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| Subject | Forward declarations of template specilizations. | ||||||||||||||||||||||||
| From | Ryan Bloomfield <_sir_maniacREMOVE_ME@yahoo.com> | ||||||||||||||||||||||||
| Date | Fri, 14 Nov 2008 15:30:38 -0500 | ||||||||||||||||||||||||
| Newsgroups | digitalmars.D | ||||||||||||||||||||||||
I have an interesting issue, that makes me curious on how D handles it. consider the following: ====================== module List; template NodeTraits(NodeType) { static NodeType getNext(NodeType v) { return v.next; } } class List(NodeType, Traits=NodeTraits!(NodeType)) { private NodeType head; NodeType next() { return Traits.getNext(head); } } ====================== module main; class Node { Node myNext; } template NodeTraits(NodeType : C) { static NodeType getNext(NodeType v) { return v.myNext; } } main() { alias List!(Node) L; L l = new L(); C next = l.next(); // error: no property 'next' for type 'main.Node' } ============================================== Templates are evaluated in the scope they are defined, which makes sense, but when should the compiler check those definitions? It makes sense that given 2 modules, one module shouldn't be able to change the behavior of another module, having said that, c++ does allow this, and it proves very useful in customizing the behavior of library classes. I noticed that if I change it to the following it works correctly(Traits argument becomes an alias, and is passed in main): ====================== module List; template NodeTraits(NodeType) { static NodeType getNext(NodeType v) { return v.next; } } class List(NodeType, alias Traits) { private NodeType head; NodeType next() { return Traits.getNext(head); } } ====================== module main; class Node { Node myNext; } template NodeTraits(NodeType : Node) { static NodeType getNext(NodeType v) { return v.myNext; } } main() { alias NodeTraits!(Node) Traits; // a template's alias arguments only take a single identifier alias List!(Node, Traits) L; L l = new L(); C next = l.next(); // Successfully accesses l.myNext } ============================================== Does the evaluation of a template have to know only the scope it was instantiated in? No forward declaration(of a module that imports it)? | |||||||||||||||||||||||||
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Forward declarations of template specilizations. (Current message)
