define fragile union

1 files changed, 22 insertions, 2 deletions

diff --git a/language/type-definition.rst b/language/type-definition.rst
index 7b90934..04cc858 100644
--- a/language/type-definition.rst
+++ b/language/type-definition.rst
@@ -86,6 +86,26 @@ Defining Types
     However, there is no internal indication of which type of data is actually being stored in the union.
     As such, in non-trivial cases no compiler can predict which field is or is not valid, and any statement which reads a field of a fragile union must itself be a :crowbar:ref:`FragileStatement`.
 
-    .. todo::
+    The size of a fragile union is the largest size of any of its members.
+    The address of each member is the address of the union object itself.
+    The member which was most recently set will retain its value.
+    Reading another member with size no larger than the most recently set member will interpret the first bytes of the most recently set member as a value of the type of the member being read.
+
+    For example, the functions ``test1`` and ``test2`` are equivalent::
+
+        fragile union Example {
+            float32 float_data;
+            uint32 uint_data;
+        }
 
-        explain memory layout of fragile unions
+        uint32 test1(float32 arg) {
+            union Example temp;
+            temp.float_data = arg;
+            fragile return temp.uint_data;
+        }
+
+        uint32 test2(float32 arg) {
+            float32* temp = &arg;
+            fragile uint32* temp_casted = (uint32*)temp;
+            return *temp_casted;
+        }