有没有办法找出一个类是否是另一个类的直接基？ [英] Is there a way to find out whether a class is a direct base of another class?

问题描述

我不知道是否有发现一个类是否是另一个类，即在升压型性状方面的直接基地 is_direct_base_of 函数的方法。据我所看到的，升压似乎并不支持这种功能，这使我觉得这是不可能与当前的C ++标准。

I'm wondering whether there is a way to find out whether a class is a direct base of another class i.e. in Boost type trait terms a is_direct_base_of function. As far as I can see, Boost doesn't seem to support this kind of functionality, which leads me to think that it's impossible with the current C++ standard.

我希望它的原因是为了做一些验证在该用于一个反射系统，以指定一个类是从另一个导出两个宏检查，如在下面的例子中code

The reason I want it is to do some validation checking on two macros that are used for a reflection system to specify that one class is derived from another, as in the example code below.

header.h：

#define BASE     A
#define DERIVED  B

class A {};
class B : public A 
{
   #include <rtti.h>
};

rtti.h：

rtti.h:

// I want to check that the two macro's are correct with a compile time assert
Rtti<BASE, DERIVED> m_rtti;

虽然宏这个简单的例子似乎是不必要的，在我的现实世界中的场景 rtti.h 是一个复杂得多。

一个可能途径将是this指针的大小与该指针转换到基本类型的大小比较，不知怎么想弄清楚它是否是基类本身远或东西的尺寸。 （是的，你说得对，我不知道如何将工作，要么！）

One possible avenue would be to compare the size of the this pointer with the size of a this pointer cast to the base type and somehow trying to figure out whether it's the size of the base class itself away or something. (Yeah, you're right, I don't know how that would work either!)

推荐答案

我问自己，难道直接继承与间接区分什么C ++结构？它配备介意C ++派生类型的构造函数直接调用了他们唯一的直接基础（S）构造函数。所以，code是这样的：

I asked myself, "What C++ constructs do differentiate between direct inheritance vs. indirect?" It comes to mind that C++ constructors of derived types directly call constructors for their direct base(s) only. So code like this:

Derived::Derived() : Base() {}

只有当基本有效期为的直接碱的。而且，由于你是注射 rtti.h 的code进入人体导出，你能容忍限制，这项技术仅仅是直接可见的派生类本身（即它并不像一般的假想 type_traits :: is_direct_base_of ，但不必是）。

Is only valid if Base is is a direct base of Derived. And since you are injecting the code of rtti.h into the body of Derived, you can tolerate the restriction that this technique is only directly visible within the derived class itself (i.e. it is not as general as a hypothetical type_traits::is_direct_base_of, but does not need to be).

所以，因为我们可能不希望混乱与周围的默认构造本身，怎么样加入一些特殊用途的？

So since we probably don't want to mess around with the default constructors per se, how about adding some special-purpose ones?

#define BASE     A
#define DERIVED  B

struct rtti_tag {}; // empty type

class A
{
protected:
    A(rtti_tag) { assert(false); } // never actually called
};

#include <rtti.h>
class B : public A
{
    IS_DIRECT_BASE_OF(DERIVED, BASE); // fails to compile if not true
};

rtti.h ：

#define IS_DIRECT_BASE_OF(_B_, _A_) _B_(rtti_tag tag) : _A_(tag) \
    { assert(false); } // never actually called

这code编译我使用g ++ 4.2;如果我在继承层次插入一个新的类，断言休息和编译失败，我认为是一个合理的描述性诊断：

This code compiles for me with g++ 4.2; if I insert a new class in the inheritance hierarchy, the assertion breaks and compilation fails with what I think is a reasonably descriptive diagnostic:

In constructor ‘B::B(rtti_tag)’:
error: type ‘A’ is not a direct base of ‘B’
...

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