是否可以使用“这个”？在 c++在成员函数中创建本地对象？

发布于 2024-11-29 15:41:05 字数 601 浏览 1 评论 0原文

我有一个关于“this”关键字的问题。我有一个名为“BiPoly”的类，它表示二元多项式。有一个名为 BiPoly::DifferentiateX() 的成员函数，它对 X 进行偏微分，并且它是自我修改的。

template <class NT>
BiPoly<NT> & BiPoly<NT>::differentiateX() { 
    if (ydeg >= 0)
      for (int i=0; i<=ydeg; i++) 
    coeffX[i].differentiate();

    return *this;
}//partial differentiation wrt X

在另一个名为 BiPoly::eval1() 的成员函数中，我需要获取调用 BiPoly::eval1() 的对象的 DifferentiateX() 结果。由于 DifferentiateX() 是自我修改的，因此我必须创建一个临时变量才能在 eval1() 中获取结果。我的问题是：我可以使用 "this" 在成员函数中创建临时对象吗？如果是这样，我该怎么做？

原文

I have a question regarding "this" keyword. I have a class called "BiPoly", which represents bivariate polynomial. There is a member function called BiPoly<NT>::DifferentiateX(), which gets Partial Differentiation wrt X, and it's self-modified.

template <class NT>
BiPoly<NT> & BiPoly<NT>::differentiateX() { 
    if (ydeg >= 0)
      for (int i=0; i<=ydeg; i++) 
    coeffX[i].differentiate();

    return *this;
}//partial differentiation wrt X

In another member function called BiPoly::eval1(), I need to get the result of DifferentiateX() of the object who calls BiPoly<NT>::eval1(). Since DifferentiateX() is self-modified, I have to create a temp variable to get the result within eval1(). My question is: can I use "this" to create a temp object within member function? If so, how do I do that?

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一笔一画续写前缘 2024-12-06 15:41:05

您可以在 *this 上使用复制构造函数，生成多项式对象的副本，然后对它进行微分和评估：

BiPoly<NT> copy(*this);
copy.DifferentiateX();
NT val = copy.eval1(arg);

取决于您如何存储系数（例如在标准向量中）），您甚至可能不需要实际编写复制构造函数。

You can use copy constructor on *this, yielding a copy of your polynomial object which you then differentiate and evaluate:

BiPoly<NT> copy(*this);
copy.DifferentiateX();
NT val = copy.eval1(arg);

Depending on how do you store the coefficients (e.g. in standard vector), you might not even need to actually write the copy constructor.

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天涯离梦残月幽梦 2024-12-06 15:41:05

您不应返回对函数内创建的临时对象的引用。它将在函数退出时被销毁。

您宁愿将您的函数声明为这样：

template <class NT>
BiPoly<NT> BiPoly<NT>::differentiateX() { 
    BiPoly nv = *this;     
    if (nv.ydeg >= 0)
      for (int i=0; i<=nv.ydeg; i++) 
    nv.coeffX[i].differentiate();

    return nv;
}//partial differentiation wrt X

注意它返回对象（nv 的副本）但不引用它。

You should not return reference to temporary object that was created inside your function. It will be destroyed on function exit.

You'd rather declare your function as this:

template <class NT>
BiPoly<NT> BiPoly<NT>::differentiateX() { 
    BiPoly nv = *this;     
    if (nv.ydeg >= 0)
      for (int i=0; i<=nv.ydeg; i++) 
    nv.coeffX[i].differentiate();

    return nv;
}//partial differentiation wrt X

Note it returns object (copy of nv) but not reference to it.

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