glfw non-standard syntax; use '&' to create a pointer to member - c++

Question

i keep getting non-standard syntax; use '&' to create a pointer to member
i dont know why its not working.
it works if Resize function is global
minimal example:
#include <GLFW/glfw3.h>
class test {
public:
test(){}
~test(){}
void resize(GLFWwindow* window, int new_width, int new_height) {}
}resizer;
int main(){
auto newwindow = glfwCreateWindow(1, 1, "test", NULL, NULL);
glfwSetWindowSizeCallback(newwindow, resizer.resize);
return 0;
}
the initial problem got kinda solved using static as the function but this created diferent errors in what i want to do here is the simplified problem the problem:
//rough replication of lib functions cannot change these
typedef void(* windowsizefun)(int,int);
void setWindowSizeCallback(windowsizefun fun){}
//the problem
class windowhandler{
private:
int width, height;
static void resize(int new_width, int new_height) {
width =new_width; height =new_height; //error
}
public:
test(){
width =100; height =100;
setWindowSizeCallback(windowhandler::resize);
}
}
int main(){
windowhandler newWindow();
return 0;
}

Answer 1

Your function pointer doesn't match the declaration that's needed to pass into glfwSetWindowSizeCallback().
It must be a static function, and the scope operator :: needs to be applied correctly:
#include <GLFW/glfw3.h>
class test {
public:
test(){}
~test(){}
static void resize(GLFWwindow* window, int new_width, int new_height) {}
// ^^^^^^
} /*resizer*/;
// ^^^^^^^^^^^ No need for this.
int main(){
auto newwindow = glfwCreateWindow(1, 1, "test", NULL, NULL);
glfwSetWindowSizeCallback(newwindow, test::resize);
return 0;
}