QFile Class

The following example reads a text file line by line:

    QFile file("in.txt");
    if (!file.open(QIODevice::ReadOnly | QIODevice::Text))
        return;

    while (!file.atEnd()) {
        QByteArray line = file.readLine();
        process_line(line);
    }

The QIODevice::Text flag passed to https://() tells Qt to convert Windows-style line terminators ("\r\n") into C++-style terminators ("\n"). By default, https:// assumes binary, i.e. it doesn't perform any conversion on the bytes stored in the file.

The next example uses https:// to read a text file line by line:

    QFile file("in.txt");
    if (!file.open(QIODevice::ReadOnly | QIODevice::Text))
        return;

    QTextStream in(&file);
    while (!in.atEnd()) {
        QString line = in.readLine();
        process_line(line);
    }

https:// takes care of converting the 8-bit data stored on disk into a 16-bit Unicode https://. By default, it assumes that the file is encoded in UTF-8. This can be changed using https://().

To write text, we can use operator<<(), which is overloaded to take a https:// on the left and various data types (including https://) on the right:

    QFile file("out.txt");
    if (!file.open(QIODevice::WriteOnly | QIODevice::Text))
        return;

    QTextStream out(&file);
    out << "The magic number is: " << 49 << "\n";

https:// is similar, in that you can use operator<<() to write data and operator>>() to read it back. See the class documentation for details.

Unlike other https:// implementations, such as https://, https:// does not emit the https://(), https://(), or https://() signals. This implementation detail means that https:// is not suitable for reading and writing certain types of files, such as device files on Unix platforms.

https:// use UTF-16 based QStrings to represent file paths. Standard C++ APIs (<cstdio> or <iostream>) or platform-specific APIs however often need a 8-bit encoded path. You can use https://() and https://() to convert between both representations.

On Unix, there are some special system files (e.g. in /proc) for which https://() will always return 0, yet you may still be able to read more data from such a file; the data is generated in direct response to you calling https://(). In this case, however, you cannot use https://() to determine if there is more data to read (since https://() will return true for a file that claims to have size 0). Instead, you should either call https://(), or call https://() or https://() repeatedly until no more data can be read. The next example uses https:// to read /proc/modules line by line:

    QFile file("/proc/modules");
    if (!file.open(QIODevice::ReadOnly | QIODevice::Text))
        return;

    QTextStream in(&file);
    QString line = in.readLine();
    while (!line.isNull()) {
        process_line(line);
        line = in.readLine();
    }

File permissions are handled differently on Unix-like systems and Windows. In a non https:// directory on Unix-like systems, files cannot be created. This is not always the case on Windows, where, for instance, the 'My Documents' directory usually is not writable, but it is still possible to create files in it.

Qt's understanding of file permissions is limited, which affects especially the https://() function. On Windows, Qt will set only the legacy read-only flag, and that only when none of the Write* flags are passed. Qt does not manipulate access control lists (ACLs), which makes this function mostly useless for NTFS volumes. It may still be of use for USB sticks that use VFAT file systems. POSIX ACLs are not manipulated, either.

On Android, some limitations apply when dealing with https://:

• Access permissions might be needed by prompting the user through the https:// which implements Android's native file picker.

• Aim to follow the https:// guidelines, such as using app specific directories instead of other public external directories. For more information, also see https://.

• Due to the design of Qt APIs (e.g. https://), it's not possible to fully integrate the latter APIs with Android's https:// APIs.

See also https://, https://, https://, https://, https://

Constructs a https:// object.

Constructs a new file object to represent the file with the given name.

Constructs a new file object to represent the file with the given name.

This function was introduced in Qt 6.0.

Constructs a new file object with the given parent.

Constructs a new file object with the given parent to represent the file with the specified name.

Constructs a new file object with the given parent to represent the file with the specified name.

This function was introduced in Qt 6.0.

Destroys the file object, closing it if necessary.

Copies the file named https://() to newName.

This file is closed before it is copied.

