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

This compiles because although the WidgetHolder is a const object, this const-ness does not automatically apply to objects pointed to (referenced by) the WidgetHolder. Think of it at a machine level – if the WidgetHolder object itself were held in read-only memory, you could still write to things that were pointed to by the WidgetHolder.

The problem appears to lie in this line:

WidgetHolder(Widget a, Widget b, Widget c, Widget d): A(a), B(b), C(c), D(d){} 

As Frank mentioned, your references inside the WidgetHolder class are going to hold invalid references after the constructor returns. Therefore, you should change this to:

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

After you do that, it won’t compile, and I leave it as an exercise for the reader to work out the rest of the solution.