Const Struct&

Question

I'm having a little trouble figuring out exactly how const applies in a specific case. Here's the code I have:

struct Widget
{
    Widget():x(0), y(0), z(0){}

    int x, y, z;
};

struct WidgetHolder //Just a simple struct to hold four Widgets.
{
    WidgetHolder(Widget a, Widget b, Widget c, Widget d): A(a), B(b), C(c), D(d){}

    Widget& A;
    Widget& B;
    Widget& C;
    Widget& D;
};

class Test //This class uses four widgets internally, and must provide access to them externally.
{
    public:
        const WidgetHolder AccessWidgets() const
        {
            //This should return our four widgets, but I don't want anyone messing with them.
            return WidgetHolder(A, B, C, D);
        }

        WidgetHolder AccessWidgets()
        {
            //This should return our four widgets, I don't care if they get changed.
            return WidgetHolder(A, B, C, D);
        }

    private:
        Widget A, B, C, D;
};

int main()
{
    const Test unchangeable;

    unchangeable.AccessWidgets().A.x = 1; //Why does this compile, shouldn't the Widget& be const?
}

Basically, I have a class called test. It uses four widgets internally, and I need it to return these, but if test was declared const, I want the widgets returned const also.

Can someone explain to me why the code in main() compiles?

Thank you very much.

Answer 1

You need to create a new type specifically for holding const Widget& objects. Ie:

struct ConstWidgetHolder
{
    ConstWidgetHolder(const Widget &a, const Widget &b, const Widget &c, const Widget &d): A(a), B(b), C(c), D(d){}

    const Widget& A;
    const Widget& B;
    const Widget& C;
    const Widget& D;
};

class Test
{
public:
    ConstWidgetHolder AccessWidgets() const
    {
        return ConstWidgetHolder(A, B, C, D);
    }

You will now get the following error (in gcc 4.3):

widget.cc: In function 'int main()':
widget.cc:51: error: assignment of data-member 'Widget::x' in read-only structure

A similar idiom is used in the standard library with iterators ie:

class vector {
    iterator begin();
    const_iterator begin() const;