I'm trying to write some things to a text file but it won't even create the file. I will appreciate a lot if you can help me with this. Thanks.
#include <fstream>
#include <iostream>
int main(){
std::ofstream file;
file.open("path/to/file");
//write something to file
file << "test";
//printing to screen
std::cout << file.rdbuf();
//closing file
file.close();
return 0;
}
The following line is your culprit:
std::cout << file.rdbuf();
You cannot use rdbuf to output a file that was opened for write operations only.
Remove that line and your file will be written correctly.
If you want to read your file after you've finished writing to it:
Solution 1:
Open file for both read and write operations using fstream:
std::fstream file("path/to/file", std::ios::in | std::ios::out);
// ... write to file
file.seekg(0); // set read position to beginning of file
std::cout << file.rdbuf();
Solution 2:
Create a new std::ifstream to read from file:
// ... write to file
file.close(); // call `close` to make sure all input is written to file
std::ifstream inputFile("path/to/file");
std::cout << inputFile.rdbuf();
salesFile.open("C:\\Users\\Tebsan\\Desktop\\Coding\\c++\\re\\salesFile.txt"); // ...try to open existing file
if( !salesFile.is_open() ) // ...else, create new file...
salesFile.open("C:\\Users\\Tebsan\\Desktop\\Coding\\c++\\re\\salesFile.txt", ios_base::in | ios_base::out | ios_base::trunc);
You have to call fstream::open with an explicit openmode argument of
ios_base::in | ios_base::out | ios_base::trunc
Otherwise open will fail due to ENOENT.
Thanks for the help!!!
I Learned new things from this. The funny thing is that the issue was the name of the file, apparently the name was too long, and for the new file that included file stream I just added stream at the end of the name, so the compiler kept running the first file (without file stream)...
Related
I was trying to create a file in both read and write modes, but it doesn't create the file, what can be the problem?
This is code:
fstream file("NameFile.txt", ios::out| ios::in);
The program will start, but it will not create any files.
When you open the file using fstream:
To read, the file is required to exist;
To write you need to specify a write mode, ofstream would do that for you, but with fstream you need to do it yourself:
Replaces the contents of the file when you write (ofstream default mode).
std::fstream file("NameFile.txt", std::ios::out | std::ios::in | std::ios::trunc);
^^^^^^^^^^^^^^^
Appends to the existing data in the file when you write.
std::fstream file("NameFile.txt", std::ios::out | std::ios::in | std::ios::app);
^^^^^^^^^^^^^
Note that after reading or writing you'll need to set the offset position in the file, for instance:
std::string s = "my string";
std::string in;
file << s;
file >> in;
file >> in will not read anything, the position indicator is at the end of the file after file << s, you'll need to reset it if you want to read previously written data, for example:
file << s;
file.seekg(0);
file >> in;
This resets the read position indicator to the beggining of the file, before the file is read from, more about it here:
https://en.cppreference.com/w/cpp/io/basic_fstream
well, you initialized an object, now to create a file use
file.open();
see
fstream won't create a file
I am trying to find a way using which I can Edit the contents in a binary file, without reading the whole file.
Suppose this is my file
abdde
And I want to make it
abcde
I tried following:-
Attempt 1)
ofstream f("binfile", ios::binary);
if(f.is_open()){
char d[]={'c'};
f.seekp(2,ios::beg);
f.write(d, 1);
f.close();
}
//the file get erased
Output:
**c
Attempt 2)
ofstream f("binfile", ios::binary | ios::app);
if(f.is_open()){
char d[]={'c'};
f.seekp(2,ios::beg);
f.write(d, 1);
f.close();
}
//the file simple gets append seekp() does nothing
Output:
abddec
Attempt 3)
ofstream f("binfile", ios::binary | ios::app);
if(f.is_open()){
char d[]={'c'};
f.seekp(2);
f.write(d, 1);
f.close();
}
//same as before the file simple gets append seekp() does nothing
Output:
abddec
And if I just try to replace the 1st byte of the file, which is 'a' with 'h'
ofstream f("binfile", ios::binary);
if(f.is_open()){
char d[]={'c'};
f.seekp(ios::beg);
f.write(d, 1);
f.close();
}
//file is erased
Output:
h
What do I do? Is it even possible for the OS to allow a program to edit a file at any point own its own?
std::ios::app means the cursor is put at the end of the file before every write. Seeking has no effect.
