Re: const correct member variable access thru function?

"Victor Bazarov" <>
30 Sep 2006 11:17:45 -0400
<efk5sb$9q3$> wrote:

The proper way to do it (IMO) would be to use a proxy class that
would know whether it's on the right or on the left side of the
assignment op. It's not as simple as introducing two overloaded
functions, like here:

Thanks, I cant believe i did not think of overloading..

because in this case if 'A' is non-const, then the non-const version
will be called if you use it in an expression (even though you're not
trying to assign to it):

That is interseting. So if I wanted to log read and write access to
the variable then overloading would not work.. hmm, I had a go at
making a proxy object emulating the variable, instead of having
functions emulating the variable. The same problem seems to creep up
though. That is, if Z is a non const object and I do a 'int tmp =
z.x;' then the non const assignment operator is called even though
this is a "read only" operation. Anyone got any tips or links to
related threads?

Example code (take 2):

template <class T>
 struct Proxy {

 Proxy(T &x): x_ref_(x) {} //init ref

 //assignment operator..
 Proxy<T>& operator = (const T& rhs) {x_ref_ = rhs; /*cout<<"non-const
lhs";*/ return *this;}

 //conversion operator..
 operator T& () { /*cout<<"non-const rhs";*/ return x_ref_; }
 operator const T& () const { /*cout<<"const rhs";*/ return x_ref_; }

 T &x_ref_;

struct A{
 int x;

struct Z {
 int x_;
 Proxy<int> x;

 Z(): x_(0), x(x_) {;}

void main() {

 int tmp;

 A a = A();
 a.x = 1;
 tmp = a.x;

 const A a_const = A();
 tmp = a_const.x;

 Z z = Z();
 z.x = 1;
 //tmp = z.x; // error C2593: 'operator =' is ambiguous. // ehh!?

 Z z = Z();
 z.x = 1;
 int tmp = z.x; // calls non-const operator=() // calling non const
even though this is read only !?

 const Z z_const = Z();
 int tmp = z_const.x; // calls const operator =


OK, I don't have related threads, but here is what I managed to whip
up in ten minutes or so:

class A
     int x_;

     void change_x_value_to(int x) { x_ = x; }
     int get_x_value() const { return x_; }

     A() : x_(666) {}

     struct x_proxy
         A const& a;
         bool canchange;
         x_proxy(A& a) : a(a), canchange(true) {}
         x_proxy(A const& a) : a(a), canchange(false) {}

         x_proxy& operator =(int i) {
             if (!canchange) throw "cannot change a constant object";
             return *this;

         operator int() const { return a.get_x_value(); }

     x_proxy x() { return x_proxy(*this); }
     x_proxy x() const { return x_proxy(*this); }

int main ()
     A a;
     A const ca;

     a.x() = 42;

     int b, cb;

     b = a.x();
     cb = ca.x();

     try {
         ca.x() = 77;
     catch (...) {

     return 0;

Disclaimer: I didn't test it with all compilers avaiable to me,
and this is for illustration only, probably contains some big
errors I've missed.

It does not have a proper const checking for 'ca.x() = 77;', for
example, it can be achieved by introducing a sibling proxy to
'x_proxy', name it 'x_const_proxy', which should be returned
from the 'x' member of a constant object. I'll leave it for you
to implement.

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