复制和分配构造函数的问题 [英] Problems with Copy and assignment constructor

问题描述

我有以下代码，用作链接列表的一部分：

I have the following code, used as a part of a Linked List:

// copy constructor:
LinkedList<T>(const LinkedList<T> &list) 
{
    // make a deep copy
    for (LinkedList<T>::Iterator i = list.begin(); i != list.end(); i++)
    {
        add(*i);
    }
}


// assignment constructor
LinkedList<T>& operator= (const LinkedList<T> &list) 
{
    // make a deep copy
    for (LinkedList<T>::Iterator i = list.begin(); i != list.end(); i++)
    {
        add(*i);
    }
}

但是在编译时出现以下错误（是当我将其用作分配构造函数时）：

But when I compile I get the following errors (this is when I use it as an assignment constructor):

1>------ Build started: Project: AnotherLinkedList, Configuration: Debug Win32 ------
1>main.cpp
1>c:\users\ra\source\repos\sandbox\container\anotherlinkedlist\linkedlist.h(57): error C2662: 'LinkedList<int>::Iterator LinkedList<int>::begin(void)': cannot convert 'this' pointer from 'const LinkedList<int>' to 'LinkedList<int> &'
1>c:\users\ra\source\repos\sandbox\container\anotherlinkedlist\linkedlist.h(57): note: Conversion loses qualifiers
1>c:\users\ra\source\repos\sandbox\container\anotherlinkedlist\linkedlist.h(55): note: while compiling class template member function 'LinkedList<int> &LinkedList<int>::operator =(const LinkedList<int> &)'
1>c:\users\ra\source\repos\sandbox\container\anotherlinkedlist\main.cpp(20): note: see reference to function template instantiation 'LinkedList<int> &LinkedList<int>::operator =(const LinkedList<int> &)' being compiled
1>c:\users\ra\source\repos\sandbox\container\anotherlinkedlist\main.cpp(14): note: see reference to class template instantiation 'LinkedList<int>' being compiled
1>c:\users\ra\source\repos\sandbox\container\anotherlinkedlist\linkedlist.h(57): error C2662: 'LinkedList<int>::Iterator LinkedList<int>::end(void)': cannot convert 'this' pointer from 'const LinkedList<int>' to 'LinkedList<int> &'
1>c:\users\ra\source\repos\sandbox\container\anotherlinkedlist\linkedlist.h(57): note: Conversion loses qualifiers
1>Done building project "AnotherLinkedList.vcxproj" -- FAILED.
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

开始和结束的迭代器代码如下：

The iterator code for begin and end looks like this:

// get root
    Iterator begin()
    {
        return Iterator(sp_Head);
    }

    // get end
    Iterator end()
    {
        return Iterator(nullptr);
    }

我怎么办？

推荐答案

根据错误消息，您的 LinkedList 似乎没有 begin（）和 end（）可以在const对象上调用。但是，复制构造函数和赋值运算符的参数是const。您必须添加 begin（）和 end（）的const版本。

Based on the error message, it would seem that your LinkedList does not have variants of begin() and end() that can be called on a const object. The parameters to your copy constructor and assignment operator are const, however. You will have to add const versions of begin() and end().

想必您正在寻找这样的东西：

Presumably, you're looking for something like this:

    ConstIterator begin() const { Iterator(sp_Head); }
    Iterator begin() { Iterator(sp_Head); }

    ConstIterator end() const { ConstIterator(nullptr); }
    Iterator end() { Iterator(nullptr); }

其中 ConstIterator 是您的版本迭代const元素的迭代器类型…

Where ConstIterator is a version of your iterator type that iterates over const elements…

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