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; }