Implement a cast_to_float method for convenience; at a cost of a couple of bytes in the vtable this will save many callers having to play type introspection games.

It might be possible to implement a set of virtual cast operator overrides, but only float lets us return NAN to indicate that a cast can't be made, so for now we'll stick with just this one.

git-svn-id: https://arducopter.googlecode.com/svn/trunk@1614 f9c3cf11-9bcb-44bc-f272-b75c42450872

libraries/AP_Common/AP_Var.cpp

@@ -19,6 +19,7 @@ extern "C" { extern void delay(unsigned long); }
 
 #include <AP_Common.h>
 
+#include <math.h>
 #include <string.h>
 
 // Global constants exported for general use.
@@ -313,6 +314,14 @@ AP_Var::key(void)
     return var_header.key;
 }
 
+// Default implementation of cast_to_float, which always fails.
+//
+float
+AP_Var::cast_to_float(void) const
+{
+    return NAN;
+}
+
 // Return the next variable in the global list.
 //
 AP_Var *

libraries/AP_Common/AP_Var.h

@@ -335,6 +335,17 @@ public:
     ///
     Key key(void);
 
+    /// Casts the value to float, if possible.
+    ///
+    /// This is a convenience method for code that would rather not try to
+    /// meta-cast to the various subtypes.  It cannot guarantee that a useful value
+    /// will be returned; the caller must check the returned value against NAN
+    /// before using it.
+    ///
+    /// @return             The value cast to float, or NAN if it cannot be cast.
+    ///
+    virtual float cast_to_float() const;
+
 private:
     AP_Var_group        *_group;            ///< Group that the variable may be a member of
     AP_Var              *_link;             ///< linked list pointer to next variable
@@ -547,10 +558,23 @@ public:
         return *this;
     }
 
+    /// AP_VarT types can implement AP_Var::cast_to_float
+    ///
+    virtual float cast_to_float() const;
+
 protected:
     T _value;
 };
 
+// Implement AP_Var::cast_to_float.
+//
+template<typename T>
+float
+AP_VarT<T>::cast_to_float() const
+{
+    return (float)_value;
+}
+
 /// Template class for array variables.
 ///
 /// Objects created using this template behave like arrays of the type T,
@@ -671,7 +695,6 @@ protected:
     T _value[N];
 };
 
-
 /// Convenience macro for defining instances of the AP_VarT template.
 ///
 #define AP_VARDEF(_t, _n)   typedef AP_VarT<_t> AP_##_n;