The Nullity Of strings and Its Meaning

[kdevel via Digitalmars-d-learn]

Yesterday I noticed that std.uri.decodeComponent does not 
'preserve' the
nullity of its argument:

    1 void main ()
    2 {
    3    import std.uri;
    4    string s = null;
    5    assert (s is null);
    6    assert (s.decodeComponent);
    7 }

The assertion in line 6 fails. This failure gave rise to a more 
general
investigation on strings. After some research I found that one
"cannot implicitly convert expression (s) of type string to bool" 
as in

    1 void main ()
    2 {
    3    string s;
    4    bool b = s;
    5 }

Nonetheless in certain boolean contexts strings convert to bool 
as here:

    1 void main ()
    2 {
    3    import std.stdio;
    4    string s; // equivalent to s = null
    5    writeln (s ? true : false);
    6    s = "";
    7    writeln (s ? true : false);
    8 }

The code prints

    false
    true

to the console. This lead me to the insight, that in D there are 
two
distinct kinds of empty strings: Those having a ptr which is null 
and
the other. It seems that this ptr nullity not only determines 
whether
the string compares equal to null in an IdentityExpression [1] 
but also
the result of the above mentioned conversion in the boolean 
context.

I wonder if this distinction is meaningful and---if not---why it 
is
exposed to the application programmer so prominently.

Then today I found this piece of code

    1 void main ()
    2 {
    3    string s = null;
    4    string t = "";
    5    assert (s is t);
    6 }

which, according to the wording in [1]

   "For static and dynamic arrays, identity is defined as 
referring to
    the same array elements and the same number of elements."

shall succeed but its assertion fails [2]. I anticipate the
implementation compares the ptrs even in the case of zero 
elements.

A last example of 'deviant behavior' I found is this:

     1 import std.stdio;
     2 import std.file;
     3 void main ()
     4 {
     5    string s = null;
     6    try
     7       mkdir (s);
     8    catch (Exception e)
     9       e.msg.writeln;
    10
    11    s = "";
    12    try
    13       mkdir (s);
    14    catch (Exception e)
    15       e.msg.writeln;
    16 }

Using DMD v2.073.2 the first expression terminates the programm 
with a
segmentation fault. With 2.074.1 the program prints

    : Bad address
    : No such file or directory

I find that a bit confusing.

[1] https://dlang.org/spec/expression.html#identity_expressions
[2] https://issues.dlang.org/show_bug.cgi?id=17623