c++ - boost::signals2::signal gives wrong output? -

i'm new boost library, while practicing example on bind, wrote following code. but, seems 'res' computed but, correct result not transmitted signal. kindly help, wrong in following snippet. the code compiled , run on http://cpp.sh/

#include <iostream> #include<boost/signals2.hpp>  using namespace std; class mathobject{ public:     int addops(int op1, int op2); };  int mathobject::addops(int op1, int op2){     int res = op1 + op2;     cout << "result of addops: " << res << endl;     return res; }  int main(void){     mathobject mobj;     boost::signals2::signal<int(int)> incrementer;     incrementer.connect(boost::bind(&mathobject::addops, &mobj, 1, _1));     boost::signals2::signal<int(int)> doubler;     doubler.connect(boost::bind(&mathobject::addops, &mobj, _1, _1));     cout << "incrementer of 5" << incrementer(5) << endl;     cout << "doubler of 5" << doubler(5) << endl; } 

output:

result of addops: 6 incrementer of 51 result of addops: 10 doubler of 51 

when signal called, may or may not have handlers connected it. therefore may or may not produce result. default behaviour return result of last-connected handler, if any; or return value indicating "no result". return value of incrementer(5) , doubler(5) not int, rather boost::optional<int>.

this leads output observe because boost::optional<int> can implicitly converted bool, converted output of 1 or 0.

you need first check result present, , result out of boost::optional<int> returned:

#include <iostream> #include<boost/signals2.hpp>  using namespace std; class mathobject{ public:     int addops(int op1, int op2); };  int mathobject::addops(int op1, int op2){     int res = op1 + op2;     cout << "result of addops: " << res << endl;     return res; }  int main(void){     mathobject mobj;     boost::signals2::signal<int(int)> incrementer;     incrementer.connect(boost::bind(&mathobject::addops, &mobj, 1, _1));     boost::signals2::signal<int(int)> doubler;     doubler.connect(boost::bind(&mathobject::addops, &mobj, _1, _1));     boost::optional<int> incremented = incrementer(5);     if (incremented) {         cout << "incrementer of 5: " << *incremented << endl;     }     boost::optional<int> doubled = doubler(5);     if (doubled) {         cout << "doubler of 5: " << *doubled << endl;     } }