Return C-style string from function?

[howa]

#include <iostream>

using namespace std;

const char* test(char *src) {

string s(src);

return s.c_str();
}

int main() {

char src[] = "test123";

cout<<test(src) ;

return 0;
};

how to get the "test123" from the function?

thanks.

[Papastefanos Serafeim]

Re: Return C-style string from function?


"howa" <howachen@gmail .comwrote in message
news:1161613411 .186962.87780@b 28g2000cwb.goog legroups.com...

#include <iostream>
>
using namespace std;
>
const char* test(char *src) {
>
string s(src);
>
return s.c_str();
}
>
int main() {
>
char src[] = "test123";
>
cout<<test(src) ;
>
return 0;
};
>
how to get the "test123" from the function?
>
thanks.
>

When the function finishes the string s goes out of scope,
and, since its in the stack, it becomes garbage.

A quick fix is to change the line
return s.c_str();
to
return strdup(s.c_str( ) );

Serafeim

[Daniel T.]

Re: Return C-style string from function?

In article <1161613411.186 962.87780@b28g2 000cwb.googlegr oups.com>,
"howa" <howachen@gmail .comwrote:

#include <iostream>
>
using namespace std;
>
const char* test(char *src) {
>
string s(src);
>
return s.c_str();
}
>
int main() {
>
char src[] = "test123";
>
cout<<test(src) ;
>
return 0;
};
>
how to get the "test123" from the function?
>
thanks.

const char* test( char* src ) {
return src;
}

--
To send me email, put "sheltie" in the subject.

[howa]

Re: Return C-style string from function?


Papastefanos Serafeim 寫道：

"howa" <howachen@gmail .comwrote in message
news:1161613411 .186962.87780@b 28g2000cwb.goog legroups.com...

#include <iostream>

using namespace std;

const char* test(char *src) {

string s(src);

return s.c_str();
}

int main() {

char src[] = "test123";

cout<<test(src) ;

return 0;
};

how to get the "test123" from the function?

thanks.

>
When the function finishes the string s goes out of scope,
and, since its in the stack, it becomes garbage.
>
A quick fix is to change the line
return s.c_str();
to
return strdup(s.c_str( ) );
>
Serafeim

Thanks...

is this a usual/normal way to pass a string out of a function?

[Daniel T.]

Re: Return C-style string from function?

"howa" <howachen@gmail .comwrote:

Papastefanos Serafeim wrote:

>"howa" <howachen@gmail .comwrote:

>>#include <iostream>
>>>
>>using namespace std;
>>>
>>const char* test(char *src) {
>> string s(src);
>> return s.c str();
>>}
>>>
>>int main() {
>> char src[] = "test123";
>> cout<<test(src) ;
>> return 0;
>>}
>>>
>>how to get the "test123" from the function?
>>>
>>thanks.
>>>

>>
>When the function finishes the string s goes out of scope,
>and, since its in the stack, it becomes garbage.
>>
>A quick fix is to change the line
>return s.c str();
>to
>return strdup(s.c str() );
>>
>Serafeim

>
Thanks...
>
is this a usual/normal way to pass a string out of a function?

No. strdup isn't even defined in the language. The usual way to do it is
by returning a string that was passed in. See: strcpy for an example.

--
To send me email, put "sheltie" in the subject.

[loufoque]

Re: Return C-style string from function?

howa wrote:

is this a usual/normal way to pass a string out of a function?

No, this is just a way to stop using RAII, hence creating non
exception-safe code and a memory leak in various situations.
This also has many other obvious problems related to pointers.

[Frederick Gotham]

Re: Return C-style string from function?

howa posted:

#include <iostream>
>
using namespace std;
>
const char* test(char *src) {
>
string s(src);
>
return s.c_str();
}
>
int main() {
>
char src[] = "test123";
>
cout<<test(src) ;
>
return 0;
};
>
how to get the "test123" from the function?

The memory which the string occupies must still be accessible after the
funciton returns. Therefore, either:

(1) Have the calling function allocate the memory, and pass a pointer so
that the called function will alter the memory.

(2) Have the called function allocate the memory dynamically, and return a
pointer which the calling function shall have to delete.

(3) Use global/static data...

--

Frederick Gotham

[Martin Steen]

Re: Return C-style string from function?

howa wrote:

#include <iostream>
>
using namespace std;
>
const char* test(char *src) {
>
string s(src);
>
return s.c_str();
}
>
int main() {
>
char src[] = "test123";
>
cout<<test(src) ;
>
return 0;
};
>
how to get the "test123" from the function?
>
thanks.

const char* test(char *src)
{
return src;
}

OR

const char* test(char *src)
{
static string s(src);
return s.c_str();
}

OR

const char* test(char *src)
{
string* s = new string;
*s = src;
return s->c_str();
}

Best regards,
-Martin

[Martin Steen]

Re: Return C-style string from function?

const char* test(char *src)

{
string* s = new string;
*s = src;
return s->c_str();
}

I forgot to tell you: that's not good,
because I create a memory leak here.
String s cannot be deleted anywhere.

-Martin

[Jim Langston]

Re: Return C-style string from function?

"howa" <howachen@gmail .comwrote in message
news:1161613411 .186962.87780@b 28g2000cwb.goog legroups.com...

#include <iostream>
>
using namespace std;
>
const char* test(char *src) {
>
string s(src);
>
return s.c_str();
}
>
int main() {
>
char src[] = "test123";
>
cout<<test(src) ;
>
return 0;
};
>
how to get the "test123" from the function?
>
thanks.

You're going to have problems no matter what method you decide to return a
c-style string. The best solution is to return a std::string. That's why
std::strings were invented, to get rid of this problem. What can returning
a c-style string get you that returning a std::string can't?

[Daniel T.]

Re: Return C-style string from function?

In article <453cd2a7$1_2@n ews.arcor-ip.de>,
Martin Steen <a0.spam001.10. msteen@spamgour met.comwrote:

howa wrote:

#include <iostream>

using namespace std;

const char* test(char *src) {

string s(src);

return s.c_str();
}

int main() {

char src[] = "test123";

cout<<test(src) ;

return 0;
};

how to get the "test123" from the function?

thanks.

>
const char* test(char *src)
{
return src;
}
>
OR
>
const char* test(char *src)
{
static string s(src);
return s.c_str();
}
>
OR
>
const char* test(char *src)
{
string* s = new string;
*s = src;
return s->c_str();
}

Note, that last one leaks memory every time it is called.

--
To send me email, put "sheltie" in the subject.