非静态成员作为非静态成员函数的默认参数 [英] Nonstatic member as a default argument of a nonstatic member function

问题描述

struct X
{
   X():mem(42){}
   void f(int param = mem) //ERROR
   {
      //do something
   }
private: 
   int mem;
};

任何人都可以给我一个原因，为什么这是非法的C ++？也就是说，我知道这是一个错误，我知道错误的意思，我只是不明白为什么会是非法的！

Can anyone give me just one reason as to why this is illegal in C++?! That is to say, I know that it is an error, I know what the error means, I just can't understand why would this be illegal!

推荐答案

您的代码（简化版）：

struct X
{
   int mem;
   void f(int param = mem); //ERROR
};

您希望将非静态成员数据用作成员函数的参数的默认值。第一个问题是：这个类的特定实例的默认值 mem 属于？

You want to use a non-static member data as default value for a parameter of a member function. The first question which comes to mind is this : which specific instance of the class the default value mem belongs to?

X x1 = {100};  //mem = 100
X x2 = {200};  //mem = 200

x1.f(); //param is 100 or 200? or something else?

您的答案可能是 100 as <$对 x1 调用 f（），其中 mem = 100 。如果是这样，那么它需要实现 f（）：

Your answer might be 100 as f() is invoked on the object x1 which has mem = 100. If so, then it requires the implementation to implement f() as:

void f(X* this, int param = this->mem);

这又要求在初始化其他参数之前首先初始化第一个参数。但是C ++标准不指定函数参数的任何初始化顺序。因此，这是不允许的。它的原因与C ++ Standard不允许的原因相同：

which in turn requires the first argument to be initialized first before initialization of other argument. But the C++ standard doesn't specify any initialization order of the function arguments. Hence that isn't allowed. Its for the same reason that C++ Standard doesn't allow even this:

int f(int a, int b = a); //§8.3.6/9

事实上，§8.3.6/ 9明确说， / p>

In fact, §8.3.6/9 explicitly says,

在调用函数时，每个
计算默认参数。 函数参数的计算
的顺序是
unspecified 。 因此，函数的参数
不应在
默认参数表达式中使用，即使
未评估。

Default arguments are evaluated each time the function is called. The order of evaluation of function arguments is unspecified. Consequently, parameters of a function shall not be used in default argument expressions, even if they are not evaluated.

本节的其余部分是一个有趣的阅读。

And rest of the section is an interesting read.

与默认参数相关的一个有趣的主题（与此主题无关​​）：

An interesting topic related to "default" arguments (not related to this topic though):

• 参数列表中间的默认参数？

• Default argument in the middle of parameter list?

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