From 97928d136a15b30954c4bf7d4b57ba62301a830e Mon Sep 17 00:00:00 2001
From: Chris Lattner <sabre@nondot.org>
Date: Fri, 12 Nov 2010 00:19:41 +0000
Subject: describe the preferred approach to silencing 'unused variable
 warnings' due to asserts.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@118863 91177308-0d34-0410-b5e6-96231b3b80d8
---
 docs/CodingStandards.html | 32 ++++++++++++++++++++++++++++++++
 1 file changed, 32 insertions(+)

(limited to 'docs')

diff --git a/docs/CodingStandards.html b/docs/CodingStandards.html
index e3ec3ff..416c29b 100644
--- a/docs/CodingStandards.html
+++ b/docs/CodingStandards.html
@@ -851,6 +851,38 @@ return 0;
 </pre>
 </div>
 
+<p>Another issue is that values used only by assertions will produce an "unused
+ value" warning when assertions are disabled.  For example, this code will warn:
+</p>
+
+<div class="doc_code">
+<pre>
+  unsigned Size = V.size(); 
+  assert(Size &gt; 42 &amp;&amp; "Vector smaller than it should be");
+
+  bool NewToSet = Myset.insert(Value);
+  assert(NewToSet &amp;&amp; "The value shouldn't be in the set yet");  
+</pre>
+</div>
+
+<p>These are two interesting different cases: in the first case, the call to 
+V.size() is only useful for the assert, and we don't want it executed when
+assertions are disabled.  Code like this should move the call into the assert
+itself.  In the second case, the side effects of the call must happen whether
+the assert is enabled or not.  In this case, the value should be cast to void
+to disable the warning.  To be specific, it is preferred to write the code
+like this:</p>
+
+<div class="doc_code">
+<pre>
+  assert(V.size() &gt; 42 &amp;&amp; "Vector smaller than it should be");
+
+  bool NewToSet = Myset.insert(Value); (void)NewToSet;
+  assert(NewToSet &amp;&amp; "The value shouldn't be in the set yet");  
+</pre>
+</div>
+
+
 </div>
 
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