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Re: MSC problems: pick_ != const [message #20932 is a reply to message #20931]

Mon, 20 April 2009 00:25

mirek
Messages: 14267
Registered: November 2005

Ultimate Member

Mindtraveller wrote on Sun, 19 April 2009 17:07

For example, this code causes error:

struct AOp : Moveable<AOp>
{
	One<AOpHardware> hardware;
};

class AOps : public Vector<AOp>
{
public:
	void Xmlize(XmlIO xml) {XmlizeContainer(xml, "aop", *this);}
};

This is caused by the fact that pick != const in MSC compiler. And somewhere within U++ Core we have copying of Vector member with const argument (not pick_!):

//Core/Topt.h @ 135
template <class T>
inline void DeepCopyConstruct(void *p, const T& x) {
	::new(p) T(x);
}

This is strange. Class One supports pick behaviour, so accroding to U++ principles this code should compile without errors.
Am I right?

Yes and no.

pick definitely is not equal to semantics of const. (const guarantees no change).

That, after all, is why there is #define pick_...

Anyway, we are sort of at odds with C++ standard here as we would like to have pick_ constructors used when returning containers from functions. According to C++ standard, this is only possible if they are const (the critical rule is "nonconst references cannot be bound to temporaries").

Surprisingly, MSC++ has relaxed rules (you can call it a bug) w.r.t. to this, probably because some old MFC code was breaking the rule too. As we know when we are compiling with MSC, I decided make #define pick_ empty in that case; it better matches wanted pick_ semantics and is able to catch a bug here are there, like if you are trying to use DeepCopyConstruct on pick type.

Mirek

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