Passing non-static member function using bind

Question

I'm using the GxEPD2 library to communicate with an E-Paper display. I now wanted to use the drawPaged method of this library with an object display.

I also have written my own class from which I want to print stuff on the display.

Here's the relevant code for the MyClass.h:

class MyClass{
    //some variables and methods
    void drawClass(); //this is public
    void drawClassCallback(const void* v1); //this is private
    //    this is actually a uint8_t --^
}

And here's the MyClass.cpp:

void MyClass::drawClass(){
    //some display preparation
    uint8_t var; //A variable to pass information from the callback to this method since it is needed later
    display.drawPaged(std::bind(&MyClass::drawClassCallback, this, &var));
    //some code where I need var with info from the callback
}

I'm getting the following error:

no matching function for call to 'GxEPD2_BW::drawPaged(std::_Bind_helper::type)'

note: candidate: void GxEPD2_BW::drawPaged(void ()(const void), const void*) [with GxEPD2_Type = GxEPD2_290; short unsigned int page_height = 296u] void drawPaged(void (drawCallback)(const void), const void* pv)

I've also tried display.drawPaged(std::bind(&MyClass::drawClassCallback, this, std::placeholders::_1), &var) without success.

I've stumbled upon this post already, however the solution presented there (and the one used above) does not work in this case.

Why is that?

Answer 1

In your MyClass example, the drawPaged() call is given only a single argument, while according to the library, it needs two:

void drawPaged(void (*drawCallback)(const void*), const void* pv)

The first argument is the function pointer (or the one wrapped by std::bind!). The second argument seems to be some data that will be passed on to that function. (according to the [linked API)

The error actually explains it, it suspects that you are trying to call void GxEPD2_BW::drawPaged(void ()(const void), const void*). Hence it mentions this as a candidate.

What you need to do is something like this:

display.drawPaged(std::bind(&MyClass::drawClassCallback, this, std::placeholders::_1), &var);

std::placeholders::_1 is the placeholder that represents the var argument that will be passed on when it is called.