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POeasing c++ to objective-c/cocoa bridging via metaprogramming?
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  1. POeasing c++ to objective-c/cocoa bridging via metaprogramming?
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    copied!<p>In a pure C++ world we can generate interfacing or glue code between different components or interfaces at compile time, using a combination of template-based compile-time and runtime-techniques <em>(to e.g. mostly automatically marshall to/from calls using legacy types)</em>. </p> <p>When having to interface C++ applications with Objective-C/Cocoa for GUI, system integration or IPC though, things become harder due to the less strict typing - yet often not more then a flat repitive interface layer is needed: thin bridging delegates have to be defined or conversion code to language bridging calls has to be written. </p> <p>If you have to deal with interfaces of non-trivial size and want to avoid script-based code generation this quickly becomes cumbersome and is just a pain every time refactorings have to take place. Using a combination of (template) metaprogramming and the Objective-C runtime library, it should be possible to reduce the amount of code considerably... </p> <p>Before i go to reinvent the wheel <em>(and possibly waste time)</em>, does anyone know about techniques, best-practices or examples in that direction?</p> <hr> <p>As for an example, lets say we need a delegate that supports this informal protocol:</p> <pre><code>- (NSString*)concatString:(NSString*)s1 withString:(NSString*)s2; - (NSNumber*) indexOf:(CustomClass*)obj; </code></pre> <p>Instead of implementing an Obj-C class now that explicitly bridges to a C++-instance, i'd like to do <em>something like this</em> instead:</p> <pre><code>class CppObj { ObjcDelegate m_del; public: CppObj() : m_del(this) { m_del.addHandler &lt;NSString* (NSString*, NSString*)&gt; ("concatString", &amp;CppObj::concat); m_del.addHandler &lt;NSNumber* (CustomClass*)&gt; ("indexOf", &amp;CppObj::indexOf); } std::string concat(const std::string&amp; s1, const std::string&amp; s2) { return s1.append(s2); } size_t indexOf(const ConvertedCustomClass&amp; obj) { return 42; } }; </code></pre> <p>All that should be needed from the user to support additional types would be to specialize a conversion template function:</p> <pre><code>template&lt;class To, class From&gt; To convert(const From&amp;); template&lt;&gt; NSString* convert&lt;NSString*, std::string&gt;(const std::string&amp; s) { // ... } // ... </code></pre> <p>The example above of course does ignore support for formal protocols etc. but should get the point across. Also, due to the type-information for Objc-runtime-types being mostly decayed into <em>some-native-types or class-type</em> i don't think the explicit specification of parameter and return types for the delegate-methods can be avoided.</p>
 

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