Meanwhile, std::ios::binary goes into "truncation" mode by default for an output stream.
You want neither of those.
I suggest std::ios::out | std::ios::in, perhaps by just creating a std::fstream fs(path, std::ios::binary) rather than using an std::ofstream.
Yes, it's a bit confusing.
(Reference)
Can anyone tell me what is wrong with this code? I always get not open.
#include <iostream>
#include <fstream>
using namespace std;
int main(){
fstream fs;
fs.open("fsfile2",ios::in|ios::out|ios::binary);
if(fs.is_open()){
fs.write("wow",sizeof("wow"));
char str[20];
fs.read((char*)str,sizeof(str));
cout<<str<<endl;}
else
cout<<"Not open\n";
return 0;
}
Try this code
fs.open("fsfile2", ios::app|ios::in|ios::out|ios::binary);
By using the open() like you are that file will not be created if that is your goal.
If you want to create a new file please look at: fstream won't create a file
If the file exists, you are not looking for it in the right path. Or change the file name to the full path or put the executable in the folder where the file is.
Hope this helps.
Probably, you do not have permissions to create files in the directory, where your executable is.
Solution:
Please add a file extension to the filename.
If it's a text file, it will be
"fsfile2.txt"
Then, I tried removing
ios::in
since the first process only writes to file, and by removing that, the file is created and "wow" is also written at it.
In order for these lines
fs.read((char*)str,sizeof(str));
cout<<str<<endl;
to work,
You need to close the stream after writing to it, then open the stream in read mode, then read the contents. Take note that closing the stream will save the edited file.
Additional:
You can also change
fs.write("wow",sizeof("wow"));
to
fs << "wow";
You can do the same when reading from file,
fs >> str;
You can also use the string class of C++, instead of char array so that the number of characters inside the file won't be your problem anymore.
#include <string>
string str;
Checking for EOF (end-of-file) is recommended since files are read line by line. Once you add a new line and add a character to the line, the code that doesn't loop until EOF will only read the first line of the file.
In order to solve this, it is recommended to loop until EOF is reached.
while(!fs.eof()) {
fs >> str;
cout << str << endl;
}
So here is the improved snippet:
#include <string>
fs.open("fsfile2.txt", ios::out); // ios::out for write only
if(fs.is_open()) {
// writes "wow" to file
fs << "wow";
// closes the file
fs.close();
// ios::in for read only
fs.open("fsfile2.txt", ios::in);
// better to define variable just before using it
string str;
// loops until end-of-file
while(!fs.eof()) {
// reads a line from file, stores it to str
fs >> str;
// shows str to screen
cout << str << endl;
}
}
*Note: I removed
ios::binary
Since your code is not dealing with binary files yet.
I tried these and it worked fine! Have a nice day!
fstream fs; does not create a new file for you.
You need to make sure that the file exists in your project directory.
On the other hand, if you were to use ofstream fs("file.txt"); it would create the file for you. Or use only ios::out when you open fstream fs, this will create the file for you.
When saving something to a text file is there a way to not override whats in the file already?
EX.
blah.txt
this is an example
fout << "of saving to a file.";
i want the "of saving to a file" to be added on to "this is an example" not to override it.
use the append flag, like this:
fstream fout("blah.txt", ios::app);
if you are opening it after declaration, use something like this:
fout.open("blash.txt", fstream::app);
You should open the file with the appropriate write mode in order to append to it instead of overwriting:
pFile = fopen ("myfile.txt","a");
(C style), or
fstream filestr ("myfile.txt", fstream::ate | fstream::out);
(C++ style). In the latter case, you may want to use fstream::app instead of fstream::ate - this sets the file pointer to the end of the file before each output operation.
