In article <jc.830939057@atcmpg> jc@atcmp.nl (Jan Christiaan van Winkel)
writes:

|> While using the libg++ 2.7.1 implementation of the string library,
|> I encountered a problem.

|> Suppose the length of a string has decreased:

|> string s("hello");
|> s="hi";

|> what should s.data() return compared to s.c_ptr() ?

s.data should return a pointer to two bytes, containing 'h', 'i'.
s.c_ptr should return a poitner to three bytes, containing 'h', 'i',
'\0'.

|> Example:
|> #include <string>
|> #include <iostream.h>

|> main() {
|>  string s("hello");

|>  s="hi";

|>  cout << "Length: " << s.length() << endl;

|>  cout << s << endl;

|>  cout << s.data() << endl;

|>  cout << s.c_str() << endl;

|> }

|> g++ 2.7.2 with libg++-2.7.1 gives:
|> Length: 2
|> hi
|> hillo
|> hi

|> Is this correct?

Yes and no.  I don't think you can fault g++'s behavior here, but the
line `cout << s.data() << endl' is undefined behavior.  Outputting
"hillo" seems as good an undefined behavior as any.  (Ideally, this line
would generate a core dump.  But g++ is certainly not alone in not doing
so:-).  Does anyone know of an implementation which will systematically
generate a core dump in such cases?)

(For those who miss the point: `operator<<( ostream& , char const* )'
requires a null terminated string as second argument.  `string::data()'
does not return a pointer to a null terminated string, but an array of
string::length() char's.)
--
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GABI Software, Sarl., 8 rue des Francs Bourgeois, 67000 Strasbourg, France
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jc@atcmp.nl (Jan Christiaan van Winkel) writes:
>While using the libg++ 2.7.1 implementation of the string library,
>I encountered a problem.

It appears that libg++ is matching a later version of the draft
standard than April 1995.

>Suppose the length of a string has decreased:
>
>string s("hello");
>s="hi";
>
>what should s.data() return compared to s.c_ptr() ?

s.data() should point to an 'h' that is followed by an 'i' that can be
followed by anything ("hillo" is one possibility).  s.c_ptr() must point
to the null-terminated C string "hi".  In the libg++ implementation
c_ptr() adds the null terminator.  I've seen others (e.g. ObjectSpace)
where c_ptr copies the whole string somewhere else.

>The 4/95 WP sais data() returns c_str() if size() is nonzero,
>otherwise a null pointer.

That language is gone from the January 96 draft; the description of
data() is completely different:

  const charT* data() const;

  Returns:
    If size() is nonzero, the member returns a pointer  to  the  initial
    element  of  an  array  whose first size() elements equal the corre-
    sponding elements of the string controlled by *this.  If  size()  is
    zero, the member returns a non-null pointer that is copyable and can
    have zero added to it.
  Requires:
    The program shall not alter any of the values stored in the  charac-
    ter  array.   Nor  shall  the  program treat the returned value as a
    valid pointer value after any subsequent call to a non- const member
    function of basic_string that designates the same object as this.

I am puzzled by the clause "that is copyable and can have zero added to it".
I suspect that the author was thinking of an implementation where the
effect of c_str is to add a null character at data()+size(), but I don't
know what is meant by putting this strange phrase into the standard.
--
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Work for something because it is good,
not just because it stands a chance to succeed.    -- Vaclav Havel
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Hello,

While using the libg++ 2.7.1 implementation of the string library,
I encountered a problem.

Suppose the length of a string has decreased:

string s("hello");
s="hi";

what should s.data() return compared to s.c_ptr() ?

Example:
#include <string>
#include <iostream.h>

main() {
 string s("hello");

 s="hi";

 cout << "Length: " << s.length() << endl;

 cout << s << endl;

 cout << s.data() << endl;

 cout << s.c_str() << endl;

}

g++ 2.7.2 with libg++-2.7.1 gives:
Length: 2
hi
hillo
hi

Is this correct?

The 4/95 WP sais data() returns c_str() if size() is nonzero,
otherwise a null pointer.

This does not explain the output above...

JC
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___  __  ____________________________________________________________________
   |/  \   Jan Christiaan van Winkel      Tel: +31 24 3527252 jc@atcmp.nl
   |       AT Computing   P.O. Box 1428   6501 BK Nijmegen    The Netherlands
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