C Style Strings

[scroopy]

Hi,

I've always used std::string but I'm having to use a 3rd party library
that returns const char*s. Given:

char* pString1 = "Blah ";
const char* pString2 = "Blah Blah";

How do I append the contents of pString2 to pString? (giving "Blah
Blah Blah")

I looked at strcat but that crashes with an unhandled exception.

Thanks

[jpalecek@web.de]

Re: C Style Strings


scroopy wrote:[color=blue]
> Hi,
>
> I've always used std::string but I'm having to use a 3rd party library
> that returns const char*s. Given:
>
> char* pString1 = "Blah ";
> const char* pString2 = "Blah Blah";
>
> How do I append the contents of pString2 to pString? (giving "Blah
> Blah Blah")
>
> I looked at strcat but that crashes with an unhandled exception.[/color]

That's easy, do it the C way:

char* result=new char[strlen(s1)+strl en(s2)+1];
strcpy(result,s 1);
strcat(result,s 2);

Regards
Jiri Palecek

[Kai-Uwe Bux]

Re: C Style Strings

scroopy wrote:
[color=blue]
> Hi,
>
> I've always used std::string but I'm having to use a 3rd party library
> that returns const char*s. Given:
>
> char* pString1 = "Blah ";
> const char* pString2 = "Blah Blah";
>
> How do I append the contents of pString2 to pString? (giving "Blah
> Blah Blah")[/color]

a) Keep using std::string for your own stuff. The std::string class has
methods that allow you to interact with C-style string interfaces of
libraries.

b) Note that in your example above, pString1 is initialized to a const
string. Any attempt to modify that string would be undefined behavior. The
danger lies in not declaring pString1 as a char const *.

c) You could do:

#include <string>
#include <algorithm>

char * strdup ( std::string str ) {
char * result = new char [ str.length() +1 ];
std::copy( str.begin(), str.end(), result );
result[ str.length() ] = 0;
return ( result );
}

int main ( void ) {
char * pString1 = "Blah ";
char const * pString2 = "Blah Blah";
pString1 = strdup( std::string( pString1 ).append( pString2 ) );
}


However, I would prefer to use std::string for my own stuff:

#include <string>

int main ( void ) {
std::string pString1 = "Blah "; // rhs returned by some library
std::string pString2 = "Blah Blah"; // rhs returned by some library
pString1.append ( pString2 );
}


Best

Kai-Uwe Bux

[Roland Pibinger]

Re: C Style Strings

On Thu, 01 Jun 2006 08:35:36 +0100, scroopy <scroopy@nospam .com>
wrote:[color=blue]
>I've always used std::string but I'm having to use a 3rd party library
>that returns const char*s.[/color]

First you need to find out if you 'own' the returned string, i.e. if
you must free (maybe delete) the returned char*. Good C libraries
usually don't require the user to call free.
[color=blue]
>Given:
>
>char* pString1 = "Blah ";
>const char* pString2 = "Blah Blah";
>
>How do I append the contents of pString2 to pString? (giving "Blah
>Blah Blah")[/color]

You can append a const char* to a std::string:

string myString = "Blah ";
const char* s = myLibFunc (...);
if (s) {
myString += s; // or myString.append (s);
}
// free (s); // if it's a bad lib

Best wishes,
Roland Pibinger

[kwikius]

Re: C Style Strings

scroopy wrote:[color=blue]
> Hi,
>
> I've always used std::string but I'm having to use a 3rd party library
> that returns const char*s. Given:
>
> char* pString1 = "Blah ";
> const char* pString2 = "Blah Blah";
>
> How do I append the contents of pString2 to pString? (giving "Blah
> Blah Blah")[/color]

#include <malloc.h>
#include <cstring>

char* concat(const char * str1, const char* str2)
{
char * result = (char*) malloc(strlen( str1) + strlen (str2) + 1);
if( result != NULL){
strcpy(result,s tr1);
strcat(result, str2);
}
return result;
}

#include <iostream>
#include <string>

char* pString1 = "Blah ";
const char* pString2 = "Blah Blah";


int main()
{
// C-style
char* str = concat(pString1 ,pString2);
if(str != NULL){
std::cout << str <<'\n';
free(str);
}

// C++ style
std::string str1=std::strin g(pString1) + pString2;
std::cout << str1 <<'\n';
}

I'm not sure if that is the optimal C method. Its interesting to note
how much better the C++ version is though!

