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]

#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

 

[Martin Steen]

#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]

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

 

[Daniel T.]

#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;
}

 

[howa]

Papastefanos Serafeim 寫é“：

#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.]

#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.

 

[loufoque]

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]

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...

 

[Jim Langston]

#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.]

#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.