为什么在复制赋值运算符的定义中需要删除？ [英] Why delete is needed in the definition of the copy-assignment operator?

问题描述

我是一个C ++初学者。我在C ++ Primer（第5版）中做练习。我找到了一个引用
从Github练习13.8（这里），如下所示。

I am a C++ beginner. And I am doing the exercises in C++ Primer (5th Edition). I found a reference to Exercise 13.8 from Github (Here), which is shown below.

#include <string>
#include <iostream>

using std::cout;
using std::endl;

class HasPtr {
public:
    HasPtr(const std::string &s = std::string()) : ps(new std::string(s)), i(0) { }
    HasPtr(const HasPtr &hp) : ps(new std::string(*hp.ps)), i(hp.i) { }
    HasPtr& operator=(const HasPtr &hp) {
        std::string *new_ps = new std::string(*hp.ps);
        delete ps;          // I don't know why it is needed here? 
                            // But when I delete this line, it also works.
        ps = new_ps;
        i = hp.i;
        return *this;
    }

    void print() {
        cout << *(this->ps) << endl;
        cout << this->i << endl;
    }

private:
    std::string *ps;
    int i;
};

int main() {
    HasPtr hp1("hello"), hp2("world");
    hp1.print();
    hp1 = hp2;
    cout << "After the assignment:" << endl;
    hp1.print();
}

令我困惑的是 HasPtr& operator =（const HasPtr& hp）函数。我不知道为什么删除ps; 在这里需要。我认为这是一个错误，但它工作时，我编译代码。但是，当我删除 delete ps; 的行时，它也工作。所以，我不知道是否需要 delete ps; ，如果是保留的话有什么优点。

What makes me confusing is the HasPtr& operator=(const HasPtr &hp) function. I don't know why delete ps; is needed here. I thought it was an error, but it worked when I compiled the code. However, it also works when I delete the line of delete ps;. So, I do not know whether delete ps; is needed and what is the advantage if it is reserved.

推荐答案

HasPtr :: ps 是一个堆分配的 std :: string 指针。

HasPtr::ps is an heap-allocated std::string pointer.

新 所有 hasPtr 构造函数。因此，当 HasPtr :: ps 被替换为另一个堆分配的指针时，必须使用 delete 以避免内存泄漏。

It is allocated and constructed using new in all HasPtr constructors. Therefore, when HasPtr::ps gets replaced by another heap-allocated pointer, the existing memory has to be released using delete to avoid memory leaks.

注意在现代C ++中，您应该几乎从不使用 new 和 delete 喜欢这个。请改用智能指针，例如 std :: unique_ptr 或 std :: shared_ptr

Note that, in modern C++, you should almost never use new and delete for managing objects like this. Use smart pointers instead, like std::unique_ptr or std::shared_ptr, which take care of memory management for you, safely and conveniently.

我仍然建议您熟悉新和 delete ，因为大量的现有代码使用它们。 cppreference.com 是查找语言的详细信息的好地方

I still suggest getting familiar with new and delete, as an huge amount of existing code makes use of them. cppreference.com is a great place to find detailed information about the language.

由于 Jan Hudec 在评论中提到，在堆上存储 std :: string 通常很愚蠢 - std :: string 是一个包装器一个堆分配的字符数组，它已经为你管理了内存。

As Jan Hudec mentioned in the comments, it is generally pretty silly to store an std::string on the heap - std::string is a wrapper around an heap-allocated character array, which already manages the memory for you.

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