[cfe-commits] [PATCH] [libcxx] avoid hardcoding buffer lengths

[Marshall Clow]

 On Dec 26, 2012, at 4:11 PM, Saleem Abdulrasool <compnerd at compnerd.org> wrote:

> Hi Bigcheese, chandlerc,
> 
> Introduce a new countof template "macro" to determine the count of elements in
> a statically allocated array.  Use this to remove the various static hardcoded
> values for array element counts (strftime_l takes number of characters to write,
> not the size of the buffer, though they are equivalent with char).
> 
> Provide an overload which takes two pointers (iterators) and returns the count
> of elements that would fit in the array bounded by the iterators.  Do this to
> avoid having to add a pointless static assertion along the lines of:
> 
>   static_assert(is_same<decltype(__ib),
>                         typename remove_reference<decltype(__ie)>::type>::value,
>                 "iterator type mismatch");
> 
> to ensure that the iterator types match when performing the calculation using
> sizeof on either of the iterators to determine the size of a single element:
> 
>   (__ie - __ib) / sizeof(*__ib)

You should definitely include Howard Hinnant on this - since he's the primary author of libc++.

Also, I think your second routine is incorrect:

> template <typename T>
> _LIBCPP_ALWAYS_INLINE size_t countof(const T * const begin, const T * const end)
> {
>    return (end - begin) / sizeof(T);
> }


Consider calling it with ( ptr, prt + 1 )

It should return 1 in all cases - no matter what the type of "ptr" is.
But your code does not; it will return '0' for every type _except_ for ones where sizeof(T) == 1

	char *cp;
	int *ip;

	std::cout << "Char " << countof ( cp, cp + 1 ) << std::endl;
	std::cout << "Int  " << countof ( ip, ip + 1 ) << std::endl;

prints:
	Char 1
	Int  0

-- Marshall

Marshall Clow     Idio Software   <mailto:mclow.lists at gmail.com>

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