Function pointer with struct parameter inside struct

Question

I have a struct which currently looks like this (abbreviated to show only the essential parts):

typedef struct {
    uint32_t baudrate;
    ... some other internally used values here
    void (*request_received)(void* hbus); //< this is what I'm talking about
} hbus_options_t;

This works. Basically it contains a function pointer which takes a pointer to a parameter of type void.

What I would actually like is for this to be easier to understand:

typedef struct {
    uint32_t baudrate;
    ... some other internally used values here
    void (*request_received)(hbus_options_t* hbus); //< this doesn't work
} hbus_options_t;

Obviously the compiler needs to know the struct before I can use it. How is this done usually? Using a void pointer works but it's harder to understand.

StoryTeller - Unslander Monica · Accepted Answer · 2018-10-24 08:06:16Z

4

It's done by not being remiss and providing a struct tag:

typedef struct hbus_options {
    uint32_t baudrate;
    ... some other internally used values here
    void (*request_received)(struct hbus_options * hbus); 
} hbus_options_t;

Besides readability, the compiler will also complain if you pass a pointer to something other than the intended struct type.

Adding a tag also allows for looser coupling of components. One can forward declare a structure, but not a type alias.

answered Oct 24, 2018 at 8:06

StoryTeller - Unslander Monica

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3 Comments

Tom L. Over a year ago

Great answer, thank you. Yes, I had to do an explicit cast to void* and the other way round to stop the compiler from complaining.

StoryTeller - Unslander Monica Over a year ago

@TomL. - You had to cast to void*? That's rather odd. That one is an implicit conversion. Or do you mean you had to cast the function pointer? That was risky.

Tom L. Over a year ago

I meant the function pointer, sorry. And yes, that was the reason why I didn't like it.

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