C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Const operator= in proxy class?          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message glenlow@pixelglow.com Guest Posted: Wed Jul 27, 2005 3:30 pm    Post subject: Const operator= in proxy class? Hi All Consider a typical proxy class: template <typename T> class proxy { operator T () const; proxy& operator= (const T& rhs); }; Such an object might be returned by an iterator dereference or a matrix indexer access for example, and would be the moral equivalent (if not exact synactic equivalent) of a C++ reference. Some people (James Kanze, Sep 2004 in this group?) seem to favor declaring the operator= as a const member function e.g. proxy& operator= (const T& rhs) const; What are the pros and cons of each approach? Cheers, Glen Low, Pixelglow Software www.pixelglow.com [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top kanze@gabi-soft.fr Guest Posted: Thu Jul 28, 2005 11:14 am    Post subject: Re: Const operator= in proxy class? [email]glenlow (AT) pixelglow (DOT) com[/email] wrote: Quote: Consider a typical proxy class: template <typename T> class proxy { operator T () const; proxy& operator= (const T& rhs); }; Such an object might be returned by an iterator dereference or a matrix indexer access for example, and would be the moral equivalent (if not exact synactic equivalent) of a C++ reference. Some people (James Kanze, Sep 2004 in this group?) seem to favor declaring the operator= as a const member function e.g. proxy& operator= (const T& rhs) const; What are the pros and cons of each approach? First, "favor" isn't really the correct word. If you asked me why I declared it const, I'd answer "because I can". If I'm not mistaken, I started declaring it const when I saw that Scott Meyers did so. (The most important thing for a good software engineer is to know who to copy from:-).) On the whole, I think I rather like the const because it flags that something unusual is going on; if you see an assignment operator declared const, you know it isn't just the usual, run of the mill assignment operator. But in a code review, if someone hadn't declared it const, I wouldn't bother mentionning the point unless there was a house rule about it. -- James Kanze GABI Software Conseils en informatique orientée objet/ Beratung in objektorientierter Datenverarbeitung 9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34 [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top tony_in_da_uk@yahoo.co.uk Guest Posted: Fri Jul 29, 2005 12:44 am    Post subject: Re: Const operator= in proxy class? Hi Glen, A basic design decision is whether a const proxy should indicate that the controlled object is to be deemed const, such that it can't be modified through the proxy. This seems a natural enough model. Implementation's something like: class Proxy { public: Proxy(T& t) : t_(t) { } operator const T&() const { return t_; } operator T&() { return t_; } Proxy& operator=(const T& rhs) { t_ = rhs; return *this; } private: T& t_; }; Why does your proxy return a copy of the controlled object in its casting operator, rather than providing access by reference? Doesn't this create an unnamed temporary, preventing things like "my_proxy_to_int *= 2;" even if the proxy isn't const? Is there a special case where this is useful? Cheers, Tony [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top glenlow@pixelglow.com Guest Posted: Sat Jul 30, 2005 10:12 am    Post subject: Re: Const operator= in proxy class? Yes but not declaring it const has some benefits -- a const proxy then gets treated differently from a non-const proxy, much like a normal C++ const reference gets treated differently from a non-const reference. E.g. template <typename T> class array { const proxy <T> operator[] (int index) const; proxy <T> operator[] (int index); }; So I was wondering what the benefits are for the other approach, declaring proxy::operator= const. Cheers, Glen Low, Pixelglow Software www.pixelglow.com [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top glenlow@pixelglow.com Guest Posted: Sat Jul 30, 2005 10:20 am    Post subject: Re: Const operator= in proxy class? Quote: Why does your proxy return a copy of the controlled object in its casting operator, rather than providing access by reference? Doesn't this create an unnamed temporary, preventing things like "my_proxy_to_int *= 2;" even if the proxy isn't const? Is there a special case where this is useful? So that the proxy doesn't surrender control of the controlled object. (Especially important for operator T& () which allows the caller to do anything to the controlled object without going through the proxy again.) Or when there's no discrete controlled object per se e.g. the proxy of vector <bool>, which has no discrete bool object. So operator= and operator T do something other than straight assignment and value copy. Cheers, Glen Low, Pixelglow Software www.pixelglow.com [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top glenlow@pixelglow.com Guest Posted: Sat Jul 30, 2005 10:20 am    Post subject: Re: Const operator= in proxy class? Quote: Why does your proxy return a copy of the controlled object in its casting operator, rather than providing access by reference? Doesn't this create an unnamed temporary, preventing things like "my_proxy_to_int *= 2;" even if the proxy isn't const? Is there a special case where this is useful? So that the proxy doesn't surrender control of the controlled object. (Especially important for operator T& () which allows the caller to do anything to the controlled object without going through the proxy again.) Or when there's no discrete controlled object per se e.g. the proxy of vector <bool>, which has no discrete bool object. So operator= and operator T do something other than straight assignment and value copy. Cheers, Glen Low, Pixelglow Software www.pixelglow.com [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top kanze@gabi-soft.fr Guest Posted: Mon Aug 01, 2005 10:25 am    Post subject: Re: Const operator= in proxy class? [email]glenlow (AT) pixelglow (DOT) com[/email] wrote: Quote: Yes but not declaring it const has some benefits -- a const proxy then gets treated differently from a non-const proxy, much like a normal C++ const reference gets treated differently from a non-const reference. E.g. template <typename T> class array { const proxy <T> operator[] (int index) const; proxy <T> operator[] (int index); }; So I was wondering what the benefits are for the other approach, declaring proxy::operator= const. That's an interesting idea: using the const-ness of the proxy to determine whether it can modify the actual object. I actually adopted my current idiom before it was decided what const meant, if anything, on a return value, and have gotten into the habit of providing two proxies, one const, and one not. Note that the above solution does allow the user to break const, by declaring an explicit instance of proxy. Since proxy is copiable: array<t>::proxy p = a[ i ] ; should do the trick, even if a is const. I rather doubt that this is a real problem, however; I can't imagine anyone accidentally doing it, and of course, if the intentional goal is to break const-ness, then a const_cast will do the trick just as well. -- James Kanze GABI Software Conseils en informatique orientée objet/ Beratung in objektorientierter Datenverarbeitung 9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34 [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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