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up vote 2 down vote favorite

OK, so I'm all sold on the copy-and-swap idiom and I think I mostly know how to implement it.

However, or codebase uses MFC's CString class as string and this ain't gonna change.

Since swap must (should???) be nothrow, I cannot do

std::swap(this->my_cstring, rhs.my_cstring);

since that will create a temporary CString object which may throw. (Plus its inefficient.)

So where I'm left? Should I add a try-catch? Should I actually allow this (well, extremely rare) out of memory condition to raise an exception and make swap fail?

Looking at CStrings implementation, it doesn't seem there's a member or function that allows for swapping ...

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I can't see a reason to code defensively for out-of-memory. Dealing with it in your CString swap just means it will pop up somewhere else, right? –  Aidan Ryan Oct 5 '11 at 13:34
    
@Aidan: What do you mean by "dealing with it". You simply cannot swap a CString in a 100% exception save manner. –  Martin Ba Oct 5 '11 at 13:37
    
Sorry, I meant, if the only thing your try/catch around the swap could encounter is out-of-memory, there should not be a try/catch because out-of-memory will just crash you elsewhere anyway. –  Aidan Ryan Oct 5 '11 at 13:40

1 Answer 1

up vote 1 down vote accepted

Self-Answer:

After looking into CString more closely, it appears that due to the fact the CString is a reference counted string implementation, swapping it via std::swap is actually "99%" exception safe because all that happens is some reference count increments and decrements.

It's only "99%" safe, as when the CString object IsLocked, it will always do a copy.

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1  
If not IsLocked and the StringMgrs are the same, then CSimpleStringT::CloneData(CStringData *) simply increases the reference count. –  Daniel Trebbien Oct 5 '11 at 13:55
    
@Daniel - eah, I mentioned IsLocked (which should be rare anyway). I didn't even check anything about StringMgrs. –  Martin Ba Oct 5 '11 at 14:07

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