Marco Costalba wrote:
Here it is another undocumented behaviour of boost::fusion.
Did you know that dereferencing a fusion iterator to a mpl sequence causes a duplication of the pointed object?
Following the test code:
static int cnt = 0;
struct Test { Test() { cnt++; } };
using namespace boost::fusion;
int main(int, char**) { typedef boost::mpl::vector<Test,Test> Mpl_vec;
Mpl_vec mpl_vec;
result_of::begin<Mpl_vec>::type iter = begin(mpl_vec);
cout << cnt << endl; // prints 0
deref(iter);
cout << cnt << endl; // prints 1
*iter;
cout << cnt << endl; // prints 2
return 0; }
I found this while traversing a sequence with fusion::for_each because code started to create a lot of copies of the sequence elements, indeed the whole sequence is duplicated each time it's traversed with fusion::for_each !!
Is this foreseen or it's a bug.
It's a feature ;-)... Here's why: mpl sequences do not have storage. So how do you get a value? You get it whenever you dereference an iterator. It's computed on the fly, so to speak. It's generated. If that's not what you want, first make a fusion /container/ from the mpl sequence. Regards, -- Joel de Guzman http://www.boost-consulting.com http://spirit.sf.net