How to define Empty Char in Delphi

Question

Just for curiosity,

Why in Delphi, if we defined an empty char by:

a:Char;
a:='';

we get an error: Incompatible types: 'Char' and 'string'

However, if we placed

a:='a';

it will be fine?

Is it necessary to define an empty char by: a:=#0?

In the .net world, you could have an "empty" Char because they have Nullable types (via their generics system). And in the database world, database fields can be Nullable, but all Delphi Ordinal types, including Integer, Char, Byte, and the rest, while they might have some sentinel (flag) or zero values (#0) for Char, for example, can not be "empty" or "null". Since strings and arrays are not Ordinal types the rules are different. A string is like a "dynamic array of characters". — Warren P
– Warren P, Commented Mar 8, 2012 at 14:13
You are confused about '' constant expression. It is of type string and not compatible with type char. In contrary 'a' is compatible. So, you have to define "empty char" convention. — OnTheFly
– OnTheFly, Commented Mar 8, 2012 at 14:29

Andreas Rejbrand · Accepted Answer · 2012-03-08 13:52:49Z

29

A char is a single (that is, exactly one) character. So 'a', '∫', and '⌬' are all OK, but not 'ab' (a two-character string), 'Hello World!' (a twelve-character string), or '' (a zero-character string).

However, the NULL character (#0) is a character like any other.

In addition, the character datatype is implemented as a word (in modern versions of Delphi), that is, as two bytes. If all these values 0, 1, ..., 2^16 - 1 are used for real characters, how in the world would you represent your 'empty char'?

edited Mar 8, 2012 at 13:52

answered Mar 8, 2012 at 13:47

Andreas Rejbrand

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15 Comments

Zeina Over a year ago

u r right with the last edited phrase, I meant by empty char a null character, but i thought that when I place a:='', the compiler will take it as #0 by default, and didn't expect such error, so i wanted to know why is that. Thanks in advance!

Andreas Rejbrand Over a year ago

@Zeina: Of course, the Delphi compiler could treat '' synonymously with #0, but I would find that confusing. In addition, does it really take that much longer to write #0 than ''?

Andreas Rejbrand Over a year ago

@Marco: 'Legacy C systems'? The Windows API?

Remy Lebeau Over a year ago

I have always disliked that Delphi uses single quotes for both Char and String literals. It may make coding easier, but it also causes confusion like this. I use C++ primarily, and it uses single quotes for characters and double quotes for strings. More explicit, and intuitive to me.

Andreas Rejbrand Over a year ago

@JonathanElkins: No, that is not the case. '' is simply the empty string, and it is perfectly legal to use that combination, like in ShowMessage(''). But the empty string consists of zero characters, so it is not a character.

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David Heffernan · Accepted Answer · 2012-03-08 15:37:01Z

20

There is no such thing as an empty char. A char has to have a value. It is an ordinal type, a simple value type. Just as an integer, say, always has a value, so does a char.

The value #0 is not an empty char, it is the character with value 0, commonly known as NUL.

edited Mar 8, 2012 at 15:37

answered Mar 8, 2012 at 13:47

David Heffernan

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1 Comment

OnTheFly Over a year ago

In Pascal thats ordinal type.

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