Yes. You need to open the file in append mode.
ofstream fout("blah.txt", fstream::out | fstream::app);
You need to open the file output stream in append mode:
ofstream fout(<path>, ios::app);
You can manually set the write position if you want to
u need:
#include <stdio.h>
...
FILE *file = fopen("my_file.txt", "w");
fputs ("This is some text." , file);
fseek (file , 15, SEEK_SET);
fputs (" sam" ,file);
fclose (file);
Is it possible to open an fstream on a file that does not exist with both ios::in & ios::out without getting an error?
To open an fstream on a file that does not exist for input and output (random access) without getting an error, you should provide the flags fstream::in | fstream::out | fstream::trunc in the open (or constructor) call. Since the file does not already exist, truncating the file at zero bytes is no drama.
You may want an error when opening a file that doesn't exist when specifying only ios::in since you'll never be able to read from the stream so failing early in this case will prevent surprise failures later on.
The answer to your question unfortunately is: "No", this is not possible in a single C++ statement.
Yes, many people will answer, that you can use the combined flags fstream::in | fstream::out | fstream::trunc. But that answer is nonsense. fstream::trunc means, that the output file will be truncated to size zero upon opening. But then, why would you like to open an empty file for reading and writing? Except for the rare case, that you need a file as a temporary store for some of your application's data, that you will first write and later read back, there is no use for this flag combination.
Some people recommend to first try to open with fstream::in | fstream::out (and possible further flags like fstream:app or fstream::binary as needed) and then check the file's error status: If the file could not be opened, then re-try the open operation including | fstream::trunc. This solution has several caveats, however. For example, if your file system is mounted via NFS, the first attempt to open the file in read/write-mode might fail due to temporary network issues. If the second attempt (the one including the fstream::trunc flag) then succeeds, there goes your wounderful data, that you have collected so far.
The safe solution is to first open the file for appending only (which will create the file, if it doesn't exist, but will not truncate it) and then close it immediately and open it a second time in read-write mode. This can be achieved with the following code: Note, that an ofstream is first constructed and then immediately discarded.
std::string filename { "test.txt" };
(void) std::ofstream(filename, std::ostream::app);
std::fstream file(filename);
Alternatively, if you need further flags, like binary, use:
std::string filename { "test.txt" };
(void) std::ofstream(filename, std::ofstream::app | std::fstream::binary);
std::fstream file(filename, std::fstream::in | std::fstream::out | std::fstream::binary);
I hope, that in C++25 (or whichever standard is next), they finally add a flag std::fstream::create to create non-existant output files, if read-write-mode is requested.
#include <fstream>
ofstream out("test", ios::out);
if(!out)
{
cout << "Error opening file.\n";
return 1;
}
ifstream in("test", ios::in);
if(!in)
{
cout << "Error opening file.\n";
return 1;
}
If an error occurs the message is displayed and one (1) is returned. However it is possible to compile and execute just ofstream out("test", ios::out); and ifstream in("test", ios::in); without any errors. Either way the file test is created.
#include <iostream>
#include <fstream>
using namespace std;
int main () {
fstream f("test.txt", fstream::in | fstream::out);
cout << f.fail() << endl;
f << "hello" << endl;
f.close();
return 0;
}
This code will print 1 and will not create "test.txt" file, if it does not exit. So it is not possible to open and fstream on a file that does not exist without getting an error.
std::fstream f("test.txt", std::ios_base::out);
f.close(); //file now exists always
f.open("test.txt", fstream::in | std::ios_base::out);
//f is open for read and write without error
I haven't checked to guarantee that it will open without error, but I feel pretty confident that it should.