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    <p>It's because of:</p> <pre><code>String s("test"); </code></pre> <p>What this does is to pass the <code>const char *</code> <code>"test"</code> to your constructor which then simply stores the address of that string.</p> <p>Later, when you try to modify the memory that the pointer points to, that's undefined behaviour.</p> <p>Typically, string literals will be stored in read-only memory to allow certain optimisations to take place, and any attempt to modify them will result in a access violation.</p> <p>If you were to change:</p> <pre><code>String s("test"); </code></pre> <p>into:</p> <pre><code>char cp[] = "test"; String s(cp); </code></pre> <p>you may well find it will work.</p> <p>However, your class should really be making a <em>copy</em> of the string for its own purposes - a mere pointer is unsafe, since the code that passed you that pointer can change the contents of it as well.</p> <p>Things you should change in your code to make it safer:</p> <ul> <li><code>s</code> should be a private member, not a public one.</li> <li>your constructor should make its own copy of the string:<br><code>s = new char[strlen(x)+1];</code><br><code>strcpy (s,x);</code>.</li> <li>add a destructor to take care of that as well:<br><code>String::~String() { delete[] s; }</code>.</li> <li>consider having your constructor receive a <code>const char *</code> (since you're <em>not</em> changing it).</li> <li>consider using <code>toupper(ch)</code> instead of <code>ch - 'a' + 'A'</code>. While your formula works for ASCII, I don't believe it's guaranteed by the standard.</li> <li><code>cout</code> stuff should be handled by the class rather then code outside of it (this will be mandatory once you make <code>s</code> private.</li> <li>consider having a no-argument constructor so that string arrays will work okay.</li> </ul>
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    1. COThank you all for the answer, I was not aware of this behavior.
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      1. USYesMan85
    2. CO@Rogier21: if you aren't already, you urgently need to read a book about C++. Unlike some other languages, you can't write working C++ code by guessing.
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    3. COUpdated my answer with your comment :)
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