wrong about fstream file(s.c_str(),std::ios_base::in | std::ios_base::app);

[DaVinci]

//error happen even when there existe a a.txt file
#include<fstream>
#include <string>
#include <iostream>
using std::string;
using std::fstream;
int main()
{
string s = "a.txt";
fstream file(s.c_str(),std::ios_base::in | std::ios_base::app);
if(!file)
{
std::cerr<<"error happen\n";
}
else
{
string readToStr;
file>>readToStr;
file<<"abc";
std::cout<<"successed"<<std::endl;
}
}

what's the problem is ?
thanks very much.

[Bo Persson]

"DaVinci" <ap***********@gmail.com> skrev i meddelandet
news:11*********************@j33g2000cwa.googlegro ups.com...

//error happen even when there existe a a.txt file
#include<fstream>
#include <string>
#include <iostream>
using std::string;
using std::fstream;
int main()
{
string s = "a.txt";
fstream file(s.c_str(),std::ios_base::in | std::ios_base::app);
if(!file)
{
std::cerr<<"error happen\n";
}
else
{
string readToStr;
file>>readToStr;
file<<"abc";
std::cout<<"successed"<<std::endl;
}
}

what's the problem is ?
thanks very much.

In what directory do you expect the file a.txt to be found? How does
the program know that?
Bo Persson

[DaVinci]

Bo Persson wrote:

"DaVinci" <ap***********@gmail.com> skrev i meddelandet
news:11*********************@j33g2000cwa.googlegro ups.com...

//error happen even when there existe a a.txt file
#include<fstream>
#include <string>
#include <iostream>
using std::string;
using std::fstream;
int main()
{
string s = "a.txt";
fstream file(s.c_str(),std::ios_base::in | std::ios_base::app);
if(!file)
{
std::cerr<<"error happen\n";
}
else
{
string readToStr;
file>>readToStr;
file<<"abc";
std::cout<<"successed"<<std::endl;
}
}

what's the problem is ?
thanks very much.

In what directory do you expect the file a.txt to be found? How does
the program know that?

the current working directory.

Bo Persson

[Jacek Dziedzic]

DaVinci wrote:

//error happen even when there existe a a.txt file
#include<fstream>
#include <string>
#include <iostream>
using std::string;
using std::fstream;
int main()
{
string s = "a.txt";
fstream file(s.c_str(),std::ios_base::in | std::ios_base::app);
if(!file)
{
std::cerr<<"error happen\n";
}
else
{
string readToStr;
file>>readToStr;
file<<"abc";
std::cout<<"successed"<<std::endl;
}
}

what's the problem is ?

Are you sure you have the permissions to open this file
for read/write? IOW, the existence of the file does not
automatically imply you will manage to open it successfully.

HTH,
- J.