I am trying to open a file stored on my c drive with name test.txt.I am getting a lot of errors.I am new to filing in C++.Please help me out thanks.
// writing on a text file
#include <iostream>
#include <fstream>
using namespace std;
int main ()
{
ofstream mystream;
mystream.open("C:\\test",ios::in||ios::out);
/*Check if the file is opened properly*/
return 0;
}
This
mystream.open("C:\\test",ios::in || ios::out);
should be
mystream.open("C:\\test",ios::in | ios::out);
You are using the logical OR operator (||) instead of the bitwise OR operator (|). The former returns a boolean value, while the latter returns the bitwise OR of the two values.
You probably also want to fully qualify the filename. For example:
mystream.open("C:\\test.txt", ios::in | ios::out);
Related
I found this code and I don't know what the !ist means.
#include <iostream>
#include <fstream>
using namespace std;
#include <string>
int main()
{
string readname;
cin >> readname;
ifstream ist{ readname };
if (!ist)
{
//insert any text here
}
}
I don't know what the (!ist) is for. I have tried to figure out what this means but i cant.
std::basic_ifstream inherits std::basic_ios<CharT,Traits>::operator bool:
Checks whether the stream has no errors.
Returns true if the stream has no errors and is ready for I/O
operations. Specifically, returns !fail().
So the code is equivalent with (all of the following):
if (!static_cast<bool>(ist))
if (!ist.operator bool())
if (!!ist.fail())
if (ist.fail())
! is the boolean "not" operator, so this tests ist to see if it is not valid -- if it failed to be able to open the specified file and read from it.
I have created a file name c.txt with open() and if I want to make changes in the file then I am going to open with open() then what is the difference?
Could someone please help me out.
#include "stdafx.h"
#include<iostream>
#include<fstream>
using namespace std;
int main()
{
ofstream myfile;
myfile.open("c.txt");
myfile << "gm all";
myfile.close();
}
It's very simple!
ofstream: Output stream.
ifstream: Input stream.
Create the file with ofstream, and open it with ifstream.
The basic syntax I use to write to a .txt file is this:
ofstream myfile;
myfile.open ("data.txt", ios::trunc);
outfile<<"writing";
Now, supposing i were to let the user decide which file should be accessed, could it be done through a string?
#include <iostream>
#include <fstream>
using namespace std;
int main (){
ofstream myfile;
string filename;
char x;
cin>>x;
if (x==1)
filename="data2.txt";
myfile.open (filename, ios::trunc);
myfile<<"writing";
return 0;
}
I already tested this and it didn't work. My question is: is it possible to do such a thing? If yes, how? When i compile it, the error i get is the following:
undefined reference to 'std::basicofstream<char, std::char_traits<char> >::open(std::string const&, std::_Ios_Openmode)'
I can't understand what it is that's causing it.
Your error says it can't find open method that uses std::string.
Try this:
myfile.open(filename.c_str(), ios::trunc);
Some versions of C++ do not allow the std::string for open method, so you will have to use the c_str() method of std::string.
You should add an else branch since if x isn't equal to 1 open has nothing to open.
Also you forgot to declare ofstream myfile; in your second snippet. Maybe that's the reason this doesn't work (it shouldn't even compile).
Ok so I haven't used C++ since highschool (used to work in borland C++)
Now I want to solve a problem in C++, yet I don't understand why fstream doesn't work
For some reason ios::in doesn't work.
#include <fstream>
fstream f("Cities.txt,ios::in);
How do I use Fstream properly?
Thanks in advance!
Note : I'm using Visual Studio 2008
change from
fstream f("Cities.txt,ios::in);
to
std::fstream f("Cities.txt" , std::ios::in);
^^^ ^ ^^^
namespace you miss" namespace
done!
What you have learned in your highschool probably was way before C++ was standardized in '97. As per the standard, all C++ library functions are part of the std namespace. In order to use fstream which is part of the standard namespace, you have to qualify it with std:: so, that makes your syntax as
#include <fstream>
std::fstream f("Cities.txt",std::ios::in);
As an alternative to std::fstream, consider std::ifstream (and std::ofstream):
#include <fstream>
…
std::ifstream f("Cities.txt");
std::ofstream o("output.txt");
std::string s;
while( f >> s )
o << s;
Personally, I find this more convenient than specifying the open mode.
You have to first create an object of ifstream class and then open the file.
Do it this way.
#include <fstream>
std :: ifstream f ("Cities.txt",ios::in) ;
Then check whether it is open and start working with it.
You are also missing the " after file name.
You can also write
#include <fstream>
using namespace std;
fstream f("Cities.txt",ios::in);
The using directive allows you to not write std:: before everything. Beware, it might be bad practice, but in small programs it should not be an issue.
I have really strange problem. In Visual C++ express, I have very simple code, just:
#include <fstream>
using namespace std;
int main()
{
fstream file;
file.open("test.txt");
file<<"Hello";
file.close();
}
This same code works OK in my one project, but when I create now project and use this same lines of code, no file test.txt is created. Please, what is wrong?¨
EDIT: I expect to see test.txt in VS2008/project_name/debug - just like the first functional project does.
Canonical code to write to a file:
#include <fstream>
#include <iostream>
using namespace std;
int main() {
ofstream file;
file.open("test.txt");
if ( ! file.is_open() ) {
cerr << "open error\n";
}
if ( ! ( file << "Hello" ) ) {
cerr << "write error\n";
}
file.close();
}
Whenever you perform file I/O you must test every single operation, with the possible exception of closing a file, which it is not usually possible to recover from.
As for the file being created somewhere else - simply give it a weird name like mxyzptlk.txt and then search for it using Windows explorer.
Perhaps the executable is run in a different directory than it was before, making test.txt appear somewhere else. Try using an absolute path, such as "C:\\Users\\NoName\\Desktop\\test.txt" (The double backslashes are needed as escape characters in C strings).
fstream::open() takes two arguments: filename and mode. Since you are not providing the second, you may wish to check what the default argument in fstream is or provide ios_base::out yourself.
Furthermore, you may wish to check whether the file is open. It is possible that you do not have write permissions in the current working directory (where 'test.txt' will be written since you don't provide an absolute path). fstream provides the is_open() method as one way of checking this.
Lastly, think about indenting your code. While you only have a few lines there, code can soon become difficult to read without proper indentation. Sample code:
#include <fstream>
using namespace std;
int main()
{
fstream file;
file.open("test.txt", ios_base::out);
if (not file.is_open())
{
// Your error-handling code here
}
file << "Hello";
file.close();
}
You can use Process Monitor and filter on file access and your process to determine whether the open/write is succeeding and where on disk it's happening.
Theres two ways to fix this. Either do:
file.open("test.txt", ios::out)
#include <fstream>
using namespace std;
int main()
{
fstream file;
file.open("test.txt", ios::out);
file<<"Hello";
file.close();
}
Or you can create an ofstream instead of fstream.
#include <fstream>
using namespace std;
int main()
{
ofstream file;
file.open("test.txt");
file<<"Hello";
file.close();
}