QByteArray Class Reference
[QtCore module]

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 Qt for Embedded Linux).

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().

 QByteArray x("free");
 QByteArray y("dom");
 x.append(y);
 // x == "freedom"

 QByteArray ba("STARTTLS\r\n");
 ba.chop(2);                 // ba == "STARTTLS"

 QByteArray ba("Hello world");
 char *data = ba.data();
 while (*data) {
     cout << "[" << *data << "]" << endl;
     ++data;
 }

 QString tmp = "test";
 QByteArray text = tmp.toLocal8Bit();
 char *data = new char[text.size()]
 strcpy(data, text.data());
 delete [] data;

 QString tmp = "test";
 QByteArray text = tmp.toLocal8Bit();
 char *data = new char[text.size() + 1]
 strcpy(data, text.data());
 delete [] data;

 QByteArray url("http://qt.nokia.com/index.html");
 if (url.endsWith(".html"))
     ...

 QByteArray ba("Istambul");
 ba.fill('o');
 // ba == "oooooooo"

 ba.fill('X', 2);
 // ba == "XX"

 QByteArray text = QByteArray::fromBase64("UXQgaXMgZ3JlYXQh");
 text.data();            // returns "Qt is great!"

 QByteArray text = QByteArray::fromHex("517420697320677265617421");
 text.data();            // returns "Qt is great!"

 QByteArray text = QByteArray::fromPercentEncoding("Qt%20is%20great%33");
 text.data();            // returns "Qt is great!"

  static const char mydata[] = {
     0x00, 0x00, 0x03, 0x84, 0x78, 0x9c, 0x3b, 0x76,
     0xec, 0x18, 0xc3, 0x31, 0x0a, 0xf1, 0xcc, 0x99,
     ...
     0x6d, 0x5b
 };

 QByteArray data = QByteArray::fromRawData(mydata, sizeof(mydata));
 QDataStream in(&data, QIODevice::ReadOnly);
 ...

 QByteArray x("sticky question");
 QByteArray y("sti");
 x.indexOf(y);               // returns 0
 x.indexOf(y, 1);            // returns 10
 x.indexOf(y, 10);           // returns 10
 x.indexOf(y, 11);           // returns -1

 QByteArray ba("ABCBA");
 ba.indexOf("B");            // returns 1
 ba.indexOf("B", 1);         // returns 1
 ba.indexOf("B", 2);         // returns 3
 ba.indexOf("X");            // returns -1

 QByteArray ba("Meal");
 ba.insert(1, QByteArray("ontr"));
 // ba == "Montreal"

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

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

 QByteArray x("crazy azimuths");
 QByteArray y("az");
 x.lastIndexOf(y);           // returns 6
 x.lastIndexOf(y, 6);        // returns 6
 x.lastIndexOf(y, 5);        // returns 2
 x.lastIndexOf(y, 1);        // returns -1

 QByteArray ba("ABCBA");
 ba.lastIndexOf("B");        // returns 3
 ba.lastIndexOf("B", 3);     // returns 3
 ba.lastIndexOf("B", 2);     // returns 1
 ba.lastIndexOf("X");        // returns -1

 QByteArray x("Pineapple");
 QByteArray y = x.left(4);
 // y == "Pine"

 QByteArray x("apple");
 QByteArray y = x.leftJustified(8, '.');   // y == "apple..."

 QByteArray x("Five pineapples");
 QByteArray y = x.mid(5, 4);     // y == "pine"
 QByteArray z = x.mid(5);        // z == "pineapples"

 int n = 63;
 QByteArray::number(n);              // returns "63"
 QByteArray::number(n, 16);          // returns "3f"
 QByteArray::number(n, 16).toUpper();  // returns "3F"

 QByteArray ba = QByteArray::number(12.3456, 'E', 3);
 // ba == 1.235E+01

 QByteArray x("ship");
 QByteArray y("air");
 x.prepend(y);
 // x == "airship"

 QByteArray ba("Montreal");
 ba.remove(1, 4);
 // ba == "Meal"

 QByteArray ba("ab");
 ba.repeated(4);             // returns "abababab"

 QByteArray x("Say yes!");
 QByteArray y("no");
 x.replace(4, 3, y);
 // x == "Say no!"

 QByteArray ba("colour behaviour flavour neighbour");
 ba.replace(QByteArray("ou"), QByteArray("o"));
 // ba == "color behavior flavor neighbor"

 QByteArray x("Pineapple");
 QByteArray y = x.right(5);
 // y == "apple"

 QByteArray x("apple");
 QByteArray y = x.rightJustified(8, '.');    // y == "...apple"

 QByteArray ba;
 int n = 63;
 ba.setNum(n);           // ba == "63"
 ba.setNum(n, 16);       // ba == "3f"

 QByteArray ba("  lots\t of\nwhitespace\r\n ");
 ba = ba.simplified();
 // ba == "lots of whitespace";

 QByteArray ba("Hello");
 int n = ba.size();          // n == 5
 ba.data()[0];               // returns 'H'
 ba.data()[4];               // returns 'o'
 ba.data()[5];               // returns '\0'

 QByteArray url("ftp://ftp.qt.nokia.com/");
 if (url.startsWith("ftp:"))
     ...

 QByteArray text("Qt is great!");
 text.toBase64();        // returns "UXQgaXMgZ3JlYXQh"

 QByteArray string("1234.56");
 double a = string.toDouble();   // a == 1234.56

 QByteArray str("FF");
 bool ok;
 int hex = str.toInt(&ok, 16);     // hex == 255, ok == true
 int dec = str.toInt(&ok, 10);     // dec == 0, ok == false

 QByteArray str("FF");
 bool ok;
 long hex = str.toLong(&ok, 16);   // hex == 255, ok == true
 long dec = str.toLong(&ok, 10);   // dec == 0, ok == false

 QByteArray x("Qt by NOKIA");
 QByteArray y = x.toLower();
 // y == "qt by nokia"

 QByteArray text = "{a fishy string?}";
 QByteArray ba = text.toPercentEncoding("{}", "s");
 qDebug(ba.constData());
 // prints "{a fi%73hy %73tring%3F}"

 QByteArray x("Qt by NOKIA");
 QByteArray y = x.toUpper();
 // y == "QT BY NOKIA"

 QByteArray ba("  lots\t of\nwhitespace\r\n ");
 ba = ba.trimmed();
 // ba == "lots\t of\nwhitespace";

 QByteArray ba("Stockholm");
 ba.truncate(5);             // ba == "Stock"

 QByteArray x("free");
 QByteArray y("dom");
 x += y;
 // x == "freedom"

 QByteArray ba;
 for (int i = 0; i < 10; ++i)
     ba[i] = 'A' + i;
 // ba == "ABCDEFGHIJ"