Comment by pcfwik

7 hours ago

Being taught this rule in undergrad really hampered my appreciation of C. As I've said in a previous comment, the real key that unlocked understanding C declarations for me is the mantra "declaration follows use." You declare a variable in C in exactly the same way you would use it: if you know how to use a variable, then you know how to read and write a declaration for it. Once I understood this elegant idea, it became hard to enjoy using statically typed languages that eschew it.

It is explained in more detail at this link: https://eigenstate.org/notes/c-decl

23 comments

pcfwik

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nitrix  6 hours ago

Some more examples:

  int v, *w, x[5], *y[5], (*z[5])(int, int);

Where v is an int, w is a pointer, x is an array, y is an array of pointers, z is an array of function pointers, etc.

Similarly, typedef is also just a keyword in front of a regular declaration.

  int foo[5];
  typedef int foo[5];

  int bar(void);
  typedef int bar(void);

Now you can use `bar *` as a function pointer.

The entire language works like this.

  • arjvik  5 hours ago

    The way I've learned to read it is

        int v;

    means that `v` is an `int`.

        int *w;

    means that `*w` is an `int`, meaning `w` is a pointer to an `int`.

        int *y[5]

    (note that `◌[]` has higher precedence than `*◌`, so this is `*(y[5])`) means that `*y[5]` is an `int`, so `y[5]` is a pointer to an `int`, meaning `y` is an array of `int` pointers.

        int (*(*kitchensink[5])(int, int))[6];

    means that `(*(*kitchensink[5])(int, int))[6]` is an int, so

    - `*(*kitchensink[5])(int, int)` is an array of `int`.

    - `(*kitchensink[5])(int, int)` is a pointer to array of `int`.

    - `kitchensink[5]` is a function pointer to a function that takes `(int, int)` and returns a pointer to an array of `int`.

    - `kitchensink` is an array of function pointers to functions that take `(int, int)` and return a pointer to an array of `int`.

IronFox05  6 hours ago

Call me a hater but I don't like the spiral rule and I like "declaration follows use" even less.

How do you make an std::array of a given type? Wrap the existing type in an extra layer of std::array, we all know this, it makes sense, there's no reasonable alternative. How do you make a C-array of a given type? Oh boy, "prepend the array specifier before the list of existing array specifiers" (actually it's worse because you have to find the right possibly-empty array of existing array specifiers first, just because there's a list of array specifiers somewhere in the type doesn't mean it's the one you should be prepending to).

"Declaration follows use" immediately goes out the window when faced with typedeffed types being used as the base type, or (as mentioned) generics in descendant languages of C. Instead you get "declaration builds up a type by wrapping layers around a core, use breaks down a type layer by layer starting from the outside" (so, necessarily, they mirror each other). C could have worked that way, and it would have made more sense.

"Declaration follows use" is the type level equivalent of taking off your socks before taking off your shoes because that's the order in which you put them on.

  • clifflocked  5 hours ago

    > we all know this, it makes sense, there's no reasonable alternative

    There absolutely are reasonable alternate ways to represent ordered data that don't involve templates. The way that C does it makes sense in most cases, and if you are looking at something that you cannot understand, you are looking at bad code.

    > "Declaration follows use" immediately goes out the window when faced with typedeffed types being used as the base type

    Typedefs are an abstraction. If you create a typedef, it is usually because you only want to handle the data as a whole, passing it to helper functions that remove the typedef. Also, declaration of use does not break down with typedefs:

        typedef char *(*fn)(int, char *);
    fn my_fn;

    char *s = (*my_fn)(0, ""); // Proper use

    > "Declaration follows use" is the type level equivalent of taking off your socks before taking off your shoes because that's the order in which you put them on.

    Please give me an example of some C code where this is the case.

  • stackghost  5 hours ago

    >How do you make an std::array of a given type? Wrap the existing type in an extra layer of std::array, we all know this

    huh? where is the extra layer?

        std::array<int, 5> array_of_ints = { 1, 2, 3, 4, 5 };

    >How do you make a C-array of a given type? Oh boy, "prepend the array specifier before the list of existing array specifiers"

    ?

        int c_style_array[5] = {2, 3, 5, 7, 11};

    • uecker  5 hours ago

      I assume he means multi-dimensional arrays:

          std::array<std::array<int, 3>, 5> array_of_ints;
    int c_style_array[5][3];

      But the C-style array is more readable, so I am not sure what the complaint really is about.

      2 replies →

kazinator  4 hours ago

If you explain declarations as following use, you are just moving the spiral parse from declarations to expressions. :)

  • pcfwik  4 hours ago

    I think this is a valid point. I would much prefer if the spiral rule were presented as a helpful mnemonic for remembering C operator precedence rather than something uniquely connected to declarations!