你要求可以被转换成这个简单的例子

The essence of what you are asking can be distilled into this simple example

void foo(int a, int b = a);

这在C ++中是不允许的。 C ++不允许默认参数依赖于其他参数。

This is not allowed in C++. C++ does not allow default arguments to depend on other parameters.

使用类成员作为默认参数只是上面的一种特殊情况，因为类成员通过 this 指针和这个指针只是每个非静态成员函数的另一个隐藏参数。

Using class members as default arguments is just a particular case of the above, since class members are accessed through this pointer and this pointer is just another hidden parameter of each non-static member function.

所以，问题是为什么

void foo(int a, int b = a);

不允许。

一个明显的潜在原因是不允许这样做，因为它会对参数赋值的顺序施加额外的要求。如你所知，在C ++中，函数参数求值的顺序是未指定的 - 编译器可以以任何顺序评估参数。然而，为了支持上述默认参数功能，编译器必须确保 a $ c>在 b 。这感觉像一个过度的要求，这限制了我们习惯在C ++中看到的评价顺序的典型***。

One obvious potential reason to disallow this is that it would impose additional requirements on the order of argument evaluation. As you know, in C++ the order of function argument evaluation is unspecified - the compiler can evaluate arguments in any order. However, in order to support the above default argument functionality the compiler would have to make sure that a is evaluated before b. This feels like an excessive requirement, which restricts the typical freedom of evaluation order that we are used to seeing in C++.

注意这里

int a;

void foo(int b = a);

。显然，它没有显示上述的评估问题的顺序。

is allowed in C++. And, obviously, it does not exhibit the aforementioned order of evaluation issue.