Using std::bind on template class member

Question

Consider the code below

#include <iostream>
#include <functional>
class Solver{
public:
    int i = 0;
    void print(){
        std::cout << "i solved" << std::endl;
    }
};

template <typename T> class ThingHandler{
public:
    template <typename B,typename C>
    void handleThing(T& solver,B paramOne,C paramTwo){
        std::cout << "i handled something " << std::endl;
        solver.print();
        std::cout << paramOne << paramTwo;
    }
};

class CantHandle{
public:
    void needHelp(std::function<void(int,int)> handleThing){
        int neededInt = 0;
        int neededIntTwo = 2;
        handleThing(neededInt,neededInt);
    }
};


int main() {
    ThingHandler<Solver> thingHandler;
    CantHandle cantHandle;
    Solver solver;
    solver.i = 10;

    auto fp = std::bind(&ThingHandler<Solver>::handleThing<Solver,int,int>, 
    thingHandler,solver,std::placeholders::_1,std::placeholders::_1);
    //the row above is what I want to achieve
    cantHandle.needHelp(fp);
    return 0;
}

I'm getting the following error:

140: error: no matching function for call to 'bind(, ThingHandler&, Solver&, const std::_Placeholder<1>&, const std::_Placeholder<1>&)' 37 | auto fp = std::bind(&ThingHandler::handleThing, thingHandler,solver,std::placeholders::_1,std::placeholders::_1);

What I want to do is have a generic class that solves some problem. Then call upon a specialization of that class. So in the case above I want ThingHandler to be (Solver& solver, int paramOne, int paramTwo). I'm not quite sure how to achieve this.

Answer 1

Member function you bind takes two template type parameters, so Solver is redundant in template argument list.

Should be:

&ThingHandler<Solver>::handleThing<int,int>

Some remarks: your code binds handleThing for a copy of thingHandler instance.

Also first bound parameter - solver , is copied into functor generated by bind . If you want to avoid these two copies, use & or std::ref :

auto fp = std::bind(&ThingHandler<Solver>::handleThing<int,int>, 
    &thingHandler,std::ref(solver),std::placeholders::_1,std::placeholders::_2);