C++ 复制构造函数的奇怪行为

Question

给定以下代码:

class monomial {
public:
    mp_real coe;
    int exp;        
    monomial *next; 
};

class polynomial  
{  
private:
    monomial *start;
public:
    polynomial();
    ~polynomial();
    void destroy_poly(monomial *);
    polynomial & operator=(const polynomial &);
    polynomial(const polynomial&);
    monomial * get_start();
};  
polynomial::polynomial() {
    start = NULL;
}
polynomial::~polynomial() {
    if (start != NULL) {
        destroy_poly(start);
    }
    start = NULL;
}
void
polynomial::destroy_poly(monomial * m) {
    monomial * cm = m;
    if (cm->next != NULL) {
        destroy_poly(cm->next);
    }
    delete m;
}
polynomial::polynomial(const polynomial& p) {
    if (p.start != NULL) {
        start = new monomial();
        start->coe = p.start->coe;
        start->exp = p.start->exp;

        monomial * tmpPtr = p.start;
        monomial * lastPtr = start;

        while (tmpPtr->next != NULL) {
            monomial * newMon = new monomial();
            newMon->coe = tmpPtr->next->coe;
            newMon->exp = tmpPtr->next->exp;

            lastPtr->next = newMon;
            lastPtr = lastPtr->next;
            tmpPtr = tmpPtr->next;
        }
    }
}
polynomial & polynomial::operator=(const polynomial &p) {
    if (p.start != NULL) {
        start = new monomial();
        start->coe = p.start->coe;
        start->exp = p.start->exp;

        monomial * tmpPtr = p.start;
        monomial * lastPtr = start;

        while (tmpPtr->next != NULL) {
            monomial * newMon = new monomial();
            newMon->coe = tmpPtr->next->coe;
            newMon->exp = tmpPtr->next->exp;

            lastPtr->next = newMon;
            lastPtr = lastPtr->next;
            tmpPtr = tmpPtr->next;
        }
    }
    return *this;
}

然后在main.cpp中:

main() {
    polynomial poly;
    // code that initializes / sets values for 
    // various monomials pointed to by poly.start.

    map<set<unsigned int>, polynomial> hash;
    set<unsigned int> tmpSet;

    tmpSet.insert(0);

    hash[tmpSet] = poly;
}

我可以在复制构造函数的第一行设置断点，当我跨过 hash[tmpSet] = poly 行后，复制构造函数中的 p.start 为 NULL。然后，它会被第二次调用，此时 p.start 中设置了奇怪的值。

有什么线索吗？

谢谢，埃里希

编辑 1:认为将赋值运算符添加到多项式类可以修复它，但事实并非如此。仍然有同样的问题。

Answer 1

您违反了 Rule of Three :
由于您重载了复制构造函数，您应该为您的类重载复制赋值运算符和析构函数。

以上陈述是针对一般情况的，在您的代码示例中，您可以将应该替换为必须。