Question

I’ve created simple snippet to show strange behavior I noticed. That’s it:

#include <QCoreApplication>
#include <QLineEdit>

class MyObject : public QWidget
{
public:
    explicit MyObject(QWidget *parent = 0) : QWidget(parent) {
        editor = new QLineEdit(this);
    }

    void setValue(const QString &s) const {
        editor->setText(s);
        editor->setReadOnly(true);
        editor->setMaxLength(s.length());
    }

private:
    QLineEdit *editor;
};

int main(int argc, char **argv)
{
    QCoreApplication app(argc, argv);
    return app.exec();
}

MyObject::setValue function has const specifier, but this code is compiled well. Note that setText, setReadOnly and setMaxLength functions aren’t const:

void setText(const QString &);
void setReadOnly(bool);
void setMaxLength(int);

I just want to know what causes such behavior?
I use Qt 4.7.4 with MingGW 4.6.2.

Answer 1

(This is not Qt related. This is a general C++ question.)

The compiler is correct, because you are not modifying editor. What you are modifying is *editor; you are only modifying the object it points to. The const specifier will only disallow changing members directly contained in the object. The object editor points to is not a member and thus can be modified:

void setValue(const QString &s) const {
    editor->setText(s); // OK
    editor = new QLineEdit; // Error: 'editor' is changed.
}