关于具有自引用的C ++类 [英] About C++ classes with self reference

问题描述

我有点困惑...

为什么C ++允许这样做：

  static int t = 0; 
 
 class A 
 {
 public：
 A（）：m（t ++）
 {
 
 cout< ; C：< m < endl; 
 if（t> = 5）
 {
 A a; //<<<< -----这行
 throw（a）; 
} 
} 
 
 int m; 
}; 
  

但这不是：

  static int t = 0; 
 
 class A 
 {
 public：
 
 A（）：m（t ++）
 {
 
 cout < C：< m < endl; 
} 
 
 A a; //<<<< -----这行
 
 int m; 
}; 
  

第二个没有按预期编译（是的，我知道为什么它不编译：在代码 A 仍然不完整）...

但是...第一个编译得很好做它应该做，即：崩溃应用程序在一个语句如： A a [10]; ）。在构造函数中 A 是一个完整的类型吗？也可以给我一些C ++标准条目这种情况？

解决方案

此情况的标准条目？

是的，草案C ++标准说一个类没有完全定义，直到关闭} ，这在 9.2 段 段：

类在classspecifier的closing}被认为是一个完全定义的对象类型（3.9）（或完整类型）。 [...]

并且类的所有非静态数据成员必须是完整的， em>：

非静态（9.4）数据成员不应具有不完全类型。特别地，类C不应包含类C的非静态成员，但它可以包含指向类C的对象的指针或引用。

但在构造函数中也在 2 内完成：

..]

虽然静态成员可以不完整， 9.4.2 段 b
$ b

静态数据成员在其类定义中的声明不是定义，并且可能是除了cv限定的void之外的不完整类型。 ]

不允许类包含自身也是有意义的，因为这将需要无限空间，因为自引用永远不会结束， A 包含 A ，其中包含 A ...




I am a little bit confused ...

Why is this allowed in C++:

static int t = 0;

class A
{
    public:
        A() : m(t++)
        {

            cout << "C:" << m << endl; 
            if(t >= 5)
            {
                A a;                      // <<<< ----- THIS line
                throw( a);
            }
        }            

        int m;
};

But this not:

static int t = 0;

class A
{
    public:

        A() : m(t++)
        {

            cout << "C:" << m << endl; 
        }

        A a;                            // <<<< ----- THIS line

        int m;
};

The second one is not compiling as expected (yes, I know why it's not compiling: at that point in code the A is still incomplete) ...

But ... the first one compiles nicely (and does what it's supposed to do, ie: crashes the application on a statement like: A a[10]; ). Is the A a complete type in the constructor? Can also point me some C++ standard entries for this situation?

解决方案

Can also point me some C++ standard entries for this situation?

Yes, the draft C++ standard says a class is not completely defined until the closing }, this is in section 9.2 Class members paragraph 2:

A class is considered a completely-defined object type (3.9) (or complete type) at the closing } of the classspecifier. [...]

and all non-static data members of a class must be complete, from paragraph 9:

Non-static (9.4) data members shall not have incomplete types. In particular, a class C shall not contain a non-static member of class C, but it can contain a pointer or reference to an object of class C.

but it is considered complete within the constructor also within paragraph 2:

[...]Within the class member-specification, the class is regarded as complete within function bodies, default arguments,[...]

although static members can be incomplete, section 9.4.2 Static data members paragraph 2:

The declaration of a static data member in its class definition is not a definition and may be of an incomplete type other than cv-qualified void.[...]

It also makes sense to not allow a class to contain itself since this would require infinite space since the self reference would never end, A contains A which contains A ...

这篇关于关于具有自引用的C ++类的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！