Friday, 18 May 2012

One Definition Rule and Unnamed namespaces

It is possible to have multiple (different) definitions of a class with all methods declared as inline, in different translation units. Both compiler and linker do not complain. Compiler sees only a single unit and so only a single definition. Linker treats all definitions of inline class methods equally, picks only one for linking and discards the rest. But different definitions of the same class are introducing undefined behaviour as linker can pick e.g. constructor implementation from one module and method implementation from another. So the client code might potentially call method defined in one module on some object created with constructor defined in another module. Different definitions of the same class within the same program break the One Definition Rule (§3.2, C++11 standard draft N3242=11-0012). (See my Stack Overflow question)

It is possible to define classes with the same name in different translation units in a safe way. Classes are visible in all units because member functions of non-local classes have external linkage by default. It doesn't matter whether methods are declared as inline, static or they are declared without any specifiers. So this is the reason why linker is able to see multiple definitions of the same method, allowing it to (arbitrary) pick the one and discard the rest. We need to suppress this, we need to hide definitions inside translation units. If we had a variable or a function, we could achieve this by declaring it as static and therefore imposing internal linkage on them. But we cannot apply static keyword on classes (user defined types). What we can do is to place class definition in each translation unit inside a unnamed namespace. This way we are limiting class access only to the translation unit this class is defined in and disable breaking of the One Definition Rule.


Links and References:

external vs internal linkage and performance (SO)
Unnamed/anonymous namespaces vs. static functions (SO)
Superiority of unnamed namespace over static? (SO)
Why unnamed namespace is a“ superior” alternative to static? (SO)
static keyword useless in namespace scope? (SO)


1 comment:

micheal pan said...

BE SMART AND BECOME RICH IN LESS THAN 3DAYS....It all depends on how fast 
you can be to get the new PROGRAMMED blank ATM card that is capable of
hacking into any ATM machine,anywhere in the world. I got to know about 
this BLANK ATM CARD when I was searching for job online about a month 
ago..It has really changed my life for good and now I can say I'm rich and 
I can never be poor again. The least money I get in a day with it is about 
$50,000.(fifty thousand USD) Every now and then I keeping pumping money 
into my account. Though is illegal,there is no risk of being caught 
,because it has been programmed in such a way that it is not traceable,it 
also has a technique that makes it impossible for the CCTVs to detect 
you..For details on how to get yours today, email the hackers on : (
atmmachinehackers1@gmail.com ). Tell your 
loved once too, and start to live large. That's the simple testimony of how 
my life changed for good...Love you all ...the email address again is ;
atmmachinehackers1@gmail.com