Thread: Can anyone explain this?

The handler member is probably a function pointer. Functions have address' just like other "normal" variables do. You can assign the address of a function to a function pointer variable and call the function using the function pointer. Later, if necessary, you can reassign the pointer to point to a different function and make the same call again with different results.

Code:

void func1(void)
{
    printf("This is func1.\n");
}

void func2(void)
{
    printf("This is func2.\n");
}

...

void (*funcptr)(void);  // Declares a function pointer called funcptr that can be used with functions
                        // returning void and taking no parameters
funcptr = func1;        // Assign the address of func1 to funcptr
funcptr();              // Effectively calls func1
funcptr = func2;        // Assign the address of func2 to funcptr
funcptr();              // Effectively calls func2

The return type and types and numbers of arguments of the function pointer must match the definition of the function it points to. If you have a function that returns a double and accepts a float and a char* as an argument, the function pointer that is assigned the address of that function must be declared as:

Code:

double (*funcptr)(float,char*);

And you'd call it like so:

Code:

double value;

funcptr = somefunc;
value = funcptr(5.7f,"Hello World");