WIP: how to define the bool conversion / validity check for the function holders?

src/lib/meta/function-erasure.hpp

@@ -93,7 +93,24 @@ namespace typelist{
   
     
   /* ====== Policy classes ====== */
-  
+
+  template<class T>
+  struct BoolCheckable
+    {
+      typedef bool (T::*ValidityCheck)()  const;
+      typedef ValidityCheck _unspecified_bool_type;
+      ValidityCheck isValid;
+      
+      BoolCheckable() : isValid (&T::isValid) {}
+      
+      /** implicit conversion to "bool" */ 
+      operator _unspecified_bool_type()  const { T const& obj = *this;
+                                                 return  (obj.*isValid)()? isValid : 0; }  // never throws
+      bool operator! ()                  const { T const& obj = *this;
+                                                 return !(obj.*isValid)(); }                       //  ditto
+      
+    };
+    
   /** 
    * Policy for FunErasure: store an embedded tr1::function
    * Using this policy allows to store arbitrary complex functor objects
@@ -101,6 +118,7 @@ namespace typelist{
    * The price to pay is vtable access and heap storage of function arguments. 
    */
   class StoreFunction
+//    : public BoolCheckable<StoreFunction>
     {
       /** Helper: type erasure */
       struct Holder
@@ -155,6 +173,12 @@ namespace typelist{
           REQUIRE (INSTANCEOF (FunctionHolder<SIG>, &holder_));
           return static_cast<FunctionHolder<SIG>&> (holder_).get();
         }
+      
+      bool
+      isValid()  const
+        {
+          return reinterpret_cast<void*> (holder_.storage_[0]);
+        }
     };
   
   

src/tool/try.cpp

@@ -12,16 +12,20 @@
 // 5/08  - how to guard a downcasting access, so it is compiled in only if the involved types are convertible
 // 7/08  - combining partial specialisation and subclasses 
 // 10/8  - abusing the STL containers to hold noncopyable values
+// 6/09  - investigating how to build a mixin template providing an operator bool()
 
 
 #include <nobug.h>
 //#include "include/nobugcfg.h"
+#include "lib/meta/function-erasure.hpp"
 
 #include <iostream>
 //#include <typeinfo>
 #include <boost/format.hpp>
-//#include <boost/noncopyable.hpp>
+#include <cstdlib>
 
+
+using std::rand;
 using std::string;
 using std::cout;
 using boost::format;
@@ -32,6 +36,24 @@ using boost::format;
       long checksum = 0;
   }
   
+  struct TestIt1
+    : lumiera::typelist::BoolCheckable<TestIt1>
+    {
+      
+      int val_;
+      
+      TestIt1 (int v = (rand() % 10))
+        : val_(v)
+        { }
+      
+      bool
+      isValid()  const
+        {
+          return true;
+        }
+      
+    };
+    
   
   
 int 
@@ -39,13 +61,15 @@ main (int argc, char* argv[])
   {
     
     NOBUG_INIT;
+   
+    TestIt1 testrosteron (22);
+    
+    bool boo = true; //testrosteron;
+    if (!boo)
+      return -1;
     
     cout <<  "\n.gulp.\n";
-   
-   int * p = 0;
-
-   int oh = *p;
-
+    
     
     return 0;
   }