c++ - 运算符重载类型转换错误

Question

我有一个模板类，我需要重载运算符 ==。我通过以下方式执行此操作

template <typename T>
class Polynomial {
    vector<T> coefficients;

    public:
    Polynomial(vector<T> c);

    bool operator ==(const Polynomial& second) const {
            const typename vector<T>::iterator thisBegin = this->coefficients.begin();
            const typename vector<T>::iterator secondBegin = second.coefficients.begin();
            for ( ; ((thisBegin != this->coefficients.end()) &&
                                    (secondBegin != second.coefficients.end()));
                            ++thisBegin, ++secondBegin) {
                    if (*thisBegin != *secondBegin)
                            return false;
            }
            while (thisBegin != this->coefficients.end()) {
                    if (*thisBegin != 0)
                            return false;
                    ++thisBegin;
            }
            while (secondBegin != second.coefficients.end()) {
                    if (*secondBegin != 0)
                            return false;
                    ++secondBegin;
            }
            return true;
    }
};

但是，当我使用 T=int 创建此类的两个对象并尝试应用此运算符时

Polynomial<int> first(firstVector);
Polynomial<int> second(secondVector);
std::cout << (first == second) << std::endl;

我得到了错误

problem2.cpp: In instantiation of ‘bool Polynomial<T>::operator==(const Polynomial<T>&)    const [with T = int; Polynomial<T> = Polynomial<int>]’:
problem2.cpp:63:32:   required from here
problem2.cpp:23:83: error: conversion from ‘std::vector<int, std::allocator<int> >::const_iterator {aka __gnu_cxx::__normal_iterator<const int*, std::vector<int, std::allocator<int> > >}’ to non-scalar type ‘std::vector<int, std::allocator<int> >::iterator {aka __gnu_cxx::__normal_iterator<int*, std::vector<int, std::allocator<int> > >}’ requested

有人可以指出这种转换有什么问题吗？谢谢!

Answer 1

您正在尝试将 const_iterator 转换为 iterator:

const typename vector<T>::iterator thisBegin = this->coefficients.begin();

this 在此上下文中是 const，因此 this->coefficients.begin(); 返回一个 const_iterator。试试这个:

typename vector<T>::const_iterator thisBegin = this->coefficients.begin();

另请注意，thisBegin 不是 const，如您的示例所示。这是因为你然后做了这种事情:

++secondBegin;

这要求 const_iterator 是非常量(这意味着您可以修改迭代器，但不能修改它指向的东西)。