[poly_collection] throws upon insertion
Hi, I am looking at the documentation of poly_collection. I will not be able to thoroughly review and test the library. But judging from the documentation, the problem is worth solving, and the approach to solve it is sound. I have one question/observation though. I am trying to figure out how preventing an incorrect insert() works. In the docs ( http://rawgit.com/joaquintides/poly_collection/website/doc/html/poly_collect...) it says that it will sometimes throw if it cannot determine the most derived type. In the class reference, I get: ( http://rawgit.com/joaquintides/poly_collection/website/doc/html/poly_collect... ) ``` (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. Returns: An iterator to the newly inserted element. Complexity: Amortized constant time plus linear in the distance from the insertion position to the end of the segment. ``` 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? Maybe it would be better tom make the prose longer in "Effects" clause, but to make it harder for misinterpretation. Regards, &rzej;
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
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
(); // 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;
El 09/05/2017 a las 10:14, Andrzej Krzemienski via Boost escribió:
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 ...
Thanks for the suggestion, I'll write something along that line. ("Dynamic type" can't be used as the reference is common to all types of collections, and only base_collection really uses dynamic_cast). Joaquín M López Muñoz
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Andrzej Krzemienski -
Joaquin M López Muñoz