

After reading boost::fusion's code a bit, it seem that you are confused because of the Koenig's lookup of boost::fusion::operators::operator<< which is selected in case your argument is a real boost::fusion::sequence.

This is why you don't need to inject the boost::fusion namespace, nor explicitly call boost::fusion::operator<< for types defined in the boost::fusion namespace.

Some explanations:

I won't explain the whole concept of Koenig's lookup (also known as Argument Dependent Lookup - ADL) here since that is not the point, but basically, it states that in case you are using a variable whose type is inside a namespace, then the function lookup extends to the namespace of that parameter.

In this particular case, including boost/fusion/sequence/io/out.hpp will define boost::fusion::operator::operator<< which will then be injected in the boost::fusion namespace.

$ cat /usr/local/include/boost/fusion/sequence/io/out.hpp
[...]
namespace boost { namespace fusion
{
    [...]
    namespace operators
    {
        template <typename Sequence>
        inline typename
            boost::enable_if<
               fusion::traits::is_sequence<Sequence>
              , std::ostream&
            >::type // this is just a SFINAE trick to ensure
                    // the function will only be selected for
                    // actual boost::fusion::sequence
        operator<<(std::ostream& os, Sequence const& seq)
        {
            return fusion::out(os, seq); // this will print out the sequence
        }
    }
    using operators::operator<<; // here the operator<< is injected
                                 // in boost::fusion
}}

This means that calls using operator<< with parameters whose types are in the boost::fusion namespace will find the proper overload.

Calls using arguments whose type is not located in this namespace will fail to resolve the proper overload of operator<< (this is the case in your example).

You can check that by defining your type in the boost::fusion namespace.

namespace boost { namespace fusion {
struct employee
{
  std::string name;
  int age;
};
}}

BOOST_FUSION_ADAPT_STRUCT(
    boost::fusion::employee,
    (std::string, name)
    (int, age))

[...]
boost::fusion::employee e;
std::cout << e << std::endl; // ADL will work here

Side note: If you want to debug these kind of name lookup issues, you should use gdb. That way you will always know which overload was chosen. In this case:

$ cat fusion.cpp
#include <iostream>
#include <cstdlib>

#include <boost/fusion/container/vector.hpp>
#include <boost/fusion/sequence/io.hpp>

int main(int, char**)
{
  boost::fusion::vector<int, char> foo(42, '?');
  std::cout << foo << std::endl;

  return EXIT_SUCCESS;
}

$ gdb -q ./fusion
Reading symbols for shared libraries ... done
(gdb) b 10
Breakpoint 1 at 0x1000012f7: file fusion.cpp, line 10.
(gdb) r
Starting program: /Users/avallee/Projects/tmp/fusion
Reading symbols for shared libraries ++............................. done

Breakpoint 1, main (unnamed_arg=0x7fff5fbffb60, unnamed_arg=0x7fff5fbffb60) at fusion.cpp:10
10    std::cout << foo << std::endl;
(gdb) s
boost::fusion::operators::operator<< <boost::fusion::vector<int, char, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_> > (os=@0x7fff762b5f10, seq=@0x7fff5fbffb18) at out.hpp:38
38              return fusion::out(os, seq);