2017-05-08 21:01 GMT+02:00 Joaquin M López Muñoz via Boost < boost@lists.boost.org>:
El 08/05/2017 a las 16:55, Andrzej Krzemienski via Boost escribió:
In the class reference, I get: [...]
(1) c.insert(x) (2) c.insert(it,x)
Effects: If the type of the subobject of t is not registered, then if this type is T and T is acceptable, does the registration, otherwise throws. If t is not a non-const rvalue expression and the type of its subobject is not CopyConstructible, throws. Inserts an element with a subobject move constructed or copy constructed from the subobject of t: (1) at the end of the corresponding segment; (2) just before the position indicated by it, if it points to the corresponding segment, or at the end of the segment otherwise.
[...]
And maybe it is just me, but I cannot make the sense of it. First, it refers to 't' but in the signatures we have an 'x'. Next, the first sentence has two "if"s and one "otherwise" and I do not know which "if" the "otherwise" binds to. I read it as saying: if T is derived from the interface (like class `warrior`) but is not movable (which implies that T is not "acceptable"), then the program will compile fine but will later throw an exception. Is my interpretation correct? But if so, why not refuse to compile instead?
First of all the 't' bit is indeed a typo, it should be 'x'. As for the intended meaning of this admittedly awkard clause, this is:
1. If the type of the subobject of x is not registered, then: 1.1. if T is acceptable, does the registration, otherwise throws. 2. If x is not a non-const rvalue expression and the type of its subobject is not CopyConstructible, throws. 3. Inserts an element constructed or copy constructed from the subobject of x, etc.
So, going to your example, consider
boost::base_collection<sprite> c; warrior w; // warrior is not moveable c.insert(w);
This throws because the "real" (most derived) type of w is warrior and warrior is not acceptable: why not just mark this as a compile error? Because of the following:
boost::base_collection<sprite> c; moveable_warrior mw; // derived from warrior and moveable c.register_types<moveable_warrior>(); // fine, moveable_warrior can be registered warrior& w=mw; c.insert(w); // works!!
Insertion is succesful because the real type of w is a valid (and indeed registered) type. So we can't just SFINAE-supress warrior from the overload set of insert(). Did I make my self clear?
I can try to reword the reference sections into a more digestible form. Thanks for spotting the typo!
Joaquín M López Muñoz
Thanks for the explanation. I get it now. Could I suggest some rewording of the description of insert. Maybe first define something like 'ad-hoc registration': 'ad-hoc registration' of a reference `t` of a static type `T` into
collection `c` is defined as follows: * If the dynamic type of `t` is already registered in `c`: nothing, otherwise * if the dynamic type of `t` is `T` and `T` is acceptable in `c`: registers `T`, otherwise * throws exception of type `unregistered_type`.
Then in the effects clause of `insert` just use it: Effects: performs ad-hoc registration of `t` into `*this`. Then, If `t` is
not a ...
Regards, &rzej;