I don't understand the "not look too safe" example:
Class B will never be able to call a virtual method which is implemented
in C. Not within the CTOR.
And when talking about Dialogs:
Moving member initialization with 'this' to the body is not easily done.
You must change all Window members to Window* members, which means lot
of code to change, and memory fragmentation.
Malte.
Nikolai Pretzell wrote:
> Hi Carsten and all,
>
> > warning:
>> test.cxx(18) : warning C4355: 'this' : used in base member initializer list
>
>
>> class A
>> {
>> public:
>> A() {}
>> };
>>
>> class B
>> {
>> A& _a;
>> public:
>> B( A& a ) : _a(a) {}
>> };
>>
>> class C : public A
>> {
>> B _b;
>> public:
>> C() : _b( *(A*)this ) {}
>> };
>>
>> int main()
>> {
>> C c;
>> }
>>
>> Using 'this' in the ctor initializer list can be dangerous, if you don't
>> know what you do. The above example uses a safe way to provide this.
>
> This example does not look too safe to me, because:
> It is quite possible that - for example - the maintainer of B one day
> wants to use a virtual funtion of A that is implemented in C.
> And that functionality of B may be called in the constructor body of C.
>
> Pro warning:
> In cases like this, generally the maintainer of B has no chance to know
> that he must not use some functions of A (or C), because the instance of
> A (or C) is not yet completely constructed.
>
> So the warning is a valid hint to a potential maintainability nightmare.
>
> Contra warning:
> The normal workaround would be to move the "this" from the
> initialization section into the constructor body. This removes the
> warning, but not always removes the danger, because still the receiver
> of "this" may use it while not all construction is done.
>
> So one could argue, fixing the warning does not necessarily fix the
> problem, so we can remove the warning anyway.
>
>
>
>> I think we have two possible ways to solve this problem.
>> - Turn of the warning for every occurrence in the code.
>> - Switch of C4355 for Windows globally.
>
> I'd suggest to take the warning serious and to re-look on each
> occurrence about this way:
>
> 1. Can the construction be re-worked so no self-reference is necessary?
> Sometimes this is easy by clarifying a hierarchy that could be a
> directed graph but currently is a non-directed one.
> If yes, do so.
>
> 2. If not, is the use of "this" safe including maintainenance in later
> times?
> If yes, see X.
>
> 3. If no, make sure there is a comment at all places, using the
> partly-initialized instance that it may be only partly initialized!
> Then goto X.
>
> X. See, if moving "this" from initialization section into c'tor body
> does any harm. (In case of dialogues, the painting takes so much time,
> that one could assume, the minor performance drawback of moving the
> initialisation into the body is negligible - but I don't know that code,
> so it's just an assumption.)
> If yes, do so.
> If no, disable the warning locally.
>
>
> If we just generally disable the warning, we may overlook occurrences
> where the use indeed is questionable.
>
> Nikolai
>
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