regards
Andy Little

[kwikius]

Re: C Style Strings

Roland Pibinger wrote:
[color=blue]
> First you need to find out if you 'own' the returned string, i.e. if
> you must free (maybe delete) the returned char*. Good C libraries
> usually don't require the user to call free.[/color]

Out of interest ... What is a good C library resource management
strategy? ... for example (say) modifying the following example

char* concat(const char * str1, const char* str2)
{
char * result = (char*) malloc(strlen( str1) + strlen (str2) + 1);
if( result != NULL){
strcpy(result,s tr1);
strcat(result, str2);
}
return result;
}

char* pString1 = "Blah ";
const char* pString2 = "Blah Blah";

int main()
{
char* str = concat(pString1 ,pString2);
if(str != NULL){
std::cout << str <<'\n';
free(str); // How to avoid this in C-style?
}
}

regards
Andy Little

[Richard Bos]

Re: C Style Strings

"kwikius" <andy@servocomm .freeserve.co.u k> wrote:
[color=blue]
> scroopy wrote:[color=green]
> > How do I append the contents of pString2 to pString? (giving "Blah
> > Blah Blah")[/color]
>
> #include <malloc.h>
> #include <cstring>[/color]

This is not C...
[color=blue]
> char* concat(const char * str1, const char* str2)
> {
> char * result = (char*) malloc(strlen( str1) + strlen (str2) + 1);[/color]

....and this is the wrong way to do this in C. And, really, also in C++:
use new.

So don't cross-post stuff like that. Follow-ups set.

Richard

[Richard Heathfield]

Re: C Style Strings

Richard Bos said:
[color=blue]
> "kwikius" <andy@servocomm .freeserve.co.u k> wrote:
>[color=green]
>> scroopy wrote:[color=darkred]
>> > How do I append the contents of pString2 to pString? (giving "Blah
>> > Blah Blah")[/color]
>>
>> #include <malloc.h>
>> #include <cstring>[/color]
>
> This is not C...[/color]

....and it's not C++ either, so I'm not sure why you set followups to clc++.

The entire article, in fact (his, not yours), was a classic example of the
kind of thing you get in the Hackitt and Scarper school of programming.

--
Richard Heathfield
"Usenet is a strange place" - dmr 29/7/1999

http://www.cpax.org.uk

email: rjh at above domain (but drop the www, obviously)

[kwikius]

Re: C Style Strings

Richard Bos wrote:[color=blue]
> "kwikius" <andy@servocomm .freeserve.co.u k> wrote:
>[color=green]
> > scroopy wrote:[color=darkred]
> > > How do I append the contents of pString2 to pString? (giving "Blah
> > > Blah Blah")[/color]
> >
> > #include <malloc.h>
> > #include <cstring>[/color]
>
> This is not C...
>[color=green]
> > char* concat(const char * str1, const char* str2)
> > {
> > char * result = (char*) malloc(strlen( str1) + strlen (str2) + 1);[/color]
>
> ...and this is the wrong way to do this in C.[/color]

Out of interest what is the right way? BTW... It would seem to me to
make sense to cross-post this to comp.lang.c as it is somewhat O.T. for
C++. I think It makes sense in terms of a language comparison though,
don't you think?
[color=blue]
> And, really, also in C++:[/color]

hmmm... Touchy arent we? I showed a C++ way in my previous post.
[color=blue]
> use new.[/color]

Sure? .. The best way is not to allocate resources the users has to
manage , isnt it?.
[color=blue]
> So don't cross-post stuff like that. Follow-ups set.[/color]

Hmm... Are you sure I wouldnt get a better answer to the how to do it
in C question in a C newsgroup?

regards
Andy Little

[kwikius]

Re: C Style Strings

Richard Heathfield wrote:[color=blue]
> Richard Bos said:
>[color=green]
> > "kwikius" wrote:
> >[color=darkred]
> >> scroopy wrote:
> >> > How do I append the contents of pString2 to pString? (giving "Blah
> >> > Blah Blah")
> >>
> >> #include <malloc.h>
> >> #include <cstring>[/color]
> >
> > This is not C...[/color]
>
> ...and it's not C++ either, so I'm not sure why you set followups to clc++.
>
> The entire article, in fact (his, not yours), was a classic example of the
> kind of thing you get in the Hackitt and Scarper school of programming.[/color]

Thats great! Thanks! Its always good to get positive feedback!

regards
Andy Little

[Roland Pibinger]

