David Abrahams wrote:
[...]
>>> Example:
>>> optional<int> i;
>>>
>>> new (i) int(17);
>>
>>
>> Ex. 1:
>> // Class Widget has a heavy copy constructor
>> optional<Widget> i;
>>
>> new (i) Widget(this, ...);
>
>
> Soo... what are you demonstrating here? Please spell it out. Normal
> construction of a widget in optional<Widget> incurs a copy?
Yes, it incurs a copy. Maybe struct A is not a good example, but
TrollTech's QT widgets are.
struct A
{
A(int) { cout << "A(int)" << endl; }
A(A const &) { cout << "A(A const &)" << endl; }
};
int main()
{
optional<A> o(A(9));
}
[...]
> Soo... what are you demonstrating here? Please spell it out. The
> bool in optional that indicates initialization incurs space/alignment
> overhead?
Yes, exactly. Sorry if I wasn't precise enough.
The bool type will cancel type_with_alignment<> effects (at least on Intel
compatible platforms); i.e. unique alignment of each optional<T> type.
Philippe A. Bouchard
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