boost::signals2::signal gives wrong output?

Question

I'm new to boost library, while practicing an example on bind, i wrote the following code. But, it seems like the 'res' is computed properly but, the correct result is not transmitted back to the signal. Kindly help, what is wrong in the following snippet. The code was compiled and 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

score 1 · Answer 1 · answered Aug 30 '15 at 18:29

If you look at the documentation of boost::signals2, by default it returns the value returned by the last signal executed. But it's not a plain value - it's wrapped in boost::optional because there might be no signals connected to the slot and there's no reasonable value to return.

boost::optional is convertible to bool and that's what happens when you output it - it outputs 1 which means it does have a value. If instead you say cout << incrementer(5).get() you will get your expected output in your case.

score 1 · Accepted Answer · answered Aug 30 '15 at 18:34

When a signal is called, it may or may not have any handlers connected to it. Therefore it may or may not produce a result. The default behaviour is to return the result of the last-connected handler, if any; or to return a value indicating "no result". Thus the return value of incrementer(5) and doubler(5) is not an int, but rather a boost::optional<int>.

This leads to the output that you observe because a boost::optional<int> can be implicitly converted to bool, which is then converted to an output of 1 or 0.

You need to first check that a result is present, and then get the result out of the boost::optional<int> that is 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;
    }
}

boost::signals2::signal gives wrong output?

2 Answers2