c++ - 将 std::vector 作为输入/输出引用参数传递时丢失了一些数据

Question

我在函数之间传输 vector 中包含的某些数据时遇到问题。情况如下:

void generateObjects(std::vector<MyClass> &objects)
{
    objects.clear();
    //Fill objects vector
    std::vector<MyClass> p;

    //This 4-line pattern is repeated a number of times to generate all objects and store them in variable 'objects'
    p.clear();
    generateSomeOfTheObjects(p); //p is again passed by ref. in/out parameter
    for(uint j = 0; j < p.size(); p++){
        objects.push_back(p[j]);
    }

    //Print some members of the objects - works fine
    for(uint i = 0; i < objects.size(); i++){
        printf("%f ",objects[i].mymember->myElm);
    }
}

int main()
{
   std::vector<MyClass> objects;
   generateObjects(objects);
   //Print size of vector - size is correct it is the same as it is in generateObjects func
   printf("%lu\n",objects.size());
   //Again print members of the objects - some members are retained after the function call, some are lost. 
   //The one below doesn't work, mymember is a pointer to another object and its member myElm seems not initialized.
   for(uint i = 0; i < objects.size(); i++){
       printf("%f ",objects[i].mymember->myElm);
   }
   //Here I need to pass the objects to another read-only function
   ...
}

我在互联网上搜索了类似的案例，实际上发现了很多，但我无法对我的代码应用相同的修复。我正在尝试访问由 MyClass 实例成员 (objects[i].mymember->myElm) 指向的对象的成员，我可能在这里遗漏了什么？

Answer 1

可能错误在于MyClass的实现。我会说这个类包含一些用局部变量的地址初始化的指针，所以当你从一些函数返回时，指针指向一个被销毁的对象。

那将是未定义的行为，但它可能偶然起作用。当您从第一个函数返回时，堆栈内存最终被覆盖并且您的数据丢失。

更新:感谢@chris 在下面评论中的见解，最可能的原因是您的 MyClass 没有复制构造函数，但它确实有一个指针成员。

像这样:

class MyClass
{
public:
    Member *mymember;

    MyClass()
    {
        mymember = new Member;
    }
    ~MyClass()
    {
        delete mymember;
    }
};

现在，如果您使用编译器生成的默认复制构造函数(或复制运算符)会发生什么情况？

void foo()
{
    MyClass a;
    {
        MyClass b(a);
    }
    //a.mymember is no longer valid
}

a 和b 共享同一个指针mymember，所以当其中一个被销毁时，mymember > 被删除，另一个持有悬空指针。

这就是为什么我们有三原则。它指出:

Whenever you define a non-default destructor, you most likely will want also a non-default copy-constructor and a non-default copy-operator.

现在您必须决定是要共享 mymember 的所有权还是要复制它。第一个最好用智能指针 (shared_ptr) 完成，第二个用深拷贝。

例如深拷贝:

class MyClass
{
public:
    Member *mymember;

    MyClass()
    {
        mymember = new Member;
    }
    MyClass(const MyClass &c)
    {
        mymember = new Member(c.mymember);
    }
    MyClass &operator=(const MyClass &c)
    {
        if (this != &c) //be aware of self-copy
        {
            delete mymember;
            mymember = new Member(c.mymember);
        }
        return *this;
    }
    ~MyClass()
    {
        delete mymember;
    }
};

还有共享指针:

class MyClass
{
public:
    std::shared_ptr<Member> mymember; //or boost::shared_ptr if old compiler

    MyClass()
        :mymember(new Member)
    {
    }
    //no custom-made destructor -> no rule of 3
};