C ++：实现复制构造函数和复制赋值操作符 [英] C++ : Implementing copy constructor and copy assignment operator

问题描述

在阅读了关于C ++中的复制构造函数和复制赋值运算符之后，我试图创建一个简单的例子。虽然下面的代码段显然工作，我不知道我是否正在实现复制构造函数和复制赋值运算符正确的方式。

After reading about copy constructors and copy assignment operators in C++, I tried to create a simple example. Though the below snippet apparently works, I am not sure whether I am implementing the copy constructor and copy assignment operator the right way. Could you please point out if there are any mistakes/improvements or a better example to understand the relevant concepts.

class Foobase
{
    int bInt;

public:
    Foobase() {}

    Foobase(int b) { bInt = b;}

    int GetValue() { return bInt;}

    int SetValue(const int& val) { bInt = val; }
};


class Foobar
{
    int var;    
    Foobase *base;      

public:
    Foobar(){}

    Foobar(int v)
    {
        var = v;        
        base = new Foobase(v * -1);

    }

    //Copy constructor
    Foobar(const Foobar& foo)
    {       
        var = foo.var;
        base = new Foobase(foo.GetBaseValue());
    }

    //Copy assignemnt operator
    Foobar& operator= (const Foobar& other)
    {
        if (this != &other) // prevent self-assignment
        {
            var = other.var;
            base = new Foobase(other.GetBaseValue());

        }
        return *this;
    }

    ~Foobar()
    {
        delete base;
    }

    void SetValue(int val)
    {
        var = val;
    }

    void SetBaseValue(const int& val)
    {
        base->SetValue(val);
    }

    int GetBaseValue() const
    {
        return(base->GetValue());
    }

    void Print()
    {
        cout<<"Foobar Value: "<<var<<endl;
        cout<<"Foobase Value: "<<base->GetValue()<<endl;

    }   

};

int main()
{
    Foobar f(10);       
    Foobar g(f);  //calls copy constructor
    Foobar h = f; //calls copy constructor

    Foobar i;
    i = f;

    f.SetBaseValue(12);
    f.SetValue(2);    

    Foobar j = f = z; //copy constructor for j but assignment operator for f

    z.SetBaseValue(777);
    z.SetValue(77);

    return 1;
}

推荐答案

不正确的。被分配的对象泄漏其 base 指向的对象。

Your copy assignment operator is implemented incorrectly. The object being assigned to leaks the object its base points to.

您的默认构造函数也不正确： base 和 var 未初始化，因此无法知道是否有效和在析构函数中，当你

Your default constructor is also incorrect: it leaves both base and var uninitialized, so there is no way to know whether either is valid and in the destructor, when you call delete base;, Bad Things Happen.

实现复制构造函数和复制赋值操作符的最简单的方法是：要知道您已正确执行此操作，请使用复制和交换惯用语。

The easiest way to implement the copy constructor and copy assignment operator and to know that you have done so correctly is to use the Copy-and-Swap idiom.

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