Arduino static variable declaration with no datatype? [closed]

I am quite new to Arduino and try to understand the following variable declaration:

static btn_state_t nav_btn, joy_btn;


First, I would expect a datatype like integer or float or the like after "static".
I only found one answer that the variable then would be assigned a default type?
Second, I don't understand the following comma-separated "names" or other variables.
In the programm, it looks like they can be both used for "btn_state_t".

I couldn't find an answer to my question so far...or maybe someone can give me a hint on what search-words to look for?

Thanks Steve for the profound and complete answer below! Compleatly answered my question and his guessing was right ...that even with quite some search, I didn't find that datatype and "typedef" was the right codeword.
So solved for me.

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btn_state_t

nav_btn

joy_btn

btn_state_t

static int i, j;

1 Answer
1

Short answer: btn_state_t is a type, and you're declaring two variables of this type.

btn_state_t

Longer answer:

If you said

int nav_btn, joy_btn;


you would be declaring two variables of type int. The type name is int and the names of the variables are nav_btn and joy_btn.

int

int

nav_btn

joy_btn

If you said

static int nav_btn, joy_btn;


you would be doing the same thing, but declaring them statically. (This can have two different meanings, depending on whether the declaration is inside or outside a function, but let's not worry about that just now.) In this declaration, the keyword static is known as a storage class.

static

Once upon a time, things defaulted to int a lot. Therefore, you could also say

int

static nav_btn, joy_btn;


This has a storage class static and two variable names nav_btn and joy_btn, but no type name. But it ends up doing the same thing, declaring two static variables of type int, because originally in C, the missing type name defaulted to int. This style of declaration is obsolete today. But that's what the other answer you found was talking about.

static

nav_btn

joy_btn

int

int

But you've stumbled across another case. You have

static btn_state_t nav_btn, joy_btn;


If there were a comma between btn_state_t and nav_btn this would be a declaration of three static variables with their type defaulted to int. But there isn't a comma, and there's that little clue _t in btn_state_t. It turns out that btn_state_t is the name of a type.

btn_state_t

nav_btn

int

_t

btn_state_t

btn_state_t

So this is a declaration of two static variables, of type btn_state_t, named nav_btn and joy_btn.

btn_state_t

nav_btn

joy_btn

But what's this type name btn_state_t? Obviously you won't find it in any list of C types, along with int and long int and double and the rest. And the answer is that it's an example of a typedef.

btn_state_t

int

long int

double

Somewhere, in a project-specific header file, or the header file for a GUI or I/O library you're using, is probably a line like

typedef int btn_state_t;


This line defines a new type name. The new type name is btn_state_t, and the type it names is otherwise identical to type int. Now when you type

btn_state_t

int

btn_state_t btn;


or

btn_state_t nav_btn, joy_btn;


it's just as if you had said

int btn;
int nav_btn, joy_btn;


In other words, typedef is sort of like #define. Saying

typedef

#define

typedef int btn_state_t;


is sort of like saying

#define btn_state_t int


Afterwards, every time we write btn_state_t, it's just as if we'd written int. (But in more complicated cases, typedef can do things that #define can't, so they're really not at all the same mechanism.)

btn_state_t

int

typedef

#define

Typedefs are a powerful mechanism, but it's always a little surprising to have new type names floating around that you've never heard of, so there's a convention of always using the suffix _t in a typedef name. Other common examples of such typedefs are type names like size_t and time_t and int32_t.

_t

size_t

time_t

int32_t

If you've never encountered typedefs before, you may be wondering what they're good for. You may be wondering why they're necessary, since at first it looks like good old #define would work just fine. And you may be wondering what the point is, since you could always just say

#define

static int nav_btn, joy_btn;


if the "real" type behind btn_state_t is in fact int. But this answer is too long already, so if you're still curious about typedef, I'll just encourage you to do a web search on it and read about it.

btn_state_t

int

typedef