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    <p><code>stacks.end();</code> refers to the (nonexistent) element after the end of the vector. You can't dereference it; doing so will cause undefined behaviour, possibly a segmentation fault.</p> <p>It's not quite clear what you're doing there, but if you want an iterator for the last element, then either decrement it:</p> <pre><code>vector&lt;stack&lt;int&gt; &gt;::iterator it=stacks.end(); // points past the end --it; // points to last element </code></pre> <p>or use a reverse iterator (in which case, you use <code>++</code> rather than <code>--</code> to move backwards through the sequence):</p> <pre><code>vector&lt;stack&lt;int&gt; &gt;::reverse_iterator it=stacks.rbegin(); </code></pre> <p>Adding an element to a vector can invalidate it, so the <code>it-&gt;push_back(data)</code> at the end of <code>push()</code> is incorrect. You could avoid using an iterator here:</p> <pre><code>void push() { if (stacks.empty() || stacks.back().size()==stack_size) { // See below for explanation of this change stacks.push_back(stack&lt;int&gt;()); } stacks.back().push(data); } </code></pre> <p>In <code>pop()</code>, you probably want to remove the last stack if it's empty; otherwise, you'll end up with two empty stacks at the end, and your code will erroneously try to <code>pop</code> from one of those. Again, doing that could cause a segmentation fault or other undefined behavoiur. You probably want something like:</p> <pre><code>void pop() { if (stacks.empty()) { cout&lt;&lt;"\nEmpty Stack"; return ; } stacks.back().pop(); if (stacks.back().empty()) { stacks.pop_back(); } } </code></pre> <p>And now we've established an invariant that the last stack is never empty, <code>top</code> can be a bit simpler:</p> <pre><code>int top() { if (stacks.empty()) { cout&lt;&lt;"\nEmpty Stack"; return 0; } return stacks.back().top(); } </code></pre> <p>Also, you usually don't want to create objects using <code>new</code>, especially in a case like this where you're putting a copy of the object into the vector and then discarding the pointer, leaking the allocated memory. You can add an empty stack to the vector like this:</p> <pre><code>stacks.push_back(stack&lt;int&gt;()); </code></pre> <p>Sometimes (but not in this case) you might want to store pointers to allocated objects in a container; in that case, either remember to delete them when they're removed from the container, or store smart pointers such as <code>std::unique_ptr</code>. But in this case, just store objects.</p>
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