For example, I used std:: ios:: in to construct a fstream. Can I get the file mode of the fstream later;
I use flags () to get flags and use "&" to get them, but I can't get them. I can't find a clear answer in the search process.
There's no way to access the open mode through the class. You can manually store the open mode to get it later though:
static int mode_alloc = std::ios_base::xalloc();
auto mode = std::ios_base::in | std::ios_base::out;
std::fstream stream;
stream.iword(mode_alloc) = mode;
// ...
stream.iword(mode_alloc); // the value of mode
Related
I am trying to implement a file handle class similar to the one in Bjarne Stroustrup's FAQ page. (Scroll to "Why doesn't C++ provide a 'finally' construct".) Unlike his example, however, I want to use C++ file streams instead of a FILE*.
Right now, I am considering creating a FileHandleBase class, or something similarly named, and two derived classes—one for input files and one for output files. Below is the implementation I wrote as a proof-of-concept; keep in mind that it is very simple and unfinished.
class FileHandle {
public:
FileHandle(const char* fn, ios_base::openmode mode = ios_base::in | ios_base::out) {
file.open(fn, mode);
// Check to make sure file is open
}
FileHandle(const string &fn, ios_base::openmode mode = ios_base::in | ios_base::out) {
file.open(fn, mode);
// Check to make sure file is open
}
~FileHandle() {
file.close();
}
private:
fstream file;
};
I would like to know if this is a viable way of making a file handle, that is, whether my inheritance idea is good. I also want to know the best way to deal with the ios_base::openmode parameter because the C++ reference page for std::ifstream says this:
Note that even though ifstream is an input stream, its internal filebuf object may be set to also support output operations.
In what cases would an ifstream be used for output operations, and, similarly, when would an of stream be used for input operations; and should I restrict the options for the ios_base::openmode parameter for my file handle class(es)? That way my input file handle would only handle input operations, and the output version would only handle output operations.
In what cases would an ifstream be used for output operations, and, similarly, when would an ofstream be used for input operations
You would open an output file stream with an std::ios_base::in openmode and vice-versa for an input file stream if you would still like to perform those associated operations using the internal std::filebuf object, which is accessible by stream->rdbuf(). Note that the streams std::ofstream and std::ifstream will still be able to perform output and input respectively even if they are opened with opposite openmodes.
int main() {
std::ofstream stream("test.txt");
stream << "Hello" << std::flush;
stream.close();
stream.open("test.txt", std::ios_base::in);
char buffer[SIZE] = {};
stream.rdbuf()->sgetn(buffer, SIZE);
std::cout << buffer << std::endl;
}
I have something like that :
istream ifs("/path/to/my/file.ppm", ios::binary);
So now, for checking the extension file, It's necessary to get the name of the file.
I'm using my own function read :
... readPPM(std::istream& is) {}
It's is possible to get the /path/to/my/file.ppm in a string from the istream& variable ?
You almost certainly actually used
std::ifstream ifs(...);
// ^
However, even so the stream doesn't retain the name used to open it: there is rarely a need to doing so and it would be a wasted resource for most applications. That is, if you need the name later, you'll need to retain it. Also, not all streams have a name. For example, an std::istringstream doesn't have a name.
If you can't pass the stream's name separate from the stream, you can attach the name, e.g., using the pword() member:
int name_index() {
static int rc = std::ios_base::xalloc(); // get an index to be used for the name
return rc;
}
// ...
std::string name("/path/to/my/file.ppm");
std::ifstream ifs(name, ios::binary);
ifs.pword(name_index()) = const_cast<char*>(name.c_str());
// ...
char const* stream_name = static_cast<char*>(ifs.pword(name_index()));
The stream won't maintain the pointer in any shape or form, i.e., with the above setup the name needs to outlive the ifs object. If necessary the objects stored with pword() can be maintained using the various callbacks but doing so is non-trivial.
For the following code:
fstream file("file.txt", ios::in):
//some code
//"file" changes here
file.close();
file.clear();
file.open("file.txt", ios::out | ios::trunc);
how can the last three lines be changed so that the current file is not closed, but "re-opened" with everything blanked out?
If I am understanding the question correctly, you'd like to clear all contents of the file without closing it (i.e. set the file size to 0 by setting EOF position). From what I can find the solution you have presented is the most appealing.
Your other option would be to use an OS-specific function to set the end of file, for example SetEndOfFile() on Windows or truncate() on POSIX.
If you're only looking to begin writing at the beginning of the file, Simon's solution works. Using this without setting end of file may leave you in a situation where you have garbage data past the last position you wrote though.
You can rewind the file: put back the put pointer to the beginning of the file, so next time you write something, it will overwrite the content of the file.
For this you can use seekp like this:
fstream file("file.txt", ios::in | ios::out); // Note that you now need
// to open the file for writing
//some code
//"something" changes here
file.seekp(0); // file is now rewinded
Note that it doesn't erase any content. Only if you overwrite it so be careful.
I'm guessing you're trying to avoid passing around the "file.txt" parameter and are trying to implement something like
void rewrite( std::ofstream & f )
{
f.close();
f.clear();
f.open(...); // Reopen the file, but we dont know its filename!
}
However ofstream doesn't provide the filename for the underlying stream, and doesn't provide a way to clear the existing data, so you're kind of out of luck. (It does provide seekp, which will let you position the write cursor back to the beginning of the file, but that wont truncate existing content...)
I'd either just pass the filename to the functions that need it
void rewrite( std::ostream & f, const std::string & filename )
{
f.close();
f.clear();
f.open( filename.c_str(), ios::out );
}
Or package the filestream and filename into a class.
class ReopenableStream
{
public:
std::string filename;
std::ofstream f;
void reopen()
{
f.close();
f.clear();
f.open( filename.c_str(), ios::out );
}
...
