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POBetter, simpler example of 'semantic conflict'?
 

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  1. POBetter, simpler example of 'semantic conflict'?
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    <p>I like to distinguish three different types of conflict from a version control system (VCS):</p> <ul> <li>textual</li> <li>syntactic</li> <li>semantic</li> </ul> <p>A <em>textual</em> conflict is one that is detected by the merge or update process. This is flagged by the system. A commit of the result is not permitted by the VCS until the conflict is resolved.</p> <p>A <em>syntactic</em> conflict is not flagged by the VCS, but the result will not compile. Therefore this should also be picked up by even a slightly careful programmer. (A simple example might be a variable rename by <em>Left</em> and some added lines using that variable by <em>Right</em>. The merge will probably have an unresolved symbol. Alternatively, this might introduce a <em>semantic</em> conflict by variable hiding.)</p> <p>Finally, a <em>semantic</em> conflict is not flagged by the VCS, the result compiles, but the code may have problems running. In mild cases, incorrect results are produced. In severe cases, a crash could be introduced. Even these should be detected before commit by a very careful programmer, through either code review or unit testing.</p> <p>My example of a semantic conflict uses SVN (Subversion) and C++, but those choices are not really relevant to the essence of the question.</p> <p>The base code is:</p> <pre><code>int i = 0; int odds = 0; while (i &lt; 10) { if ((i &amp; 1) != 0) { odds *= 10; odds += i; } // next ++ i; } assert (odds == 13579) </code></pre> <p>The Left (<code>L</code>) and Right (<code>R</code>) changes are as follows.</p> <p><em>Left</em>'s 'optimisation' (changing the values the loop variable takes):</p> <pre><code>int i = 1; // L int odds = 0; while (i &lt; 10) { if ((i &amp; 1) != 0) { odds *= 10; odds += i; } // next i += 2; // L } assert (odds == 13579) </code></pre> <p><em>Right</em>'s 'optimisation' (changing how the loop variable is used):</p> <pre><code>int i = 0; int odds = 0; while (i &lt; 5) // R { odds *= 10; odds += 2 * i + 1; // R // next ++ i; } assert (odds == 13579) </code></pre> <p>This is the result of a merge or update, and is not detected by SVN (which is correct behaviour for the VCS), so it is not a textual conflict. Note that it compiles, so it is not a syntactic conflict.</p> <pre><code>int i = 1; // L int odds = 0; while (i &lt; 5) // R { odds *= 10; odds += 2 * i + 1; // R // next i += 2; // L } assert (odds == 13579) </code></pre> <p>The <code>assert</code> fails because <code>odds</code> is 37.</p> <p>So my question is as follows. Is there a simpler example than this? Is there a simple example where the compiled executable has a new crash?</p> <p>As a secondary question, are there cases of this that you have encountered in real code? Again, simple examples are especially welcome.</p>
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    1. COThis is not particularly relevant to C++, so I have removed that tag. And to be honest, I don't understand the question and don't see whatt it has to do with version control.
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    2. COrhubbarb probably wants to hear other people's experiences regarding these unnoticed changes in semantics so he could build some kind of checklist of things to watch out for when merging code. Might turn out to be an interesting topic if that is in fact what the point is.
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      1. POBetter, simpler example of 'semantic conflict'?
      1. USTomislav Nakic-Alfirevic
    3. CO@Tomislav: yes, that's one of the reasons. @Neil: I agree with your removal of the C++ tag; my mistake. On the other hand, this has *everything* to do with version control. Merge operations are central to VCS, and it is also important to be aware of ways in which these very helpful systems can occasionally have unexpected results.
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