C ++继承公开类成员不能用作默认参数 [英] c++ publicly inherited class member cannot be used as default argument

问题描述

我的问题的示意图...

A schematic of my problem...

class A
{
public:
    // etc.
protected:
    uint num;
};

class B : public A
{
public: 
    void foo(uint x = num); //bad
};

给出了这样的错误：

gives this error:

error: invalid use of non-static data member ‘A::num’
error: from this location

为什么会这样，我能做些什么来解决此问题？

Why does this happen, and what can I do to work around this?

推荐答案

我怀疑这种情况（根据有关非staticness投诉），因为没有这个指针它使用了解的这的B比如，它应该得到 NUM 从

I suspect this happens (based on the complaint about non-staticness) because there is no this pointer for it to use to know which instance of B it should get num from.

微软编译器（至少）允许您指定一个前pression，但不是非静态成员。从 MSDN ：

The Microsoft compiler (at least) allows you to specify an expression, but not a non-static member. From MSDN:

前pressions
  参数往往是不变
  前pressions，但是这不是一个
  需求。这位前pression能
  结合了在可视功能
  当前范围，恒
  前pressions和全局变量。该
  前pression不能包含本地
  变量和非静态的类成员
  变数。

The expressions used for default arguments are often constant expressions, but this is not a requirement. The expression can combine functions that are visible in the current scope, constant expressions, and global variables. The expression cannot contain local variables or non-static class-member variables.

的变通为此很多，其他人指出了一些。另外还有一个您可能会或可能不喜欢：

Work-arounds for this are numerous and others have pointed out a few. Here's one more which you may or may not like:

void foo(uint* x = NULL) {
  uint y = (x == NULL ? num : *x);
  // use y...
}

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