Calling a non-static variable from a static function

Question

I encountered an issue while trying to do something in the process of learning C++ and I am not sure how to handle the situation:

class Command
{
  public:
    const char *       Name;
    uint32             Permission;
    bool (*Handler)(EmpH*, const char* args); // I do not want to change this by adding more arguments
};
class MyClass : public CommandScript
{
 public:
  MyClass() : CommandScript("listscript") { }
  bool isActive = false;
  Command* GetCommands() const
  {
      static Command commandtable[] =
      {
          { "showlist", 3, &DoShowlistCommand } // Maybe handle that differently to fix the problem I've mentioned below?
      };
      return commandtable;
  }
  static bool DoShowlistCommand(EmpH * handler, const char * args)
  {
     // I need to use isActive here for IF statements but I cannot because
     // DoShowlistCommand is static and isActive is not static. 
     // I cannot pass it as a parameter either because I do not want to
     // change the structure of class Command at all

     // Is there a way to do it?
  }
};

Any help would be greatly appreciated! :)

If I understand well you need the function to be static in order to put it as a function pointer in your Command class ? — Christophe
– Christophe, Commented Feb 15, 2016 at 17:03
what do you need isActive for? Of which object do you want to know if it is active? I think there is a mistake in you logic, because without object, there is no point of calling a non-static method. I am not sure, but if you want to know if the commands inside the commandtable are active, then just call the method on those objects commandtable[i].isActive(); — 463035818_is_not_an_ai
– 463035818_is_not_an_ai, Commented Feb 15, 2016 at 17:04
@Chogart sorry, I was on my way home. It's not fully clear to me, if you intend to have one MyClass function in the table or if all the functions in the table are MyClass functions, nor what is in CommandScript. I nevertheless propose some alternatives to your issue, hope it can help. . — Christophe
– Christophe, Commented Feb 15, 2016 at 18:24

flogram_dev · Accepted Answer · 2016-02-15 16:57:08Z

3

// Is there a way to do it?

No.

Either pass it as parameter, make it static, or make DoShowlistCommand non-static.

answered Feb 15, 2016 at 16:57

flogram_dev

43k11 gold badges109 silver badges161 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

Chogart Over a year ago

Wouldn't making a bool static require const too? That would mean I won't be able to modify the bool if needed?

Lightness Races in Orbit Over a year ago

@Chogart: Why would it require const?

Chogart Over a year ago

@zenith Well I am initializing it in the class itself which would require the member to be const

HolyBlackCat Over a year ago

@Chogart Well, then you can initialize it outside.

Community · Accepted Answer · 2020-06-20 09:12:55Z

2

There are two potential answers here:

1. about use of non static items in a static functions:

As said in our previous question/answer, this is not possible, unless you'd have in the static function a specific MyClass object (and use object.isActive). Unfortunately, you can't do this here :

your code comments clearly show that you can't add a MyClass parameter to the function call;
the existing parameters don't suggest that you have already a pointer to parent class object;
it would not be adivsable to use global objects in such a context.

2. about what your're trying to do:

It seems that you want to have the function static, because you want to provide it in a table that maps script-commands to function pointers.

Alternative A

If all the function pointers used in commandtable are members of MyClass, you could think of using a pointer to a member function instead of a pointer to a function. The outside object/function that sets isActive on an object, could then refer the pointer to the member function, on the MyClass object it knows.

Alternative B

Revise the design of your code to implement your script engine by using the command design pattern: it's ideally suited for this kind of problems. It will require some refactoring of your code, but it will be so much more maintenable and extensible afterwards !

edited Jun 20, 2020 at 9:12

CommunityBot

11 silver badge

answered Feb 15, 2016 at 18:19

Christophe

75.1k7 gold badges87 silver badges182 bronze badges

2 Comments

Chogart Over a year ago

This command design pattern is neat. It helps me to date! It's amazing how often I use it. I cannot thank you enough.

Christophe Over a year ago

@Chogart I'm happy to get your feedback and to know that this answer helped you beyond this concrete question: You made my day ! :-)

Liza · Accepted Answer · 2016-02-15 17:03:21Z

2

I don't think there is any way to do it. Here is why: A static member function is not attached to any particular object, which means it cannot access other members that are not static, since they are attached to an object.

It doesn't look like you need to make it a static member. If you are sure you do - then pass it as a parameter. For example, make a

bool isActive();

function, and pass an argument from it to that function somewhere when you call this 'problematic' one.

You also could change your member variable to static, but it looks like you need it for EACH object, not one-for-all

answered Feb 15, 2016 at 17:03

Liza

1031 silver badge11 bronze badges

Collectives™ on Stack Overflow

Calling a non-static variable from a static function

3 Answers 3

4 Comments

1. about use of non static items in a static functions:

2. about what your're trying to do:

2 Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

4 Comments

1. about use of non static items in a static functions:

2. about what your're trying to do:

2 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related