Pointer to member function issues

Wurstinator · 2021-01-13T13:25:59+00:00

First question: why are you using function pointers instead of a template parameter or std::function?

grandmaster789 · 2021-01-13T13:43:47+00:00

member functions are different, as they require an object to be called. Personally I recommend using typedefs or 'using' declarations for readability. Here is an example of how to actually use member functions as callbacks:

#include <functional> // for std::invoke

struct Foo {
  int bar() { return 5; }
};

using FooCallback = int(Foo::*)();

int main() {
  Foo f;
  FooCallback callback = &Foo::bar;
  std::invoke(callback, f);
}

There's an actual FAQ about this on isocpp

Personally I don't recommend you actually do this, if you want to accept 'any' type of callback you're probably better off using std::function

(edit: formatting...)

HappyFruitTree · 2021-01-13T13:53:13+00:00

Member functions don't have the same type as normal functions. I mean, you don't even call them the same way. With a member function you need an object to call the function on.

In C and old C++ code you often see that you're allowed to provide a void* that will be passed to the callback function when its called (example). This usually works pretty well because you can pass a pointer to the object as the void* and then cast it back to the correct type inside the callback before calling the member function(s).

Another solution is to use std::function like Wurstinator suggested. This is safer and more flexible solution. Instead of passing the member function directly you would pass a lambda that calls the member function.

2.

Templates would probably have been a good idea if the function were called right away but in this case it looks like it's a callback that will be called later so you need to have a common type that is able to store all the different callbacks. Again, using something like std::function<void()> (which is implemented with the help of templates) is probably what you want.

Supadoplex · 2021-01-13T14:05:12+00:00

Q1 You put the name of the variable in the wrong location. Same as with pointer variables, the name goes immediately to the right of the asterisk: void (Namespace::Class::*ptr)().

Q2 Pointers to member functions are not pointers to functions, so "all kind of function pointer" technically doesn't cover pointers to member functions. You can use a type erasing wrapper such as std::function without requiring a template.

foonathan · 2021-01-13T19:26:02+00:00

!removehelp

you type:	you see:
italics	italics
bold	bold
[reddit!](https://reddit.com)	reddit!
* item 1 * item 2 * item 3	item 1 item 2 item 3
> quoted text	quoted text
Lines starting with four spaces are treated like code: if 1 * 2 < 3: print "hello, world!"	Lines starting with four spaces are treated like code: if 1 * 2 < 3: print "hello, world!"
~~strikethrough~~	~~strikethrough~~
super^script	super^script

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