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POMalloc pointer error with queue implementation
 

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  1. POMalloc pointer error with queue implementation
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    <p>I'm trying to implement a queue in C++. The queue holds strings.</p> <p>I'm using the following code:</p> <pre><code>#include &lt;iostream&gt; #include &lt;stdlib.h&gt; struct qnode { std::string data;//it stores a string qnode *link;//and has a pointer to the next node in the structure. }; class queue { private: qnode *front; // Pointer to first node. qnode *rear; // Pointer to last node. public: // Constructor. queue(); //Is the queue empty? bool empty() ; //peeks first element int first(std::string *c); // Adds element to the queue. int enqueue(std::string c); // Extracts an element from the queue. int dequeue(std::string *c); // Destructor. ~queue(); }; //////////// ACTUAL CODE ///////////// //Constructor, front and rear initially point to null. queue::queue() { front = NULL; rear = NULL; } // If front points to null then the queue is empty. bool queue::empty() { return (front == NULL); } int queue::first(std::string *c) { int res = 1; if (!empty()) { res = 0; *c = front-&gt;data; } return res; } //Function for adding an element to the end of queue. int queue::enqueue (std::string c){ int res = 1; qnode *tmp; // Auxiliar pointer. tmp = new qnode; //Auxiliar new node. if(tmp != NULL) { // If the queue is empty then we store the data in tmp and make its link point to null and front and rear become tmp. if(empty()){ tmp-&gt;data = c; tmp-&gt;link = NULL; front = tmp; rear = tmp; rear-&gt;link = tmp; } else { // If it is not, then last node's link point to tmp, we store the data in tmp and tmp's link point to null. rear-&gt;link = tmp; tmp-&gt;data = c; tmp-&gt;link = NULL; // Rear of the queue points to tmp now. rear = tmp; } res = 0; } return res; } // Function for extracting an element from the queue. int queue::dequeue(std::string *c){ int res = 1; // We make sure queue is not empty and access first node's data with the help of tmp. if(!empty()) { qnode *tmp; tmp = front; //The data extracted is stored in the character container c, passed by reference. *c = tmp-&gt;data; // Front now has to point to the next node and the old element has to be deleted. front = front-&gt;link; delete tmp; res = 0; } return res; } // Destructor queue::~queue() { // If it is already empty we don't have to do anything. if (empty()){ return; } qnode *tmp; // We take front element and delete it throught a tmp node till the queue is empty. while (!empty()){ tmp = front; front = front-&gt;link; delete tmp; } } //////////// MAIN /////////// int main(int argc, char *argv[]) { queue queue; std::string str2; queue.enqueue("hello"); queue.dequeue(&amp;str2); std::cout &lt;&lt; str2; } </code></pre> <p>Sadly, I get the following error:</p> <blockquote> <p>$ g++ -o issue issue.cpp $ ./issue issue(23060,0x7fff71058310) malloc: <strong>* error for object 0x7f962b403920: pointer being freed was not allocated *</strong> set a breakpoint in malloc_error_break to debug helloAbort trap: 6</p> </blockquote> <p>If I comment the queue destructor (the "while" block), the implementation works fine. The implementation also worked fine using char elements (just single letters).</p> <p>Can anyone enlighten me?</p>
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    1. USAdrián Navarro
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      1. VTUpMod
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    1. COI saw a delete there, but no new, what are you deleting, who allocated it and how?
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      1. USPlasmaHH
    2. COAm I the only one who cant find the `malloc()`???
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      1. USdhein
    3. COThere is a `new` on the fourth line of `queue::enqueue`
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      1. USJoe Z
 

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