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| Subject | A lightweight module for class extensions in D |
| From | Gregor Richards <Richards@codu.org> |
| Date | Fri, 05 Dec 2008 11:37:57 -0500 |
| Newsgroups | digitalmars.D |
| Attachment(s) | extensions.d |
I ran into a situation where I needed (essentially) the visitor pattern, but the visitor pattern sucks, so I wanted to make something like class extensions instead (that is, methods added to a class outside of the class definition). Of course, it's not possible to do this particularly cleanly, but I made a system that works (albeit using gross string mixins). Essentially, if you have a class A, class B : A, class C : B, you could do something like this: mixin(extensions("A", "void", "doFoo", "", "")); mixin(extend("A", "doFoo")); void A_doFoo(A pthis) { /* method for A */ } mixin(extend("B", "doFoo")); void B_doFoo(B pthis) { /* method for B */ } Then if you call doFoo(new A()) the call will become A_doFoo(new A()), if you call doFoo(new B()) the call will become B_doFoo(new B()), if you call doFoo(new C()) the call will become B_doFoo(new C()). If anybody has some improvements, that'd be cool. Maybe you can get rid of the dependence on string mixins ... but I don't think templates quite cut it. - Gregor Richards | |
/** * A lightweight system for class extensions in D. * * Copyright (C) Gregor Richards, 2008 * * License: * Permission is hereby granted, free of charge, to any person obtaining a copy * of this software and associated documentation files (the "Software"), to deal * in the Software without restriction, including without limitation the rights * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell * copies of the Software, and to permit persons to whom the Software is * furnished to do so, subject to the following conditions: * * The above copyright notice and this permission notice shall be included in * all copies or substantial portions of the Software. * * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN * THE SOFTWARE. */ /* Use: * To make a function int foo(int a, int b) which extends the class A: * mixin(extension("A", "int", "foo", "int a, int b", "a, b")); * * Then to declare a function as an extension for A or a subclass: * int A_foo(A pthis, int a, int b) { ... } * mixin(extend("A", "foo")); * * To call the function, use the object as the first argument: * return foo(a, 1, 2); */ module extensions; /// Maximum extension number currently int extensionCount = 0; /// Basic generator for extensions char[] extension(char[] cl, char[] rtype, char[] fun, char[] args, char[] sargs) { if (args != "") args = ", " ~ args; if (sargs != "") sargs = ", " ~ sargs; char[] ret = `void*[ClassInfo] __ext_` ~ fun ~ `; ` ~ rtype ~ ` ` ~ fun ~ `(` ~ cl ~ ` pthis` ~ args ~ `) { // go through super-classes until we lose or find the extension ClassInfo ci = pthis.classinfo; while (ci !is null) { void** ext = ci in __ext_` ~ fun ~ `; if (ext !is null) { void* f = *ext; /* found it! */`; if (rtype != "void") ret ~= "return "; ret ~= `(cast(` ~ rtype ~ ` function(` ~ cl ~ args ~ `)) f)(pthis` ~ sargs ~ `);`; if (rtype == "void") ret ~= "return;"; ret ~= ` } ci = ci.base; } throw new ExtensionLookupFailure(); }`; return ret; } /// To make a specific extension char[] extend(char[] cl, char[] fun) { return `class __ext_` ~ cl ~ `_` ~ fun ~ `_staticthis { static this() { __ext_` ~ fun ~ `[` ~ cl ~ `.classinfo] = cast(void*) &` ~ cl ~ `_` ~ fun ~ `; } }`; } class ExtensionLookupFailure : Exception { this() { super("ExtensionLookupFailure"); } } | |
A lightweight module for class extensions in D (Current message)
