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# QByteArray Class Reference
[com.trolltech.qt.core module]

The `QByteArray` class provides an array of bytes. More...

## Detailed Description

The `QByteArray` class provides an array of bytes.

`QByteArray` can be used to store both raw bytes (including '\0's) and traditional 8-bit '\0'-terminated strings. Using `QByteArray` is much more convenient than using `const char *`. Behind the scenes, it always ensures that the data is followed by a '\0' terminator, and uses implicit sharing (copy-on-write) to reduce memory usage and avoid needless copying of data.

In addition to `QByteArray`, Qt also provides the `QString` class to store string data. For most purposes, `QString` is the class you want to use. It stores 16-bit Unicode characters, making it easy to store non-ASCII/non-Latin-1 characters in your application. Furthermore, `QString` is used throughout in the Qt API. The two main cases where `QByteArray` is appropriate are when you need to store raw binary data, and when memory conservation is critical (e.g. with Qtopia Core).

One way to initialize a `QByteArray` is simply to pass a `const char *` to its constructor. For example, the following code creates a byte array of size 5 containing the data "Hello":

        QByteArray ba("Hello");

Although the `size` is 5, the byte array also maintains an extra '\0' character at the end so that if a function is used that asks for a pointer to the underlying data (e.g. a call to `data`), the data pointed to is guaranteed to be '\0'-terminated.

`QByteArray` makes a deep copy of the `const char *` data, so you can modify it later without experiencing side effects. (If for performance reasons you don't want to take a deep copy of the character data, use QByteArray::fromRawData() instead.)

Another approach is to set the size of the array using `resize` and to initialize the data byte per byte. `QByteArray` uses 0-based indexes, just like C++ arrays. To access the byte at a particular index position, you can use operator[](). On non-const byte arrays, operator[]() returns a reference to a byte that can be used on the left side of an assignment. For example:

        QByteArray ba;
        ba.resize(5);
        ba[0] = 0x3c;
        ba[1] = 0xb8;
        ba[2] = 0x64;
        ba[3] = 0x18;
        ba[4] = 0xca;

For read-only access, an alternative syntax is to use `at`:

        for (int i = 0; i < ba.size(); ++i) {
            if (ba.at(i) >= 'a' && ba.at(i) <= 'f')
                cout << "Found character in range [a-f]" << endl;
        }

`at` can be faster than operator[](), because it never causes a deep copy to occur.

To extract many bytes at a time, use `left`, `right`, or `mid`.

A `QByteArray` can embed '\0' bytes. The `size` function always returns the size of the whole array, including embedded '\0' bytes. If you want to obtain the length of the data up to and excluding the first '\0' character, call qstrlen() on the byte array.

After a call to `resize`, newly allocated bytes have undefined values. To set all the bytes to a particular value, call fill().

To obtain a pointer to the actual character data, call `data` or constData(). These functions return a pointer to the beginning of the data. The pointer is guaranteed to remain valid until a non-const function is called on the `QByteArray`. It is also guaranteed that the data ends with a '\0' byte. This '\0' byte is automatically provided by `QByteArray` and is not counted in `size`.

`QByteArray` provides the following basic functions for modifying the byte data: append(), prepend(), insert(), `replace`, and `remove`. For example:

        QByteArray x("and");
        x.prepend("rock ");         // x == "rock and"
        x.append(" roll");          // x == "rock and roll"
        x.replace(5, 3, "&");       // x == "rock & roll"

The `replace` and `remove` functions' first two arguments are the position from which to start erasing and the number of bytes that should be erased.

When you append() data to a non-empty array, the array will be reallocated and the new data copied to it. You can avoid this behavior by calling `reserve`, which preallocates a certain amount of memory. You can also call `capacity` to find out how much memory `QByteArray` actually allocated. Data appended to an empty array is not copied.

A frequent requirement is to remove whitespace characters from a byte array ('\n', '\t', ' ', etc.). If you want to remove whitespace from both ends of a `QByteArray`, use `trimmed`. If you want to remove whitespace from both ends and replace multiple consecutive whitespaces with a single space character within the byte array, use `simplified`.

If you want to find all occurrences of a particular character or substring in a `QByteArray`, use `indexOf` or `lastIndexOf`. The former searches forward starting from a given index position, the latter searches backward. Both return the index position of the character or substring if they find it; otherwise, they return -1. For example, here's a typical loop that finds all occurrences of a particular substring:

        QByteArray ba("We must be <b>bold</b>, very <b>bold</b>");
        int j = 0;
        while ((j = ba.indexOf("<b>", j)) != -1) {
            cout << "Found <b> tag at index position " << j << endl;
            ++j;
        }

If you simply want to check whether a `QByteArray` contains a particular character or substring, use `contains`. If you want to find out how many times a particular character or substring occurs in the byte array, use `count`. If you want to replace all occurrences of a particular value with another, use one of the two-parameter `replace` overloads.

QByteArrays can be compared using overloaded operators such as operator<(), operator<=(), operator==(), operator>=(), and so on. The comparison is based exclusively on the numeric values of the characters and is very fast, but is not what a human would expect. QString::localeAwareCompare() is a better choice for sorting user-interface strings.

For historical reasons, `QByteArray` distinguishes between a null byte array and an empty byte array. A null byte array is a byte array that is initialized using `QByteArray`'s default constructor or by passing (const char *)0 to the constructor. An empty byte array is any byte array with size 0. A null byte array is always empty, but an empty byte array isn't necessarily null:

        QByteArray().isNull();          // returns true
        QByteArray().isEmpty();         // returns true

        QByteArray("").isNull();        // returns false
        QByteArray("").isEmpty();       // returns true

        QByteArray("abc").isNull();     // returns false
        QByteArray("abc").isEmpty();    // returns false

All functions except `isNull` treat null byte arrays the same as empty byte arrays. For example, `data` returns a pointer to a '\0' character for a null byte array (not a null pointer), and `QByteArray` compares equal to `QByteArray`(""). We recommend that you always use `isEmpty` and avoid `isNull`.

### Note on 8-bit Character Comparisons

In `QByteArray`, the notion of uppercase and lowercase and of which character is greater than or less than another character is locale dependent. This affects functions that support a case insensitive option or that compare or lowercase or uppercase their arguments. Case insensitive operations and comparisons will be accurate if both strings contain only ASCII characters. (If `$LC_CTYPE` is set, most Unix systems do "the right thing".) Functions that this affects include `contains`, `indexOf`, `lastIndexOf`, operator<(), operator<=(), operator>(), operator>=(), `toLower` and `toUpper`.

This issue does not apply to QStrings since they represent characters using Unicode.

See also `QString` and `QBitArray`.