strange behavior of std::bind

Question

I am using this std::bind to bind function outside the class and gives a strange behavior.

It somewhat controls the value of private member variables.

//MClass.h
typedef std::function<void(void)> Action;

class MClass {

public:

  Action OnLeftClick;

//some other functions here

private:
  int totalContents;

  VScrollbar* _vscrollbar;

};

//MClass.cpp

in this kind of implementation it does not make any bug:

MClass::MClasss() {

  OnLefClick = std::bind(&VScrollbar::Scrolldown, this);

}

//but when I do this
//otherfile.h

MClass mclass;

void clickBar() {

  mclass.totalContents = 0;

}

void InitComponentns() {   

mclass.OnLeftClick = std::bind(clickBar, mclass);

}

and whenever I call the function assigned to OnLeftClick() that is clickBar() , the value of totalContents does not change in value to 0 (totalContents value is incremented by other functions of MClass).

am I doing something wrong here?

Answer 1

The this argument in the std::bind call is actually the first argument to the function. For member functions this has to be an instance of the class the function is a member of, as all member function has a hidden first argument that is the this pointer.

So for your code to work, the first argument to the VScrollbar::Scrolldown has to be an instance of a VScrollbar class, not this in the MClass . Like so:

OnLefClick = std::bind(&VScrollbar::Scrolldown, _vscrollbar);

For the second std::bind call, you should only pass it the function pointer, as it's a function that takes no arguments (hidden or not):

mclass.OnLeftClick = std::bind(clickBar);

I'm surprised the code in the question builds though, as the totalContents member variable is private, and can't be accessed from the clickBar function.

Answer 2

Well, for starters, you cannot bind mclass to clickBar, since the function takes no arguments. You could modify the function to take a reference to an MClass :

void clickBar(MClass& m) { m.totalContents = 42; }

Second, even if you could, bind would make a copy of mclass. You could pass an std::reference_wrapper instead, using std::ref :

#include <functional>

struct MClass { int totalContents = 0;};

void clickBar(MClass& m) { m.totalContents = 42; }

int main() {   
  MClass mclass;
  auto OnLeftClick = std::bind(clickBar, std::ref(mclass));
  OnLeftClick();
}