std::bind() throwing error when this is not passed - c++

Question

I am trying to understand std::bind(). I am trying to understand the code in the post https://riptutorial.com/cplusplus/example/7541/std--function-used-with-std--bind.
Code is as below.
#include <iostream>
#include <functional>
using namespace std;
class A
{
public:
std::function<void(int, const std::string&)> m_CbFunc = nullptr;
void foo()
{
if (m_CbFunc)
{
m_CbFunc(100, "event fired");
}
}
};
class B
{
public:
B(int x) : y(x)
{
auto aFunc = std::bind(&B::eventHandler, this, std::placeholders::_1, std::placeholders::_2);
anObjA.m_CbFunc = aFunc;
}
void eventHandler(int i, const std::string& s)
{
std::cout << s << ": " << i << std::endl;
}
void DoSomethingOnA()
{
anObjA.foo();
}
int y;
A anObjA;
};
int main(int argc, char *argv[])
{
B anObjB(4);
anObjB.DoSomethingOnA();
}
I couldn't understand why we are using this in bind call
auto aFunc = std::bind(&B::eventHandler, this, std::placeholders::_1, std::placeholders::_2)
eventHandler is taking 2 parameters and we are binding with placeholders. Not sure why do we need to pass this. If I remove this, I am getting below error.
/usr/include/c++/6/functional:1286:7: error: static assertion failed: Wrong number of arguments for pointer-to-member
static_assert(_Varargs::value
^~~~~~~~~~~~~
main.cpp: In constructor ‘B::B(int)’:
main.cpp:34:27: error: no match for ‘operator=’ (operand types are ‘std::function&)>’ and ‘std::_Bind&)>(std::_Placeholder<1>, std::_Placeholder<2>)>’)
anObjA.m_CbFunc = aFunc;
^~~~~
In file included from main.cpp:10:0:
/usr/include/c++/6/functional:1929:7: note: candidate: std::function<_Res(_ArgTypes ...)>& std::function<_Res(_ArgTypes ...)>::operator=(const std::function<_Res(_ArgTypes ...)>&) [with _Res = void; _ArgTypes = {int, const std::basic_string, std::allocator >&}]
operator=(const function& __x)
^~~~~~~~

Answer 1

The this is needed because eventHandler() is not a static method. std::bind() is used specifically when you want to use non-static methods of your class.
Note that personally, I find it ugly and don't like using it. I use lambdas instead.
anObjA.m_CbFunc = [=](int i, const std::string&s) { eventHandler(i,s); };
Even though the syntax for lambdas is kind of ugly, I don't think it's as ugly or obscure as bind. But that's just my preference.