Digvijoy Chatterjee wrote:

Hi,

i have a boost::variant holding some C++ types which have to be inserted into the database :

boost::variant dbColumnValueVar;

before i insert the value_type into the database I need to call a processing function which is overloaded for every datatype used in the variant ie. each of these functions are defined

process ( int ); process ( std::string ); process ( double); template <class T> process(T); .....

what i do not understand is how do i use apply_visitor or some other tactic to automatically dispatch the call to the right process function as in :

boost::variant dbColumnVar; dbColumnVar = "hello world"; process ( dbColumnVar ); // should call process std::string ....but is always calling the generic template version ?? dbColumnVar = 1.3434; process ( dbColumnVar ); //again calls generic template not specialization

struct process_visitor : boost::static_visitor<void> { template <typename T> void operator()([const] T &val) { process(val); } }; process_visitor pv; dbColumnVar.apply_visitor(pv);

cout << dbColumnVar is somehow able to get the correct overloaded << operator for the hidden datatype irrespective of datatype ??

boost::variant has an operator<< overload which calls an internal visitor (as in the previous example) to call the stored type's operator<<.