};
If you're feeling over zealous you could make ReopenableStream actually behave like a stream, so that you could write reopenable_stream<<foo; rather than reopenable_stream.f<<foo but IMO that seems like overkill.
The following code will print something to a file
std::fstream fout ("D_addr.txt", std::fstream::app);
fout << pkt->Addr() << std::endl;
flush(fout);
fout.close();
While debugging, I watched pkt->Addr() and it has some values. The fout line is passed without problem. Also the file D_addr.txt is created. However after closing the file, the file size is zero! nothing has been written to it.
Where is the problem?
This is not your actual code I guess and if it is I would start with that Addr() function of yours.
Note that fstream::close "closes the file currently associated with the object, disassociating it from the stream. Any pending output sequence is written to the physical file." flush(fout); can be omitted.
You should also specify std::fstream::out flag. "If the function is called with any value in that parameter the default mode is overridden, not combined." So instead of std::fstream::app you should pass std::fstream::app | std::fstream::out.
I'm wondering if you're not using the wrong class. If you want to write to a file, use std::ofstream, and not std::fstream. In particular, the constructor of std::ofstream forces the ios_base::out bit when calling rdbuf()->open; the constructor of std::fstream doesn't (so you're opening the file with neither read nor write access).
And you probably want to check the error status: did the open succeed, and after the close (or the flush), did all of the writes succeed. The usual way of doing this is just:
if ( fout ) {
// All OK...
}
if ( !fout ) {
// Something went wrong.
}
After the open (the constructor), you can use fout.is_open(), which has the advantage of being a little bit more explicit with regards to what you are checking for.
First of all, flush() and fout.close() do not make any harm, but are not needed here, because when fout gets destroyed the file will be closed (and flushed) as part of fstream destructor.
Second, you should use an ofstream or alternatively add the flag std::ios::out to the openmode parameter.
Try something along the lines of:
{
uint64_t x = 42;
std::fstream of("test.txt", std::ios::app);
of << x << std::endl;
}
I'm using a FileManager for a project so that reading and writing is less of a hassle for me. Or would be, if I didn't spend all this time debugging it. So, this comfort-class actually caused me stress and time. Awesome.
The problem seems to be the fstream. Before I continue further, here is the structure of my FileManager class.
class FileManager : Utility::Uncopyable
{
public:
FileManager();
void open(std::string const& filename);
void close();
void read(std::string& buffer);
void write(std::string const& data);
private:
std::fstream stream_;
};
Very simple. The buffer is loaded with data during the read function, the data parameter is what's to be written to file. Before reading and writing you must open the file or risk getting a big, fat exception in your face. Kind of like the one I'm getting now.
Scenario: Simple command-line registering of a user, then writing the data to file. I ask for a name and password. The name is copied and appended with .txt (the filename). So it looks like this:
void SessionManager::writeToFile(std::string const& name,
std::string const& password)
{
std::string filename = name + ".txt";
std::string data;
data += name +", " +password;
try
{
fileManager_->open(filename);
fileManager_->write(data);
fileManager_->close();
}
catch(FileException& exception)
{
/* Clean it up. */
std::cerr << exception.what() << "\n";
throw;
}
}
Problem: the open fails. The file is never created, and during the write I get an exception for not having an open file.
FileManager::open() function:
void FileManager::open(std::string const& filename)
{
if(stream_.is_open())
stream_.close();
stream_.open(filename.c_str());
}
and write
void FileManager::write(std::string const& data)
{
if(stream_.is_open())
stream_ << data;
else
throw FileException("Error. No file opened.\n");
}
However, if I create the file beforehand, then it has no troubles opening the file. Yet, when I check, the default std::ios::openmode is std::ios::in | std::ios::out. I can create the file just fine when I only tag std::ios::out, but I want to keep the stream in a read/write state.
How can I accomplish this?
Best method:
void FileManager::open(std::string const& filename)
{
using std::ios_base;
if( stream_.is_open() )
stream_.close();
stream_.open( filename.c_str() ); // ...try existing file
if( !stream_.is_open() ) // ...else, create new file...
stream_.open(filename.c_str(), ios_base::in | ios_base::out | ios_base::trunc);
}
So the code tests for an existing file and, if not, creates it.
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.
Table 95 of the draft C++ standard lists possible file open modes and their equivalent in stdio. The default, ios_base::out | ios_base::in is r+. The one I listed above is equivalent to w+. ios_base::out | ios_base::app is equivalent to a. All other combinations involving ios_base::app are invalid.
(At the risk of being scoffed at: you could switch to stdio and use the file mode a+, which reads from the start and appends at the end.)
You cannot use std::ios::in on a non-existing file. Use
std::ios::in | std::ios::out | std::ios::trunc
instead (but make sure it doesn't exist or it will be truncated to zero bytes).
How can I accomplish this?
std::ofstream file("file.txt");
file << data;
Isn't that simpler?
That's the way the library works: std::ios::in | std::ios::out opens it in the equivalent of stdio's "r+", that is it will only open an existing file. I don't believe there's a mode that will do what you are wanting, you'll have to us an OS-specific call or check the file for existence (again, using an OS-specific call) and open it in one mode or the other depending on whether it already exists.
Edit: I assumed that you didn't want the file truncated if it already exists. As other people have noted, if you're happy to truncate any existing file then in | out | trunc is an option.
Just get your function to open
void FileManager::open(std::string const& filename)
{
using std::ios_base;
if(stream_.is_open())
stream_.close();
stream_.open(filename.c_str(), ios_base::in | ios_base::out | ios_base::trunc);
}
if that is the mode you require.
There is no magic way to open a file for read/write creating it if it does not exist but not truncating it (removing its content) if it does. You have to do that in multiple steps.