C++ Function as Parameter using std::bind from other function Linker Error

Question

Assume that class C contains a function doSomething that is called repeatedly and its results are mandatory for a bunch of other functions of different classes. So in this case the function start of class B. This should operate from another class C. I think that should be pretty easy with std::bind.

But this only works if doSomething belongs to C. If I'm calling it from another class I'm getting a linker error. What am I missing?

#include <iostream>
#include <functional>
class A {
public:
    A() {}
    template<typename Callable>
    void doSomething(Callable f); 
};

template<typename Callable>
void A::doSomething(Callable f) {
    // do some stuff 
    int input = 1;
    f(input); // B::start should be called here
}

class B {
    public:
         B() {}
         void start(int input);
};

void B::start(int input) {
    std::cout << "dostuff" << std::endl;
    // do some other stuff
}

class C {
    private:
        A a;
    public:
        C() : a() {
            B b;
            a.doSomething(std::bind(&B::start, b, std::placeholders::_1)); 
            // if doSomething would be a member of class C then 
            // doSomething(std::bind(&B::start, b, std::placeholder//::_1) would work 
        }
};

int main() {
   C c;
}

What am I missing?

Undefined symbols for architecture x86_64:
  "void A::doSomething<std::_Bind<void (B::*(B))()> >(std::_Bind<void (B::*(B))()>)", referenced from:
      C::C(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >) in C.cpp.o
      C::C(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >) in C.cpp.o
ld: symbol(s) not found for architecture x86_64
collect2: error: ld returned 1 exit status