Re: operator== for aggregate types / member wise comparison
"Ulrich Eckhardt" <eckhardt@satorlaser.com>
Now, what is the percentage of classes/structs in a project that has a
meaningful op==?
And then, where it is based on full memberwise comparition?
There is another problem where C++ doesn't provide a satisfactory solution
and that is copying. Actually, that can become a real bummer:
Copy and op= is created to do memberwise stuff since like CFront 2.0? Yeah,
getting rid of unwanted ones is not so nice yet (look C++0x for improvements
on all fronts...)
struct X {
X(X const& rhs):
a(rhs.a)
{}
int a;
int b;
};
Imagine such a structure being returned from a function.
This "problem" is a top gotcha for ~ 2 decades. And has an extremely easy
solution: do NOT write copyctor, op=. I mean it. Not in user code!
Too bad, around the same CFront 2.0 the suggestion went the other way to
mitigate some broken *bitwise* copying compilers. With just homegrown stuff
in early '90-s it may be even justified, but since we have standard, a deal
good libs, and RAII being the norm for ages -- what is really the motivation
to write cctor by hand?
When you do it, omission is only one of the problems, you drag a full basket
of other issues. Slowly "fixing" until the motivator gets out in a base
class or member -- then just to discover you could START with using a smart
pointer or vector or string instead of a fraking raw pointer... ;-o
My experience is to just have two basic cases in user code: forbid cctor and
op= or leave the auto-created. Exceptions are in the 'writing library'
land, where it is increasingly hard to find a case of a new resource or
collection not already covered.
If RVO is in
effect, the caller will receive the correct data.
It's plain case of UB. The other usual screw variant is to list all the
members -- but forget the base class. That will pick up default init --
likely "work" just do not what ecpected.
Been there, done that, wasted lots of time.
And your conclusions on mitigation?
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