Re: C Style Strings

On 1 Jun 2006 01:29:33 -0700, "kwikius"
<andy@servocomm .freeserve.co.u k> wrote:[color=blue]
>Roland Pibinger wrote:
>[color=green]
>> First you need to find out if you 'own' the returned string, i.e. if
>> you must free (maybe delete) the returned char*. Good C libraries
>> usually don't require the user to call free.[/color]
>
>Out of interest ... What is a good C library resource management
>strategy? ... for example (say) modifying the following example
>
>char* concat(const char * str1, const char* str2)[/color]

This is almost the declaration of strcat, but strcat operates on the
given buffer and doesn't allocate anything. AFAIK, no Standard C
library functions returns anything to be freed.
Your example could be rewritten as:

char* concat (const char * str1, const char* str2, char* result,
size_t resultLen);

The function returns NULL when the 'contract' (string concatenation)
cannot be fulfilled, e.g. because the result-buffer is to small.
resultLen is for additional safety.

Best wishes,
Roland Pibinger

[kwikius]

Re: C Style Strings

Roland Pibinger wrote:[color=blue]
> On 1 Jun 2006 01:29:33 -0700, "kwikius"
> <andy@servocomm .freeserve.co.u k> wrote:[/color]

[...]
[color=blue][color=green]
> >Out of interest ... What is a good C library resource management
> >strategy? ... for example (say) modifying the following example
> >
> >char* concat(const char * str1, const char* str2)[/color]
>
> This is almost the declaration of strcat, but strcat operates on the
> given buffer and doesn't allocate anything. AFAIK, no Standard C
> library functions returns anything to be freed.
> Your example could be rewritten as:
>
> char* concat (const char * str1, const char* str2, char* result,
> size_t resultLen);
>
> The function returns NULL when the 'contract' (string concatenation)
> cannot be fulfilled, e.g. because the result-buffer is to small.
> resultLen is for additional safety.[/color]

Thanks for the answer. So the answer to the question of a library
resource management strategy in C is that there isnt one. Everything to
do with resource managment should be done manually by the user, whereas
in C++ it is usual to automate resource management thus dramatically
simplifying life for the user. That would seem to be a clear win for
C++ over C!

regards
Andy Little

[Ian Collins]

Re: C Style Strings

kwikius wrote:[color=blue]
> Richard Bos wrote:
>[color=green]
>>"kwikius" <andy@servocomm .freeserve.co.u k> wrote:
>>
>>[color=darkred]
>>>scroopy wrote:
>>>
>>>>How do I append the contents of pString2 to pString? (giving "Blah
>>>>Blah Blah")
>>>
>>>#include <malloc.h>
>>>#include <cstring>[/color]
>>
>>This is not C...
>>
>>[color=darkred]
>>>char* concat(const char * str1, const char* str2)
>>>{
>>> char * result = (char*) malloc(strlen( str1) + strlen (str2) + 1);[/color]
>>
>>...and this is the wrong way to do this in C.[/color]
>
>
> Out of interest what is the right way? BTW...[/color]

He was probably referring to the cast, which isn't used in C due to
implicit conversion from void*

--
Ian Collins.

[Roland Pibinger]

Re: C Style Strings

On 1 Jun 2006 02:20:01 -0700, "kwikius"
<andy@servocomm .freeserve.co.u k> wrote:[color=blue]
>So the answer to the question of a library
>resource management strategy in C is that there isnt one.[/color]

Let the user provide the necessary resources. Actually, IMO, that's a
very good resource management strategy. In C and C++.
[color=blue]
>Everything to
>do with resource managment should be done manually by the user, whereas
>in C++ it is usual to automate resource management thus dramatically
>simplifying life for the user. That would seem to be a clear win for
>C++ over C![/color]

Yes, the C++ advantage stems from RAII.

Best wishes,
Roland Pibinger

[Roland Pibinger]

Re: C Style Strings

On 1 Jun 2006 01:57:05 -0700, "kwikius"
<andy@servocomm .freeserve.co.u k> wrote:[color=blue]
>Richard Heathfield wrote:[color=green]
>> The entire article, in fact (his, not yours), was a classic example of the
>> kind of thing you get in the Hackitt and Scarper school of programming.[/color]
>
>Thats great! Thanks! Its always good to get positive feedback![/color]

BTW, an interesting C-string library can be found here:

http://synesis.com.au/software/cstring/index.html

Best wishes,
Roland Pibinger