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    <p>They are correct, it is undefined behaviour. You're not allowed to modify the value of a <code>const</code> variable, which is the danger of casting away the <code>const</code>ness of something: you better <em>know</em> it's not really <code>const</code>.</p> <p>The compiler, seeing that <code>maxint</code> is <code>const</code> and should <em>never</em> be modified, doesn't even have to give it an address. It can just replace all the uses of <code>maxint</code> with 100 if it sees fit. Also it might just put the constant in to a portion of memory that is read-only, as Matteo Italia points out, which is probably what's happening for you. That's why modifying it produces undefined behaviour.</p> <p>The only way you can safely cast away the <code>const</code>ness of a variable is if the variable is not actually <code>const</code>, but the <code>const</code> qualifier was added to a non-<code>const</code> variable, like this:</p> <pre><code>int f(const int&amp; x) { int&amp; nonconst = const_cast&lt;int&amp;&gt;(x); ++nonconst; } int blah = 523; f(blah); // this is safe const int constblah = 123; f(constblah); // this is undefined behaviour </code></pre> <p>Think about this example, which compiles perfectly:</p> <pre><code>int f(const int&amp; x) { int&amp; nonconst = const_cast&lt;int&amp;&gt;(x); ++nonconst; } int main() { f(4); // incrementing a number literal??? } </code></pre> <p>You can see how using <code>const_cast</code> is pretty dangerous because there's no way to actually tell whether a variable is originally <code>const</code> or not. You should avoid using <code>const_cast</code> when possible (with functions, by just not accepting <code>const</code> parameters).</p>
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    1. POconst cast to a global var and program crashed (C++)
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    1. USSeth Carnegie
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    1. COI'd say that here the compiler gave an address to the constant, but put it into a read-only segment.
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      1. USMatteo Italia
    2. COThank you! But i still got a question: why the local const variable after const_cast will not cause the program crash?
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    3. CO@OriginalWood undefined behaviour is just that - undefined. It is not defined to crash. It can do anything, from working, to crashing, to setting the atmosphere on fire. In that case, it just happened to work.
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