why can't a volatile object call nonvolatile member function

Question

Why can't a volatile object call a non-volatile member function?

In case of const, it makes sense that calling a non-const member function violates the constness of the the object and hence it is prohibited. But why in the case of volatile?

Because you're trying to violate volatile-correctness... – Lightness Races in Orbit Nov 08 '11 at 10:38 — Lightness Races in Orbit, Nov 08 '11 at 10:38
and I just spotted the anagram – Lightness Races in Orbit Nov 08 '11 at 10:38 — Lightness Races in Orbit, Nov 08 '11 at 10:38

score 5 · Accepted Answer · answered Nov 08 '11 at 10:32

In case of const, it makes sense that calling a non-const member function violates the const-ness of the the object and hence it is prohibited. But why in the case of volatile?

It's just the same for volatile. Volatile means every access to an object is a visible side effect and cannot be eliminated. If you called a nonvolatile method on a volatile object, you would violate this property (because the nonvolatile method would treat the object just as a normal object). Therefore, it is impossible.

score 4 · Answer 2 · answered Nov 08 '11 at 10:37

4

From the standard:

7.1.5.1. If an attempt is made to refer to an object defined with a volatile-quailified type through the use of an lvalue with a non-volatile-quailified type, the program behaviour is undefined.

I'm guessing your compiler posts an error to prevent undefined behavior. The standard stating so should be reason enough.

answered Nov 08 '11 at 10:37

Luchian Grigore

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I think the question is "why" – Lightness Races in Orbit Nov 08 '11 at 10:38
@TomalakGeret'kal because the standard says so :) – Luchian Grigore Nov 08 '11 at 10:40
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_Why_ does the standard say so? What is the rationale? What is the practical rationale? In a case like this you can spell it out, unlike in others where sometimes it's a case of "well, ask them" – Lightness Races in Orbit Nov 08 '11 at 10:59
Agree. (violation of volatile-correctness was hard for me to see). As mentioned by jpalecek, and Tomalak, if such call were allowed, the member function behavior would be incorrect since it would *not* access member variables as volatile. – Chethan Nov 08 '11 at 11:59

score 2 · Answer 3 · answered Nov 08 '11 at 10:33

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The volatile qualifier works much in the same way as const works. To see what can be done thanks to this, take a look at this Alexandrescu article. That article should also give you some insight into the why.

answered Nov 08 '11 at 10:33

Andrea Bergia

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it is a 2001 article and all I read now is people advising against volatile for critical section/multithreaded purposes and prefer to use mutexes. For instance: https://www.securecoding.cert.org/confluence/display/seccode/POS03-C.+Do+not+use+volatile+as+a+synchronization+primitive – Nikko Nov 08 '11 at 10:37
@Nikko: It seems you didn't read more than the title of the article referenced in the answer. Read the *full* article. – Yakov Galka Nov 08 '11 at 11:31

why can't a volatile object call nonvolatile member function

3 Answers3