队列实现出现Malloc指针错误

我正在尝试用C++实现一个队列。队列包含字符串。

我使用以下代码：

#include <iostream> 
#include <stdlib.h> 
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->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->data = c;
            tmp->link = NULL;
            front = tmp;
            rear = tmp;
            rear->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->link = tmp;
            tmp->data = c;
            tmp->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->data;
        // Front now has to point to the next node and the old element has to be deleted.
        front = front->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->link;
        delete tmp;
    }
}
//////////// MAIN ///////////
int main(int argc, char *argv[]) {
    queue queue;
    std::string str2;
    queue.enqueue("hello");
    queue.dequeue(&str2);
    std::cout << str2;
}

遗憾的是，我得到了以下错误：

$g++-o问题issue.cpp$/问题问题（23060,0x7fff71058310）malloc：*对象0x7f962b403920出错：释放的指针未分配*在malloc_error_break中设置断点以调试helloAbort陷阱：6

如果我对队列析构函数（"while"块）进行注释，则实现工作正常。使用char元素（只有单个字母），实现也很好。

有人能启发我吗？