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POImplementing function delegates in C with unions and function pointers
 

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    <p>I'd like to be able to <em>generically pass a function to a function</em> in C. I've used C for a few years, and I'm aware of the barriers to implementing proper closures and higher-order functions. It's almost insurmountable.</p> <p>I scoured StackOverflow to see what other sources had to say on the matter:</p> <ul> <li><a href="https://stackoverflow.com/questions/2535631/higher-order-functions-in-c">higher-order-functions-in-c</a></li> <li><a href="https://stackoverflow.com/questions/10405436/anonymous-functions-using-gcc-statement-expressions">anonymous-functions-using-gcc-statement-expressions</a></li> <li><a href="https://stackoverflow.com/questions/1023261/is-there-a-way-to-do-currying-in-c">is-there-a-way-to-do-currying-in-c</a></li> <li><a href="https://stackoverflow.com/questions/12991014/functional-programming-currying-in-c-issue-with-types">functional-programming-currying-in-c-issue-with-types</a></li> <li><a href="https://stackoverflow.com/questions/5524299/emulating-partial-function-application-in-c">emulating-partial-function-application-in-c</a></li> <li><a href="https://stackoverflow.com/questions/1650379/fake-anonymous-functions-in-c">fake-anonymous-functions-in-c</a></li> <li><a href="https://stackoverflow.com/questions/894167/functional-programming-in-c-with-macro-higher-order-function-generators">functional-programming-in-c-with-macro-higher-order-function-generators</a></li> <li><a href="https://stackoverflow.com/questions/14160516/higher-order-functions-in-c-as-a-syntactic-sugar-with-minimal-effort">higher-order-functions-in-c-as-a-syntactic-sugar-with-minimal-effort</a></li> </ul> <p>...and none had a silver-bullet generic answer, outside of either using varargs or assembly. I have no bones with assembly, but if I can efficiently implement a feature in the host language, I usually attempt to.</p> <h3>Since I can't have HOF easily...</h3> <p>I'd love higher-order functions, but I'll settle for delegates in a pinch. I suspect that with something like the code below I could get a workable <em>delegate implementation</em> in C.</p> <p>An implementation like this comes to mind:</p> <pre><code>enum FUN_TYPES { GENERIC, VOID_FUN, INT_FUN, UINT32_FUN, FLOAT_FUN, }; typedef struct delegate { uint32 fun_type; union function { int (*int_fun)(int); uint32 (*uint_fun)(uint); float (*float_fun)(float); /* ... etc. until all basic types/structs in the program are accounted for. */ } function; } delegate; </code></pre> <p>Usage Example:</p> <pre><code>void mapint(struct fun f, int arr[20]) { int i = 0; if(f.fun_type == INT_FUN) { for(; i &lt; 20; i++) { arr[i] = f.function.int_fun(arr[i]); } } } </code></pre> <p><br /></p> <p>Unfortunately, there are some obvious downsides to this approach to delegates:</p> <ul> <li>No type checks, save those which you do yourself by checking the 'fun_type' field.</li> <li>Type checks introduce extra conditionals into your code, making it messier and more branchy than before.</li> <li>The number of (safe) possible permutations of the function is limited by the size of the 'fun_type' variable.</li> <li>The enum and list of function pointer definitions would have to be machine generated. Anything else would border on insanity, save for trivial cases.</li> <li>Going through ordinary C, sadly, is not as efficient as, say a <em>mov -> call</em> sequence, which could probably be done in assembly (with some difficulty).</li> </ul> <p><br /></p> <h2>Does anyone know of a better way to do something like delegates in C?</h2> <p><strong>Note:</strong> The more portable and efficient, the better</p> <p><strong>Also, Note:</strong> I've heard of <a href="http://www.codeproject.com/Articles/7150/Member-Function-Pointers-and-the-Fastest-Possible" rel="nofollow noreferrer">Don Clugston's very fast delegates</a> for C++. However, I'm not interested in C++ solutions--just C .</p>
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    1. CO"Type checks introduce extra conditionals into your code, making it messier and more branchy than before." That's why a union of function pointer types doesn't make much sense, since you already have the enum, you could simply pass that to a switch statement, which in turn calls the appropriate function. Also, you don't seem to have considered where the parameters and return values fit in with all of this. And finally, as an universal programming rule of thumb: generic programming always makes the code slower.
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    2. COYou're right, I don't have an easy answer for return types. As for parameters, the most likely way to generically solve that in C would be with yet another union of the possible types. Yeah, it's ugly as sin. And when I looked at the generated assembly output for the posted approach, I saw that it was far less efficient than I would have liked it to be. Thus why I asked for suggestions on better approaches.
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