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How Do I Declare a Function Pointer in C?

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Re: How Do I Declare a Function Pointer in C?

#11

Post by bstamour »

david-given wrote: post #9
bstamour wrote: post #8

This is one of those cases where I prefer C++ template using function_ptr = add_pointer_t ; and now declarations are a bit more sane: void foo(function_ptr callback);

In C, the easiest thing to do is to just use a typedef. You get to use the same syntax as for normal function calls with no trying to remember where that blasted extra * goes, and typically end up with cleaner code anyway. typedef void function_ptr_t(int i); function_ptr_t* ptr;

Yeah, typedefs solve the problem for fixed types. I guess you could use a macro to create a generic FUNC_PTR or something as well.

Re: How Do I Declare a Function Pointer in C?

#12

Post by bstamour »

Const-me wrote: post #10
bstamour wrote: post #8

This is one of those cases where I prefer C++ template using function_ptr = add_pointer_t ; and now declarations are a bit more sane: void foo(function_ptr callback);

In C++, pointers to member functions are even more cumbersome than C function pointers.

Agreed. I rarely use pmf's, so every time I do I have to look up the syntax again.

Re: How Do I Declare a Function Pointer in C?

#13

Post by paulnechifor »

dnquark wrote: post #7

The trick to reading crazy C declarations is learning the "spiral rule": http://c-faq.com/decl/spiral.anderson.html (here are more examples, with nicer formatting: http://www.unixwiz.net/techtips/reading-cdecl.html )

The "spiral rule" doesn't work all the time. See this previous HN comment by stephencanon: https://news.ycombinator.com/item?id=12775862 .

Also this comment by Linus Torvalds (copy pasted because I don't know how to link to Google+ comments):

> I don't think that works. It breaks trivially for consecutive [] or * cases, something that he carefully didn't have in his examples.

> So the examples were made up to make it look like it's a spiral, but type parsing is about precedence, not about spirals. It so happens that the higher-precedence operators ([] and ()) are on the right-hand side, which is why it "works" to start on the right.

> And it doesn't explain why

> typedef int (fn_t[][2])(void);

> is ok, but

> typedef int (fn_t[2][])(void);

> is not.

> "Spirals"? I don't think so.

Re: How Do I Declare a Function Pointer in C?

#14

Post by TheAdamist »

The new c++ alt function syntax talked about here: https://blog.petrzemek.net/2017/01/17/pros-and-cons-of-alter...

mentions replacing function declarations for

  void (*get_func_on(int i))(int);
with

  auto get_func_on(int i) -> void (*)(int);
which looks a lot more readable to me.

Re: How Do I Declare a Function Pointer in C?

#16

Post by colanderman »

dnquark wrote: post #7

The trick to reading crazy C declarations is learning the "spiral rule": http://c-faq.com/decl/spiral.anderson.html (here are more examples, with nicer formatting: http://www.unixwiz.net/techtips/reading-cdecl.html )

No, the trick is to remember that declaration follows use. Declare a symbol using (nearly) the same exact syntax you would use to extract a value of the base type from that symbol.

See also my comment last time this subject came up: https://news.ycombinator.com/item?id=12775966

Re: How Do I Declare a Function Pointer in C?

#17

Post by ramshorns »

dnquark wrote: post #7

The trick to reading crazy C declarations is learning the "spiral rule": http://c-faq.com/decl/spiral.anderson.html (here are more examples, with nicer formatting: http://www.unixwiz.net/techtips/reading-cdecl.html )

So, applied to the examples:

    As a variable:
    returnType (*variableName)(parameterTypes) = function_name;
variableName is a pointer to a function that accepts parameterTypes and returns returnType

    As a static const variable:
    static returnType (* const variableName)(parameterTypes) = function_name;
variableName is a const pointer to a function that accepts parameterTypes and returns static returnType (or maybe variableName is the static thing?)

    As an array:
    returnType (*arrayName[])(parameterTypes) = {function_name0, ...};
arrayName is an array of pointers to functions that accept parameterTypes and return returnType

    As an argument to a function:
    int my_function(returnType (*argumentName)(parameterTypes));
my_function is a function that accepts an argumentName, which is a pointer to a function accepting parameterTypes and returning returnType, and returns an int

    As a return value from a function:
    returnType (*my_function(int, ...))(parameterTypes);
my_function is a function that accepts int and other parameters and returns a pointer to a function that accepts parameterTypes and returns returnType

    As a typedef:
    typedef returnType (*typeName)(parameterTypes);
a typeName is now a pointer to a function that accepts parameterTypes and returns returnType

Re: How Do I Declare a Function Pointer in C?

#19

Post by 2trill2spill »

petters wrote: post #15

Just use the typedef. Even if you personally find the other variants readable, chances are that your peer reading your code doesn't.

No don't use typedefs there's no real reason[1] and it may cause problems. Also if your peer can't read a function pointer I don't know what help you plan on getting from them anyway, chances are you are helping/teaching them, not the other way around.

[1]: http://yarchive.net/comp/linux/typedefs.html

Re: How Do I Declare a Function Pointer in C?

#20

Post by moron4hire »

2trill2spill wrote: post #19
petters wrote: post #15

Just use the typedef. Even if you personally find the other variants readable, chances are that your peer reading your code doesn't.

No don't use typedefs there's no real reason[1] and it may cause problems. Also if your peer can't read a function pointer I don't know what help you plan on getting from them anyway, chances are you are helping/teaching them, not the other way around. [1]: http://yarchive.net/comp/linux/typedefs.html

Just because Linus Torvalds is a successful programmer doesn't mean he is at all correct about issues of programming. The vast majority of any of the opinions I've seen him express on programming fly in the face of good engineering practice under some guise of "real, macho programmers don't need tools".

I'm not afraid to admit: I need tools. I need lots and lots of tools to do my job. The more the computer can be used to make my job easier, the better. There is no virtue in hard work for hard work's sake.

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