[Boost-users] sizeof(boost::fusion:map)

Subject: [Boost-users] sizeof(boost::fusion:map)
From: Robert Lupton the Good (rhl_at_[hidden])
Date: 2010-11-05 14:01:47

I was very excited to find the boost::fusion library, expecting that I could use it to implement structs with introspection.  I still think I can, but I was disappointed to see that I'm paying a size penalty. I'd have expected all the book-keeping to be handled at compile time.

The following code prints
        sizeof(Foo) = 12 sizeof(FooS) = 4
with g++ 4.2 or 4.4 on an os/x 10.6 or Centos box.

Here Foo is
        boost::fusion::map<boost::fusion::pair<Name, int> > 

Where are the extra 8 bytes going?  With more complex boost::fusion objects I see an overhead of up to 16 bytes.  gdb indicates that foo.data.vec has the desired size, but foo.data is larger, and foo is larger still; however, it doesn't indicate any data members --- is this some sort of alignment issue?

Is there a way to generate a "packed boost::fusion::map" without this space overhead?

                                        R

#include <iostream>
#include "boost/fusion/container.hpp"

struct Name {};

int main() {
    typedef boost::fusion::map<boost::fusion::pair<Name, int> > Foo;

    struct FooS {
        int age;
    };

    std::cout << "sizeof(Foo) = " << sizeof(Foo)
              << " sizeof(FooS) = " << sizeof(FooS)
              << std::endl;
}