If the copied file is a symbolic link (symlink), the file it refers to is copied, not the link itself. With the exception of permissions, which are copied, no other file metadata is copied.

Returns true if successful; otherwise returns false.

Note that if a file with the name newName already exists, copy() returns false. This means https:// will not overwrite it.

See Also▲

This is an overloaded function.

Copies the file named fileName to newName.

This file is closed before it is copied.

If the copied file is a symbolic link (symlink), the file it refers to is copied, not the link itself. With the exception of permissions, which are copied, no other file metadata is copied.

Returns true if successful; otherwise returns false.

Note that if a file with the name newName already exists, https://() returns false. This means https:// will not overwrite it.

See Also▲

This is an overloaded function.

This function was introduced in Qt 6.0.

This does the reverse of https://() using localFileName.

See Also▲

This is an overloaded function.

Returns the Unicode version of the given localFileName. See https://() for details.

Converts fileName to an 8-bit encoding that you can use in native APIs. On Windows, the encoding is the one from active Windows (ANSI) codepage. On other platforms, this is UTF-8, for macOS in decomposed form (NFD).

See Also▲

Returns true if the file specified by fileName exists; otherwise returns false.

This is an overloaded function.

Returns true if the file specified by https://() exists; otherwise returns false.

See Also▲

Reimplements: https://.

Returns the name set by https://() or to the https:// constructors.

See Also▲

Returns https://() as std::filesystem::path.

This function was introduced in Qt 6.0.

Returns https://() as std::filesystem::path.

This function was introduced in Qt 6.3.

Returns https://() as std::filesystem::path of fileName.

This function was introduced in Qt 6.3.

Creates a link named linkName that points to the file currently specified by https://(). What a link is depends on the underlying filesystem (be it a shortcut on Windows or a symbolic link on Unix). Returns true if successful; otherwise returns false.

This function will not overwrite an already existing entity in the file system; in this case, link() will return false and set https://() to return https://.

See Also▲

This is an overloaded function.

Creates a link named linkName that points to the file fileName. What a link is depends on the underlying filesystem (be it a shortcut on Windows or a symbolic link on Unix). Returns true if successful; otherwise returns false.

See Also▲

This is an overloaded function.

This function was introduced in Qt 6.0.

Moves the file specified by https://() to the trash. Returns true if successful, and sets the https://() to the path at which the file can be found within the trash; otherwise returns false.

This is an overloaded function.

Moves the file specified by https://() to the trash. Returns true if successful, and sets pathInTrash (if provided) to the path at which the file can be found within the trash; otherwise returns false.

Reimplements: https://.

Opens the file using OpenMode mode, returning true if successful; otherwise false.

The mode must be QIODevice::ReadOnly, QIODevice::WriteOnly, or QIODevice::ReadWrite. It may also have additional flags, such as QIODevice::Text and QIODevice::Unbuffered.

See Also▲

This is an overloaded function.

If the file does not exist and mode implies creating it, it is created with the specified permissions.

On POSIX systems the actual permissions are influenced by the value of umask.

On Windows the permissions are emulated using ACLs. These ACLs may be in non-canonical order when the group is granted less permissions than others. Files and directories with such permissions will generate warnings when the Security tab of the Properties dialog is opened. Granting the group all permissions granted to others avoids such warnings.

This function was introduced in Qt 6.3.

See Also▲

This is an overloaded function.

Opens the existing file handle fh in the given mode. handleFlags may be used to specify additional options. Returns true if successful; otherwise returns false.

Example:

#include <stdio.h>

void printError(const char* msg)
{
    QFile file;
    file.open(stderr, QIODevice::WriteOnly);
    file.write(msg, qstrlen(msg));        // write to stderr
    file.close();
}

When a https:// is opened using this function, behaviour of https://() is controlled by the AutoCloseHandle flag. If AutoCloseHandle is specified, and this function succeeds, then calling https://() closes the adopted handle. Otherwise, https://() does not actually close the file, but only flushes it.

