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skarap

This reminded me of another C syntax strangeness: "Flexible array member". It allows you to do something like this:

struct items_with_header { int header_field1; unsigned int length; double array[]; };

Then allocate enough memory and use the struct to access it.

Used it once in a hash-table implementation.

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This might be my biggest gripe with C's syntax.

typename identifier[];

means three different things, depending on where it appears: As a function argument, it is a pointer that will be accessed like an array. As a variable with automatic storage duration, it is an array whose size will be determined by the right-hand-side of an assignment from a braced initializer list. In the middle of a struct definition, it's illegal. And at the end of a struct definition, it is a flexible array member.

My favorite is similar thing, but in C++ and much more confusing:

  class Bar {};
  
  class Foo {
  public:
  	Foo(const Bar &c) { }
  	void method() { }
  };
  
  int main() {
  	Foo foo(Bar());
  	foo.method();
  	return 0;
  }

"foo.method();" doesn't compile because:

  error: request for member ‘method’ in ‘foo’, which is of non-class type ‘Foo(Bar (*)())’

That's right: "foo" is a declared function that returns instance of Foo and takes one argument - a function that returns instance of Bar :) But when you add additional parentheses in a line above:

  Foo foo((Bar()));

then "foo" is an instance of Foo created by passing a new instance of Bar to its constructor, which is more like what you'd expect by reading the code, and the code compiles. Fun!

Yes, the most vexing parse.

https://en.wikipedia.org/wiki/Most_vexing_parse

I believe this situation is usually referred to as the "Most vexing parse".

C++11 initialization syntax solves this

https://en.wikipedia.org/wiki/Most_vexing_parse#Uniform_init...

As an extern'd variable it's an array of unknown length

AFAIK this this was only made legal by c99. Though I have seen it in code that is much older. (Pre-c99 you would give the "flexible" member an element size inside the square brackets, such as 0 or 1, but allocate as if it were a larger size on the heap.)

I also used this in a similar situation, it helped me to save a lot of memory in my skiplist implementation:

https://github.com/sepeth/python-skiplist/commit/00ae7f94246...

Redis's skiplist is also using the same feature:

https://github.com/antirez/redis/blob/unstable/src/server.h#...

In my case the software used mmap-ed file for storage - I couldn't believe how easy is was to work with the data this way!

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