[cdd1695] | 1 | Proposal to add simple inhieritance to the language. |
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| 2 | |
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| 3 | Tagged structures allow for dynamic casting between types in a hierarchy. |
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| 4 | Children (rather pointers to) can be up-cast to their parents, a safe |
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[974bcdd] | 5 | conversion that may recive language level support or even be implicit. |
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[cdd1695] | 6 | Parents can be down cast to their children, which might fail if the underlying |
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| 7 | object is not of the child type, or a child of that. |
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| 8 | |
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| 9 | This does not however cause dynamic look-up. During function calls the |
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| 10 | underlying type is ignored, and the pointer type is used to type match the |
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| 11 | function call. |
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| 12 | |
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[1a42132] | 13 | The name tagged structure comes from tagged union, which carries a value to |
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| 14 | say which of the possible values is currently stored in the union. The idea |
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| 15 | here is similar, however the possibilities are more open ended. |
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[cdd1695] | 16 | |
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| 17 | |
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| 18 | Syntax: |
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| 19 | |
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| 20 | "struct" name [ "tagged" [ parent-name ] ] "{" fields "}" |
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| 21 | |
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| 22 | The keywords can change (although they currently reflect the concept name |
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[974bcdd] | 23 | closely). More formally, in terms of grammar this adds: |
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[cdd1695] | 24 | |
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| 25 | struct-or-union-specifier |
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| 26 | ... |
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| 27 | struct identifier tagged { struct-declaration-list } |
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| 28 | struct identifier tagged parent-identifier { struct-declaration-list } |
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| 29 | |
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| 30 | "tagged" by itself create a tagged structure that is the root of a new tree. |
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| 31 | It has no parent tagged structure. If "tagged" is used with a parent than |
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| 32 | that is the parent of this node. |
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| 33 | |
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| 34 | Tagged structures have fields beyond the ones listed. Root tags have a type |
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| 35 | field added which give the type of the instance. Child tags prepend all of |
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| 36 | their parent's fields to their field list so they can be upcast. |
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| 37 | |
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| 38 | |
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| 39 | Implemenation: |
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| 40 | |
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| 41 | Adding to the field list is a simple matter, should be doable during |
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| 42 | translation. The type field is just a pointer to a type object. With proper |
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| 43 | linking we can create a single unique instance of the type object for each |
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| 44 | declared tagged struct. The instance's address is used as an id for the type. |
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| 45 | It also holds data about the type, such as its parent's id/a pointer to the |
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| 46 | parent type object. |
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| 47 | |
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[1a42132] | 48 | The type field could be hidden (as best as C can hide it) or it could be |
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| 49 | visible to the user with easy access to allow the user to examine the type |
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| 50 | object directly. |
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[cdd1695] | 51 | |
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[1a42132] | 52 | Direct access is more useful if the data on the type-objects can change, other |
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| 53 | wise the build in function could handle all cases. Perhaps each root object |
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| 54 | can specify a type object to use or the type objects are themselves tagged, |
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| 55 | although there may not be a base case with the latter. |
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[cdd1695] | 56 | |
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[1a42132] | 57 | In the simplest case the type object is a pointer to the parent type object. |
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| 58 | Additional data could be added, such as a name, or a function pointer to the |
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| 59 | destructor. |
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[cdd1695] | 60 | |
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| 61 | |
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| 62 | Traits: |
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| 63 | |
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| 64 | [is_]tagged[_struct](dtype T) |
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| 65 | True if the given T is a tagged struct of some kind. This promises that it has |
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| 66 | a type object, but nothing else. |
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| 67 | |
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| 68 | [is_]tagged_under(dtype parent, dtype child) |
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| 69 | True if child is a child type of parent. Requires that both are tagged structs |
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| 70 | and that child can upcast to parent. |
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| 71 | |
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| 72 | |
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| 73 | Functions: |
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| 74 | |
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| 75 | forall(dtype T | is_tagged(T), dtype U | is_tagged(U)) |
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| 76 | T * dynamic_cast(U * value) |
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| 77 | The cast function, that safely converts the U* into a T*, returning null if |
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| 78 | the underlying object value points to is not a child type of T. A shorter name |
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| 79 | might be perfered. The runtime should be no more than linear with the depth |
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| 80 | of U in the inhiertance tree. |
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| 81 | |
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| 82 | bug#11 might require `bool dynamic_cast(T ** dst, U * src)` instead. |
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[1a42132] | 83 | |
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| 84 | |
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| 85 | Tagging Unions (Extention): |
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| 86 | |
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| 87 | Using this system as is does not really work if used on unions directly. |
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| 88 | No new options to the union can be added, as they must be able to upcast. |
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| 89 | Similarly, if options are removed, writing to an upcast union is invalid. |
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| 90 | To allow for growth each option would have to be a structure itself. |
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| 91 | |
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| 92 | Which brings us to "tagget struct union", ie. a union of tagged structures |
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| 93 | as opposed to tagging the union itself. This extention acts as a constraint. |
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| 94 | If unions are declared tagged instead of creating a new tagged type, all |
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| 95 | possible values of the union must be of that tagged type or a child type. |
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