C++为什么在堆栈中构造对象后立即调用析构函数？答案

【问题标题】：C++ Why is the destructor immediately called after the object has been constructed in the stack?C++为什么在堆栈中构造对象后立即调用析构函数？
【发布时间】：2016-03-02 19:23:28
【问题描述】：

我有两个单元测试。在第一个中，我在堆栈中创建对象 myMovie。对象被创建，然后立即调用析构函数。这会导致单元测试失败，因为当 myMovie 超出范围时，会再次调用析构函数。这会导致访问冲突。

但是，如果我在堆中创建对象，一切正常。为什么在栈中构造完对象后立即调用析构函数？

第一个这样的：

TEST_METHOD(constructingMovieWithParametersStack)
    {
        _CrtMemState s1, s2, s3;
        _CrtMemCheckpoint(&s1);
        {
            Movie myMovie = Movie("firstName", "lastName", "title");
            // Why is the destructor is called here???

            string expectedDirectorFirst = "firstName";
            string expectedDirectorLast = "lastName";
            string expectedTitle = "title";
            wchar_t* message = L"movie title wasn't set correctly";
            Assert::AreEqual(expectedTitle, myMovie.getTitle(), message, LINE_INFO());
        }
        _CrtMemCheckpoint(&s2);
        wchar_t* leakMessage = L"there is a leak";
        bool isThereALeak = _CrtMemDifference(&s3, &s1, &s2);
        Assert::IsFalse(isThereALeak, leakMessage, LINE_INFO());
    }

这样的第二个单元测试：

TEST_METHOD(constructingMovieWithParametersHeap)
    {
        _CrtMemState s1, s2, s3;
        _CrtMemCheckpoint(&s1);
        {
            Movie* myMovie = new Movie("firstName", "lastName", "title");

            string expectedDirectorFirst = "firstName";
            string expectedDirectorLast = "lastName";
            string expectedTitle = "title";
            wchar_t* message = L"movie title wasn't set correctly";
            Assert::AreEqual(expectedTitle, myMovie->getTitle(), message, LINE_INFO());
            delete myMovie;
        }
        _CrtMemCheckpoint(&s2);
        wchar_t* leakMessage = L"there is a leak";
        bool isThereALeak = _CrtMemDifference(&s3, &s1, &s2);
        Assert::IsFalse(isThereALeak, leakMessage, LINE_INFO());
    }

这是电影类：

#include "Movie.h"

using namespace std;

Movie::Movie()
{
    this->director = new Person();
    this->title = "";
    this->mediaType = 'D';    // for DVD
}

Movie::Movie(string firstName, string lastName, string title)
{
    this->director = new Person();
    this->director->setFirstName(firstName);
    this->director->setLastName(lastName);
    this->title = title;
    this->mediaType = 'D';    // for DVD
}

Movie::~Movie()
{
    delete director;
}

string Movie::getTitle()
{
    return title;
}

【问题讨论】：

@Leeor：嗨！欢迎来到评论区。这是给 cmets 的。当您想向问题作者请求更多信息以帮助您回答问题时，您可以编写 cmets。当您准备好为问题提供解决方案时，您可以编写一个答案。答案部分在下面：↓↓↓↓↓↓↓↓ 谢谢，祝您有美好的一天！
@LightnessRacesinOrbit，伙计，这种语气没有理由。当我写评论时，还没有答案，我认为关于基本概念的问题可以通过简单的“查找主题 X”评论来回答，特别是因为它肯定已经被问了几十次了。
@Leeor：什么“语气”？我特意对你好。只是没有取悦一些人。叹息

标签： c++ destructor heap-memory stack-memory

【解决方案1】：

Movie myMovie = Movie("firstName", "lastName", "title");
// Why is the destructor is called here???

这里创建了一个临时对象并用于复制初始化myMovie，然后该临时对象被销毁。

你是说

Movie myMovie("firstName", "lastName", "title");

【讨论】：

如果原始代码导致访问冲突，则强烈表明 Movie 中的复制语义被严重破坏。