Changes between Version 10 and Version 11 of EFLWebKitCodingStyle

Timestamp:

Aug 13, 2012 7:31:17 PM (12 years ago)

Author:

rakuco@webkit.org

Comment:

Add a better explanation about the usage of !!s

EFLWebKitCodingStyle

@@ -10 +10 @@
  * Use '''const''' keyword instead of '''#define''' when you define constant variable.
  * Use NULL in public C headers instead of 0
- * Use '''double not operator(!!)''' when you modify or compare with a Eina Bool member variable which stores a value in a bit field.
+ * Use a double negation '''(!!)''' when you modify or compare with a Eina Bool member variable which stores a value in a bit field.
 
 = Example =
@@ -171 +171 @@
 }}}
 
-== Use double not operator(!!) when you modify or compare with a Eina Bool member variable which stores a value in a bit field. ==
+== Use a double negation when you modify or compare with a Eina Bool member variable which stores a value in a bit field. ==
+`Eina_Bool` is an `unsigned char` typedef, which means one can in theory pass a value such as 3 or 42 to it. This can be problematic if one is storing an `Eina_Bool` in a bitfield, since only the last bit will be used. For example:
+
+{{{
+#!cpp
+struct MyStruct {
+    Eina_Bool b : 1;
+};
+
+void foo()
+{
+    Eina_Bool v1 = 2, v2 = 3;
+    MyStruct s;
+
+    s.b = v1; // b is 0, since the least significant bit is 0 (2 == 10 in binary)
+    s.b = v2; // b is 1, since the least significant bit is 1 (3 == 11 in binary)
+}
+}}}
+
+On the other hand, if an `Eina_Bool` is simply being obtained from some publicly-visible API and passed to another function that takes a `bool`, or if a bitfield is made from `bool`s instead of `Eina_Bool`s, this does not happen, since, according to the C++ standard, any value that is not zero, a null pointer or a null member pointer is converted to `true`.
+
 === Right ===
 {{{
 struct {
     ...
-    bool pageCache : 1;
+    Eina_Bool pageCache : 1;
     ...
 }