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    <p>The problem is that variables declared in one <code>case</code> are still visible in the subsequent <code>case</code>s unless an explicit <code>{ }</code> block is used, <em>but they will not be initialized</em> because the initialization code belongs to another <code>case</code>.</p> <p>In the following code, if <code>foo</code> equals 1, everything is ok, but if it equals 2, we'll accidentally use the <code>i</code> variable which does exist but probably contains garbage.</p> <pre><code>switch(foo) { case 1: int i = 42; // i exists all the way to the end of the switch dostuff(i); break; case 2: dostuff(i*2); // i is *also* in scope here, but is not initialized! } </code></pre> <p>Wrapping the case in an explicit block solves the problem:</p> <pre><code>switch(foo) { case 1: { int i = 42; // i only exists within the { } dostuff(i); break; } case 2: dostuff(123); // Now you cannot use i accidentally } </code></pre> <h2>Edit</h2> <p>To further elaborate, <code>switch</code> statements are just a particularly fancy kind of a <code>goto</code>. Here's an analoguous piece of code exhibiting the same issue but using a <code>goto</code> instead of a <code>switch</code>:</p> <pre><code>int main() { if(rand() % 2) // Toss a coin goto end; int i = 42; end: // We either skipped the declaration of i or not, // but either way the variable i exists here, because // variable scopes are resolved at compile time. // Whether the *initialization* code was run, though, // depends on whether rand returned 0 or 1. std::cout &lt;&lt; i; } </code></pre>
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    1. COSee this fixed LLVM bug report for other explanations: http://llvm.org/bugs/show_bug.cgi?id=7789
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