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QString 提供了一个采用Unicode字符编码标准（以双字节对字符编码）的的字符串。

Unicode是US-ASCII和Latin-1的超集。

QString中存储的每个字符占16-bit，以QChar表示，QString采用写时拷贝技术（copy-on-write）来减少内存的占用和避免不必要的数据拷贝，对于大部分的使用来说，QString是我们所需要的，因为对它的使用贯穿了整个Qt的API，另外，对Unicode编码的支持也可使你的程序易于翻译和拓展。对于传统以8-bit ‘\0’作为结束的字符串，Qt提供了QByteArray来支持。

构造函数一览

QString ()
 
 QString ( const QChar * unicode, int size )
 
 QString ( const QChar * unicode )
 
 QString ( QChar ch )
 
 QString ( int size, QChar ch )
 
 QString ( const QLatin1String & str )
 
 QString ( const QString & other )
 
 QString ( const char * str )
 
 QString ( const QByteArray & ba )
 

初始化字符串

一种初始化QString的方法是传递一个const char * 类型的字符串常量， QString str = "Hello";这时，QString会利用fromAscii（）函数将const char *数据转化成Unicode。默认情况下，在所有的带有const char *参数的QString 函数中，const char *型被理解成传统的C风格以‘\0’作为结束标志的字符串，而将const char * 数据设成0的参数是合法的。

你还可以以QChar型的数组来表示QString字符串， static const QChar data[4] = { 0x0055, 0x006e, 0x10e3, 0x03a3 };  QString str(data, 4);

另外一种方式是通过resize（）方法来设置一个字符串的大小然后逐一赋值，QString是以0下标作为索引的开始的，

 QString str;
  str.resize(4);
  str[0] = QChar('U');
  str[1] = QChar('n');
  str[2] = QChar(0x10e3);
  str[3] = QChar(0x03a3);

对于只读访问，一个比较有效的语法是使用at（）函数，它的效率要比[]操作符来的高，

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

QString提供了一下几个基本函数来修改字符数据，

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

对于Ｃ程序员要注意的点：
由于c++的类型系统和基于QString的implicit sharing技术，QString类型可能会被视为int或其他的基本数据类型，例如：
 QString Widget::boolToString(bool b)
  {
      QString result;
      if (b)
          result = "True";
      else
          result = "False";
      return result;
  }

QString区别null string 和 empty string 这两个概念，一个null string表示以QString的默认构造函数传递（const char *）0来完成的初始化的字符串，而an empty string则表示

其中所有字符的长度都是0，A null string is always empty, but an empty string isn't necessarily null:

 QString().isNull();               // returns true
  QString().isEmpty();              // returns true
 
  QString("").isNull();             // returns false
  QString("").isEmpty();            // returns true
 
  QString("abc").isNull();          // returns false
  QString("abc").isEmpty();         // returns false

We recommend that you always use the isEmpty() function and avoid isNull(). 

关于8-bit字符串向Unicode字符串的转化问题：
QString提供了以下四个函数来返回一个const char *版本的字符串 QByteArray: toAscii(), toLatin1(), toUtf8(), and toLocal8Bit().

由QString向8-bit字符串的转换：

To convert from one of these encodings, QString provides fromAscii(), fromLatin1(), fromUtf8(), and fromLocal8Bit(). Other encodings are supported through the QTextCodec class.
 As mentioned above, QString provides a lot of functions and operators that make it easy to interoperate with const char * strings. But this functionality is a double-edged sword: It makes QString more convenient to use if all strings are US-ASCII or Latin-1, but there is always the risk that an implicit conversion from or to const char * is done using the wrong 8-bit encoding. To minimize these risks, you can turn off these implicit conversions by defining the following two preprocessor symbols:
 QT_NO_CAST_FROM_ASCII disables automatic conversions from C string literals and pointers to Unicode.
 QT_NO_CAST_TO_ASCII disables automatic conversion from QString to C strings.
 One way to define these preprocessor symbols globally for your application is to add the following entry to your qmake project file:
  DEFINES += QT_NO_CAST_FROM_ASCII \
             QT_NO_CAST_TO_ASCII
 You then need to explicitly call fromAscii(), fromLatin1(), fromUtf8(), or fromLocal8Bit() to construct a QString from an 8-bit string, or use the lightweight QLatin1String class, for example:
  QString url = QLatin1String("http://www.unicode.org/");
 Similarly, you must call toAscii(), toLatin1(), toUtf8(), or toLocal8Bit() explicitly to convert the QString to an 8-bit string. (Other encodings are supported through the QTextCodec class.)

toAscii() returns an 8-bit string encoded using the codec specified by QTextCodec::codecForCStrings (by default, that is Latin 1).
 toLatin1() returns a Latin-1 (ISO 8859-1) encoded 8-bit string.
 toUtf8() returns a UTF-8 encoded 8-bit string. UTF-8 is a superset of US-ASCII (ANSI X3.4-1986) that supports the entire Unicode character set through multibyte sequences.
 toLocal8Bit() returns an 8-bit string using the system's local encoding.