[Assorted-commits] SF.net SVN: assorted:[1197] sandbox/trunk/src/cc/tmpl_fn_ptrs.cc

[<yan...@us...>]

Revision: 1197
          http://assorted.svn.sourceforge.net/assorted/?rev=1197&view=rev
Author:   yangzhang
Date:     2009-02-18 18:31:55 +0000 (Wed, 18 Feb 2009)

Log Message:
-----------
added question submitted to stackoverflow

Added Paths:
-----------
    sandbox/trunk/src/cc/tmpl_fn_ptrs.cc

Added: sandbox/trunk/src/cc/tmpl_fn_ptrs.cc
===================================================================
--- sandbox/trunk/src/cc/tmpl_fn_ptrs.cc	                        (rev 0)
+++ sandbox/trunk/src/cc/tmpl_fn_ptrs.cc	2009-02-18 18:31:55 UTC (rev 1197)
@@ -0,0 +1,46 @@
+    // How do I do set templated function pointers?
+    //
+    // See <http://stackoverflow.com/questions/560322/how-else-to-achieve-templated-function-pointers>.
+
+    #include <iostream>
+    using namespace std;
+
+    template<typename T> void write0(T msg) { cout << "write0: " << msg.name() << endl; }
+    template<typename T> void write1(T msg) { cout << "write1: " << msg.name() << endl; }
+
+    // This isn't so bad, since it's just a conditional (which the processor will
+    // likely predict correctly most of the time).
+    bool use_write0;
+    template<typename T> void write(T msg) { if (use_write0) write0(msg); else write1(msg); }
+
+    struct MsgA { const char *name() { return "MsgA"; } };
+    struct MsgB { const char *name() { return "MsgB"; } };
+    struct MsgC { const char *name() { return "MsgC"; } };
+    struct MsgD { const char *name() { return "MsgD"; } };
+
+    // This doesn't work: templates may not be virtual.
+    #if 0
+    struct Writer { template<typename T> virtual void write(T msg) = 0; };
+    struct Writer0 { template<typename T> virtual void write(T msg) { cout << "write0: " << msg.name() << endl; } };
+    struct Writer1 { template<typename T> virtual void write(T msg) { cout << "write0: " << msg.name() << endl; } };
+    #endif
+
+    int main(int argc, char **argv) {
+      use_write0 = argc == 1;
+
+      // I can do this:
+      write(MsgA());
+
+      // Can I achieve the following without the verbosity (manual setup, named
+      // template instantiations, etc.)?
+      void (*pwriteA)(MsgA) = use_write0 ? (void(*)(MsgA)) write0<MsgA> : (void(*)(MsgA)) write1<MsgA>;
+      void (*pwriteB)(MsgB) = use_write0 ? (void(*)(MsgB)) write0<MsgB> : (void(*)(MsgB)) write1<MsgB>;
+      void (*pwriteC)(MsgC) = use_write0 ? (void(*)(MsgC)) write0<MsgC> : (void(*)(MsgC)) write1<MsgC>;
+      void (*pwriteD)(MsgD) = use_write0 ? (void(*)(MsgD)) write0<MsgD> : (void(*)(MsgD)) write1<MsgD>;
+      pwriteA(MsgA());
+      pwriteB(MsgB());
+      pwriteC(MsgC());
+      pwriteD(MsgD());
+
+      return 0;
+    }


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