原型
QString QString::mid(int position, int n = -1) const
Returns a string that contains n characters of this string, starting
at the specified position index. Returns a null string if the position
index exceeds the length of the string. If there are less than n
characters available in the string starting at the given position, or
if n is -1 (default), the function returns all characters that are
available from the specified position.
例子：
若需要QStringRef类型的子串，可以使用函数midRef （），与mid（）类似，只是返回类型为ref，原型如下
QStringRef QString::midRef(int position, int n = -1) const
Returns a substring reference to n characters of this string, starting
at the specified position. If the position exceeds the length of the
string, a null reference is returned. If there are less than n
characters available in the string, starting at the given position, or
if n is -1 (default), the function returns all characters from the
specified position onwards.
Example:
QString 还有left() 和 right().两个函数可以取子串
原型
QString QString::left(int n) const
返回最左边n个字符的子串。当n大于等于QString.size()或n小于0的时候，返回整个字符串。
Returns a substring that contains the n leftmost characters of the
string. The entire string is returned if n is greater than or equal to
size(), or less than zero.
例子：
right()与left()类似，不阐述
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