Const Struct&

我在确定const在特定情况下的确切应用方式时遇到了一些麻烦.这是我的代码:

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?
}

基本上,我有一个名为test的类.它在内部使用了四个小部件,我需要它来返回这些小部件,但如果测试被声明为const,我希望小部件也返回const.

有人可以向我解释为什么main()中的代码编译？

非常感谢你.

您需要创建一个专门用于保存const Widget和对象的新类型.即:

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);
    }

您现在将收到以下错误(在gcc 4.3中):

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

类似的习惯用法在标准库中使用迭代器,即:

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