Function Name Variability with Pointer in Struct

Question

The pointers function declarations in the following snippets of code allows me to vary the names of each function inside the struct, correct? This is merely hypothetical, so the names are of no consequence yet.

Header File

typedef struct
{
    bool (*enabled)(uint32_void);
    void (*start)(uint32_t);
    void (*stop)(uint32_t);
    bool (*expired)(void);
} battle_star_gallactica;

C File

static bool Battle_v0_enabled (void) { ... }
static void Battle_v0_start (uint32_t) { ... }
static void Battle_v0_stop (uint32_t) { ... }
static bool Battle_v0_stop (void) { ... }

const battle_star_gallactica battle_v0 =
{
    enabled,
    start,
    stop,
    expired
};
...

One Example Use

battle_v0.start(1000);

The C file snippet would be repeated for other versions (e.g. v1 or v2 instead of v0).

How does this work?

EDIT

Corrected code:

Header File

typedef struct
{
    bool (*enabled)(uint32_void);
    void (*start)(uint32_t);
    void (*stop)(uint32_t);
    bool (*expired)(void);
} battle_star_gallactica;

extern const battle_star_gallactica battle_v0;
extern const battle_star_gallactica battle_v1;
...

C File

static bool Battle_v0_enabled (void) { ... }
static void Battle_v0_start (uint32_t val) { ... }
static void Battle_v0_stop (uint32_t val) { ... }
static bool Battle_v0_expired (void) { ... }

const battle_star_gallactica battle_v0 =
{
    Battle_v0_enabled,
    Battle_v0_start,
    Battle_v0_stop,
    Battle_v0_expired
};
...

One Example Use

battle_v0.start(1000);

@Biff - You've watched the new "Battlestar Galactica" series, right? — John Bode
– John Bode, Commented Jan 27, 2013 at 1:45

Richard Schneider · Accepted Answer · 2013-01-27 00:18:52Z

2

Yes. But you have a few issues:

1) Battle_v0_stop is defined twice 2) When initing battle_v0 you need to prefix values with Battle_v0_

You could then do:

battle_v0.enabled()

But I would have currnet battle star galactic and use it:

const battle_star_gallactica bsg = battle_v0;
bgs.enabled()

answered Jan 27, 2013 at 0:18

Richard Schneider

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5 Comments

Biff Over a year ago

Is there a name for the manner in which the pointers were used? I was going to read more about it in a C book.

Richard Schneider Over a year ago

I simply call them function pointers. see newty.de/fpt/index.html and en.wikipedia.org/wiki/Function_pointer

Biff Over a year ago

Do the names of the pointers even matter? Could I, for instance, only have void (*)(uint32_t) for the first function pointer?

Richard Schneider Over a year ago

You do need a name for fucntion pointer. How else would you reference it?

Richard Schneider Over a year ago

You should not have changed the code that you posted. Now other readers will not be able to understand my answer.

RageD · Accepted Answer · 2013-01-27 00:17:43Z

1

You have no functions named enabled, start, and so on. When initializing an object with function pointers, pass a pointer to the actual function name (i.e. Battle_v0_start). That said, it is important to notice that your types do not match. It should be Battle_v0_start(uint32_t), not void.

But yes, after being properly initialized, you will be able to simply call your_obj.your_struct_function_name(your_arguments, ...). In your case battle_v0.start(1000) will call Battle_v0_start (if you pass this function to the pointer). You can think of this like most other pointer types.

answered Jan 27, 2013 at 0:17

RageD

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Comments

John Bode · Accepted Answer · 2013-01-27 00:46:44Z

You have to assign the target function to each of your function pointers, such as

battle_v0.enabled = Battle_V0_enabled;
battle_v0.start   = Battle_V0_start;
// etc.

or

const battle_star_galactica battle_v0 = {
    Battle_V0_enabled,
    Battle_V0_start,
    Battle_V0_stop,
    Battle_V0_expired
};

Then you can call the functions through the pointer:

battle_v0.start(1000);  // or battle_v0.(*start)(1000)

will call the Battle_V0_start function with a value of 1000.

Note that the function signatures (return type and number/types of arguments) have to match; for example, you declare the function pointer for start to take an argument of type uint32_t, but Battle_V0_start takes no arguments. One or the other of these needs to change so that they match.

void Battle_V0_start(uint32_t val) { ... }

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