Warning:

1. If fh does not refer to a regular file, e.g., it is stdin, stdout, or stderr, you may not be able to https://(). https://() returns 0 in those cases. See https://() for more information.

2. Since this function opens the file without specifying the file name, you cannot use this https:// with a https://.

Note for the Windows Platform

fh must be opened in binary mode (i.e., the mode string must contain 'b', as in "rb" or "wb") when accessing files and other random-access devices. Qt will translate the end-of-line characters if you pass QIODevice::Text to mode. Sequential devices, such as stdin and stdout, are unaffected by this limitation.

You need to enable support for console applications in order to use the stdin, stdout and stderr streams at the console. To do this, add the following declaration to your application's project file:

CONFIG += console

See Also▲

This is an overloaded function.

Opens the existing file descriptor fd in the given mode. handleFlags may be used to specify additional options. Returns true if successful; otherwise returns false.

When a https:// is opened using this function, behaviour of https://() is controlled by the AutoCloseHandle flag. If AutoCloseHandle is specified, and this function succeeds, then calling https://() closes the adopted handle. Otherwise, https://() does not actually close the file, but only flushes it.

See Also▲

Reimplements: https://.

See Also▲

This is an overloaded function.

Returns the complete OR-ed together combination of QFile::Permission for fileName.

This is an overloaded function.

This function was introduced in Qt 6.0.

Removes the file specified by https://(). Returns true if successful; otherwise returns false.

The file is closed before it is removed.

See Also▲

This is an overloaded function.

Removes the file specified by the fileName given.

Returns true if successful; otherwise returns false.

See Also▲

Renames the file currently specified by https://() to newName. Returns true if successful; otherwise returns false.

If a file with the name newName already exists, rename() returns false (i.e., https:// will not overwrite it).

The file is closed before it is renamed.

If the rename operation fails, Qt will attempt to copy this file's contents to newName, and then remove this file, keeping only newName. If that copy operation fails or this file can't be removed, the destination file newName is removed to restore the old state.

See Also▲

This is an overloaded function.

Renames the file oldName to newName. Returns true if successful; otherwise returns false.

If a file with the name newName already exists, https://() returns false (i.e., https:// will not overwrite it).

See Also▲

This is an overloaded function.

This function was introduced in Qt 6.0.

Reimplements: https://.

This is an overloaded function.

Sets fileName to size (in bytes) sz. Returns true if the resize succeeds; false otherwise. If sz is larger than fileName currently is the new bytes will be set to 0, if sz is smaller the file is simply truncated.

See Also▲

Sets the name of the file. The name can have no path, a relative path, or an absolute path.

Do not call this function if the file has already been opened.

If the file name has no path or a relative path, the path used will be the application's current directory path at the time of the https://() call.

Example:

QFile file;
QDir::setCurrent("/tmp");
file.setFileName("readme.txt");
QDir::setCurrent("/home");
file.open(QIODevice::ReadOnly);      // opens "/home/readme.txt" under Unix

Note that the directory separator "/" works for all operating systems supported by Qt.

See Also▲

This is an overloaded function.

This function was introduced in Qt 6.0.

Reimplements: https://.

Sets the permissions for the file to the permissions specified. Returns true if successful, or false if the permissions cannot be modified.

See Also▲

This is an overloaded function.

Sets the permissions for fileName file to permissions.

This is an overloaded function.

This function was introduced in Qt 6.0.

Reimplements: https://.

Returns the absolute path of the file or directory referred to by the symlink (or shortcut on Windows) specified by fileName, or returns an empty string if the fileName does not correspond to a symbolic link.

This name may not represent an existing file; it is only a string. https://() returns true if the symlink points to an existing file.

This is an overloaded function.

Returns the absolute path of the file or directory a symlink (or shortcut on Windows) points to, or a an empty string if the object isn't a symbolic link.

This name may not represent an existing file; it is only a string. https://() returns true if the symlink points to an existing file.

See Also▲