? Why c_str() ?

[Ericcson]

Hi.
Compilation of this program is OK :
---------------------
#include <string>
#include <fstream>
using namespace std ;
int main() {
ofstream myfile;
string FileName="Sampl e";
myfile.open((Fi leName).c_str() );
myfile.close();
}
--------------------
but this one fails :
#include <string>
#include <fstream>
using namespace std ;
int main() {
ofstream myfile;
string FileName="Sampl e";
myfile.open(Fil eName);
myfile.close();
}
and the error message is:
no matching function for call to `std::basic_ofs tream<char
------------------------
I would like to understand what is c_str() function for.

Isn't my string the "const char * filename" that requires the open function
void open ( const char * filename, openmode mode = out | trunc ) ?

Is there a way to avoid c(str() it and only write "myfile.open(Fi leName);" ?
What does the c_str() function ?

TIA
Erkson

[Peter van Merkerk]

Re: ? Why c_str() ?

"Ericcson" <Ericcson@wanad oo.fr> wrote in message
news:bg2svh$ei6 $1@news-reader5.wanadoo .fr...[color=blue]
> Hi.
> Compilation of this program is OK :
> ---------------------
> #include <string>
> #include <fstream>
> using namespace std ;
> int main() {
> ofstream myfile;
> string FileName="Sampl e";
> myfile.open((Fi leName).c_str() );
> myfile.close();
> }
> --------------------
> but this one fails :
> #include <string>
> #include <fstream>
> using namespace std ;
> int main() {
> ofstream myfile;
> string FileName="Sampl e";
> myfile.open(Fil eName);
> myfile.close();
> }
> and the error message is:
> no matching function for call to `std::basic_ofs tream<char
> ------------------------
> I would like to understand what is c_str() function for.[/color]

It is needed to get a const char* representation of the text stored
inside a std::string class.
[color=blue]
> Isn't my string the "const char * filename" that requires the open[/color]
function[color=blue]
> void open ( const char * filename, openmode mode = out | trunc ) ?[/color]

No 'const char*' is not the same as a 'std::string' class.
[color=blue]
> Is there a way to avoid c(str() it and only write[/color]
"myfile.open(Fi leName);" ?

You would you want to?
[color=blue]
> What does the c_str() function ?[/color]

See above, it is mainly used for interfacing. Don't ask me why
ofstream::open( ) takes a const char* instead of a std::string, I have
really no clue whatsoever. Unfortunately the std::string::c_ str()
function is still needed in quite a few cases. Often GUI libraries have
their own string class with the std::string::c_ str() function often
being the only way to convert between the two.

--
Peter van Merkerk
peter.van.merke rk(at)dse.nl

[Sam Holden]

Re: ? Why c_str() ?

On Mon, 28 Jul 2003 12:12:41 +0200, Ericcson <Ericcson@wanad oo.fr> wrote:[color=blue]
> Hi.
> Compilation of this program is OK :
> ---------------------
> #include <string>
> #include <fstream>
> using namespace std ;
> int main() {
> ofstream myfile;
> string FileName="Sampl e";
> myfile.open((Fi leName).c_str() );
> myfile.close();
> }
> --------------------
> but this one fails :
> #include <string>
> #include <fstream>
> using namespace std ;
> int main() {
> ofstream myfile;
> string FileName="Sampl e";
> myfile.open(Fil eName);
> myfile.close();
> }
> and the error message is:
> no matching function for call to `std::basic_ofs tream<char
> ------------------------
> I would like to understand what is c_str() function for.[/color]

It returns a C style string representation of the data in the std::string
object - ie. a const char*.
[color=blue]
>
> Isn't my string the "const char * filename" that requires the open function > void open ( const char * filename, openmode mode = out | trunc ) ?[/color]

No. It's a std::string.
[color=blue]
>
> Is there a way to avoid c(str() it and only write "myfile.open(Fi leName);" ?[/color]

const char *FileName = "Sample";

If you want to use a std::string then no. This is silly in my opinion,
but the streams predate the strings, I guess anyway.
[color=blue]
> What does the c_str() function ?[/color]

It returns a const char* representation of the std::string object, but
you already asked that, and I already answered it :)

--
Sam Holden

[Charles]

Re: ? Why c_str() ?

On Mon, 28 Jul 2003 12:12:41 +0200, "Ericcson" <Ericcson@wanad oo.fr>
wrote:
[color=blue]
>Hi.
>Compilation of this program is OK :
>---------------------
>#include <string>
>#include <fstream>
>using namespace std ;
>int main() {
> ofstream myfile;
> string FileName="Sampl e";
> myfile.open((Fi leName).c_str() );
> myfile.close();
>}
>--------------------
>but this one fails :
>#include <string>
>#include <fstream>
>using namespace std ;
>int main() {
> ofstream myfile;
> string FileName="Sampl e";
> myfile.open(Fil eName);
> myfile.close();
>}
>and the error message is:
>no matching function for call to `std::basic_ofs tream<char
>------------------------
>I would like to understand what is c_str() function for.[/color]
The member function returns a pointer to a nonmodifiable C string
constructed by adding a terminating null element to the controlled
sequence.
[color=blue]
>
>Isn't my string the "const char * filename" that requires the open function
>void open ( const char * filename, openmode mode = out | trunc ) ?[/color]
no, your string is a <string> class object, not a pointer to a
null-terminated character array
[color=blue]
>
>Is there a way to avoid c(str() it and only write "myfile.open(Fi leName);" ?
>What does the c_str() function ?[/color]

you could...

char filename[] = "Sample";
char* file = filename;

myfile.open(fil e);
myfile.close();

Charles
[color=blue]
>
>TIA
>Erkson
>
>[/color]

[Kevin Goodsell]

Re: ? Why c_str() ?

Dabroz wrote:[color=blue]
> Uzytkownik <Charles> napisal w wiadomosci
> news:12v9iv8rfe 3c3olg5g5gqcg2r h326sdq5f@4ax.c om...[color=green]
>>
>>you could...
>>
>>char filename[] = "Sample";
>>char* file = filename;[/color]
>
>
> Why two pointers should show same memory adress?
>[/color]

filename is not a pointer. It is an array.

-Kevin
--
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To contact me please use the address from a recent posting.