Re: Ada Idioms Progress Preview

[James Rogers <jimmaureenrogers@worldnet.att.net>]

Ole-Hjalmar Kristensen wrote:
> 
> One thing which can be said in favour of having a terminator character
> is that it frees you from having to store the length explicitly. The
> length of a string is usually different from the size of the array
> used to store the string.
> So, in a sense a C string is more self-describing than a plain Ada
> string.
> Of course, as soon as you call a procedure, you can use a slice, but
> you still need the actual length to decide which slice.
> 
> On the balance, I would rather have Ada strings.

An old saying is "there is no free lunch". In other words, nothing
comes for free. In the case of a C string, you do not explicitly
carry around the length of a string. Instead, you rely on a convention
stating that the logical end of the string is indicated by a null
character.

The C approach presents two very real costs:

1) You must serially read the string to find the terminating null
   character. This operation is very expensive if you only need to
   determine the length of the string.

2) Sometimes the null character is omitted. Since C arrays are
   unbounded, this causes your program to read beyond the end of
   the string until it finds a null character. The resulting
   length will be incorrect. When copying or editing a string
   this problem will result in data corruption and undefined
   behaviors.

Another less common cost occurs when copying C strings. The most
efficient copy operation for C arrays is the memcpy function.
This function allows you to copy blocks of memory efficiently.
If you try to use memcpy to copy strings you will find some
real problems. In those cases you want to copy the actual array
of characters, not just the logical string contained in it.
The problem is that the C sizeof operator does not report the
correct size of arrays outside the immediate scope where they
are declared. Instead you will only get the size of the pointer
to the first element of the array. Therefore, to efficiently
copy C strings using memcpy you must provide a second "length"
argument, which may not be readily available.

Jim Rogers
Colorado Springs, Colorado USA