QString[]和QString.at()的区别

现象：

QString x;
qDebug() << __LINE__<< x.size() << x.length();//10 0 0
qDebug() << __LINE__<< x.[0];//11 x0
qDebug() << __LINE__<< x.[10];//12 x0
qDebug() << __LINE__<< x.at(0);
//ASSERT: "uint(i) < uint(size())" in file /usr/include/x86_64-linux-gnu/qt5/QtCore/qstring.h, line 931

QString()[k]，k为任意值都不会崩溃

QString().at(0)，则会崩溃

原因：

QString()[10] <=> QString().operator[](10);

源码：QString/operator[]

inline QCharRef QString::operator[](int i)
{ Q_ASSERT(i >= 0); return QCharRef(*this, i); }

源码：QString/QCharRef

class Q_CORE_EXPORT QCharRef {
    QString &s;
    int i;
    inline QCharRef(QString &str, int idx)
        : s(str),i(idx) {}
    friend class QString;
public:

    // most QChar operations repeated here

    // all this is not documented: We just say "like QChar" and let it be.
    inline operator QChar() const
    { return i < s.d->size ? s.d->data()[i] : 0; }

看出operator[]的函数，传任意值是不会崩溃的

备注

下图能看出来，QString的有

两个operator[]()：QString()[10]不会崩溃

两个operator[]() const：QString()[10]会崩溃

一个at() const;：QString().at(10)会崩溃

当我们外部调用operator[]函数时，优先operator[]()，其次operator[]() const

demo验证

class A{
public:
    A(){}
    int printx() const {qDebug() << "printx() const";}
    int printx()  {qDebug() << "printx() ";}
};

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    A aa;
    aa.printx();//printx()