Passing functions as arguments not working inside a class

Question

Creating a Foo object passing func to constructor works just fine in this example:

int func(int a) { return a; }

struct Foo {
  Foo( int (*func_ptr)(int) ) {};
};

Foo bar(func);

However attempting to create a Foo object inside another class does not:

class ThisIsCrap {
  Foo doesntWork(func);
};

How can I create a Foo object inside a class like I can outside a class? On the bit that doesn't compile, the error is: "cannot resolve type 'func'"

Thanks in advance.

This is fairly obvious once you realize that `MoreCrap::noFun` is *not a function*. It is a *member* function. Unlike a function, it cannot be called: `MoreCrap::noFun(10)` makes no sense. Member functions are different constructions that require a more complex set of data to be used, namely an object instance and arguments. — Kerrek SB, Mar 15 '17 at 09:31
Okay, I can see that, but what about ThisIsCrap::doesntWork ? — Bryan Fallin, Mar 15 '17 at 09:34
That one doesn't even make syntactic sense. That's like `void f(10, int);`. — Kerrek SB, Mar 15 '17 at 09:35
You can bind `this` to the member function and use it as a `std::function`, see [this answer](http://stackoverflow.com/a/7582576/104774) — stefaanv, Mar 15 '17 at 09:37
You cannot initialize class members inline with round parentheses. Your example is a far too contrived way to demonstrate that; you should have a *much* simpler example, like `struct X { int a(5); };`. — Kerrek SB, Mar 15 '17 at 09:37
@BryanFallin: ... *with round parentheses*. There are other ways in which you can do it. — Kerrek SB, Mar 15 '17 at 10:08

score 0 · Answer 1 · answered Mar 15 '17 at 09:57

0

1,000 thanks to Kerrek SB.

class ThisWorks {
    Foo* working;
    ThisWorks() {
         working = new Foo(func);
    }
}

answered Mar 15 '17 at 09:57

Bryan Fallin

23
5

This is as wrong as it gets. Why did you introduce a pointer? You didn't need a pointer but now you will have to look after it **forever**. – n. m. could be an AI Mar 15 '17 at 09:59
Oy.. Couldn't figure out how else to do it :/ – Bryan Fallin Mar 15 '17 at 10:11

Kerrek SB · Accepted Answer · 2017-03-15T10:15:52.407

0

You can provide an initializer for non-static class data members with a default member initializer (DMI):

int func(int a) { return a; }
struct Foo { Foo(int (*)(int)) {}; };

class ThisIsGreat {
  Foo one_way = func;     // DMI with copy-initialization syntax
  Foo another_way{func};  // DMI with list-initialization syntax
};

Or of course you could use a constructor:

class ThisIsSane {
  ThisIsSane()
      : third_way(func)   // constructor-initializer list
  {}

  Foo third_way;
};

Language-lawyer note for pedants: In C++11, ThisIsGreat is not an aggregate; in C++14 it is.

edited Mar 15 '17 at 10:15

answered Mar 15 '17 at 10:10

Kerrek SB

464,522
92
875
1,084

I knew that could be done with basic data types, but for whatever reason my brain seized up and didnt connect the dots to abstract data types as well. Well.. I am embarrassed to say the least lol, I need to put the coffee away, fold the laptop, and go to sleep. Maybe when I wake up later I will have regained some IQ points.... and my dignity. @Kerrek, again, Thanks! – Bryan Fallin Mar 15 '17 at 10:20
As I said, always reduce your problem to the absolute core of the question, and just by doing so you would probably discover the missing link. – Kerrek SB Mar 15 '17 at 10:21

Passing functions as arguments not working inside a class

2 Answers2