file input problem

[pentiumPunk]

string fileName;

cout<<"Enter filename of datafile you wish to open \n";
cin>>fileName;

ifstream fin;

fin.open(fileName, ifstream::ios);
if(!fin)
{
cerr<<"blah blah blah";
exit(1);
}
i get this error message in VC++ .....Error C2664...on the
fin.open(fileName, ifstream::ios); line...cannot convert parameter 1 from
std::string to const char *
i cannot put a certain filename down such as fin.open("textfile.txt"), i
need the user to enter its name, for it will change.

Thanks alot in ahead!

[John Harrison]

"pentiumPunk" <Br***@triad.rr.com> wrote in message
news:wp*********************@twister.southeast.rr. com...

string fileName;

cout<<"Enter filename of datafile you wish to open \n";
cin>>fileName;

ifstream fin;

fin.open(fileName, ifstream::ios);
if(!fin)
{
cerr<<"blah blah blah";
exit(1);
}
i get this error message in VC++ .....Error C2664...on the
fin.open(fileName, ifstream::ios); line...cannot convert parameter 1 from
std::string to const char *
i cannot put a certain filename down such as fin.open("textfile.txt"), i
need the user to enter its name, for it will change.

Thanks alot in ahead!

fin.open(fileName.c_str(), ifstream::ios);

As the error message says, open requires a const char*, c_str returns a
const char* from a string.

Its just one of the quirks of the standard library.

john

[Sandeep]

"John Harrison" <jo*************@hotmail.com> wrote in message news:<bj************@ID-196037.news.uni-berlin.de>...

"pentiumPunk" <Br***@triad.rr.com> wrote in message
news:wp*********************@twister.southeast.rr. com...

string fileName;

cout<<"Enter filename of datafile you wish to open \n";
cin>>fileName;

ifstream fin;

fin.open(fileName, ifstream::ios);
if(!fin)
{
cerr<<"blah blah blah";
exit(1);
}
i get this error message in VC++ .....Error C2664...on the
fin.open(fileName, ifstream::ios); line...cannot convert parameter 1 from
std::string to const char *
i cannot put a certain filename down such as fin.open("textfile.txt"), i
need the user to enter its name, for it will change.

Thanks alot in ahead!

When we declare any string variable it is not null terminated ie not
appended with '\0' .Since in the above program u r using fstream which
requires filename to be null terminated so we can make it null
terminated by using c_str() which convert it to null terminated string
..ie use
fin.open(filename.c_str(),ifstream::ios) instead of fin.open(fileName,
ifstream::ios)

Enjoy c++ programming........

[Thomas Matthews]

pentiumPunk wrote:

string fileName;

cout<<"Enter filename of datafile you wish to open \n";
cin>>fileName;

ifstream fin;

fin.open(fileName, ifstream::ios);
if(!fin)
{
cerr<<"blah blah blah";
exit(1);
}
i get this error message in VC++ .....Error C2664...on the
fin.open(fileName, ifstream::ios); line...cannot convert parameter 1 from
std::string to const char *
i cannot put a certain filename down such as fin.open("textfile.txt"), i
need the user to enter its name, for it will change.

Thanks alot in ahead!

1. As others have stated:
fin.open(filename.c_str());
or
ifstream fin(filename.c_str());

2. What kind of file mode is "ifstream::ios"?
Try a combination of: "ios_base::in", "ios_base::out",
or "ios_base::binary". The default mode for ifstream
is "ios_base::in" as a text file.

3. Don't use the exit() function. It has some nasty quirks to it.
Use the return statement instead.

4. Instead of returning a '1' from main() to the operating system,
use the predefined, portable constants EXIT_SUCCESS or
EXIT_FAILURE as defined in <cstdlib>.

5. You may want to change:
cin >> fileName;
to
getline(cin, filename, '\n');
The getline() function will read in the whole line. The other
expression _may_ not read in spaces or other characters.

--
Thomas Matthews

C++ newsgroup welcome message:
http://www.slack.net/~shiva/welcome.txt
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http://www.raos.demon.uk/acllc-c++/faq.html
Other sites:
http://www.josuttis.com -- C++ STL Library book

[Rolf Magnus]

Sandeep wrote:

"John Harrison" <jo*************@hotmail.com> wrote in message
news:<bj************@ID-196037.news.uni-berlin.de>...

"pentiumPunk" <Br***@triad.rr.com> wrote in message
news:wp*********************@twister.southeast.rr. com...

> string fileName;
>
> cout<<"Enter filename of datafile you wish to open \n";
> cin>>fileName;
>
> ifstream fin;
>
> fin.open(fileName, ifstream::ios);
> if(!fin)
> {
> cerr<<"blah blah blah";
> exit(1);
> }
>
>
> i get this error message in VC++ .....Error C2664...on the
> fin.open(fileName, ifstream::ios); line...cannot convert parameter
> 1 from std::string to const char *
> i cannot put a certain filename down such as
> fin.open("textfile.txt"), i need the user to enter its name, for it
> will change.
>
> Thanks alot in ahead!
>

When we declare any string variable it is not null terminated ie not
appended with '\0' .

What do you mean by "any string variable"? If you're talking about
std::string, then it's not defined (and shouldn't matter) how the data
is stored internally. The only thing that matters is that c_str()
returns a null terminated array of the string's content.
An implementation of std::string that works on null terminated
characters internally (and thus doesn't need any extra work for
c_str() would be valid, too.