How to change string into QString ?

 Using in simple method std::string  :

    QString QString::fromStdString(const std::string & str)

std::string str = "Hello world";
QString qstr = QString::fromStdString(str);

 Ascii encoded const char * by using this:

QString QString::fromAscii(const char * str, int size = -1)

const char* str = "Hello world";
QString qstr = QString::fromAscii(str);

To const char * encoded with system encoding  with QTextCodec::codecForLocale()  by applying link:

   QString QString::fromLocal8Bit(const char * str, int size = -1)

const char* str = "zażółć gęślą jaźń";  // latin2 source file and system encoding
QString qstr = QString::fromLocal8Bit(str);

 QString QString::fromUtf16(const ushort * unicode, int size = -1)

The  const ushort * containing UTF16 encoded string:

const ushort* str = read_raw("hello.txt"); // assuming hello.txt is UTF16 encoded, and read_raw() reads bytes from file into memory and returns pointer to the first byte as const ushort*
QString qstr = QString::fromUtf16(str);

QString has a static method to convert a std::string to a QString:

std::string str = "abc";
QString qstr = QString::fromStdString(str);

simplest way:

std::string s = "This is an STL string";
QString qs = QString::fromAscii(s.data(), s.size());

The.c_str()  have the std::string to copy itself in case there is no place to add the ‘\0’ at the end.