C++ 中的高效运行时类型检查 [英] Efficient run-time type checking in C++

问题描述

我有两个名为Expression"和BinExp"的类，代码如下:

I've two class named 'Expression' and 'BinExp' as following codes:

class Expression
{
public:
    virtual BinExp* IsBinaryExp() { return NULL; }
};
class BinExp : public Expression
{
public:
    virtual BinExp* IsBinaryExp() { return this; }
};

例如，我有一个 Expression* 的指针变量类型，但初始化为 new BinExp 并作为参数发送给 analysis 函数如下代码:

As example, I've a pointer variable type of Expression* but initialized as new BinExp and send as argument to a analyse function as following code:

int main()
{
    Expression* e = new BinExp;
    analyse(e);
}

在analysis函数内部，我需要知道e是指向Expression类型还是BinExp类型.在我看来，有三种方法可以做到这一点.

Inside the analyse function, I need to know whether the e is pointer to Expression type or BinExp type. In my hand, there is three way to do this.

首先:

BinExp* be = e->IsBinaryExp();
if ( be )
{
    printf("Yes, `e` is a binary expression\n");
}

第二:

BinExp* be = dynamic_cast<BinExp*>(e);
if ( be )
{
    printf("Yes, `e` is a binary expression\n");
}

第三个:

if ( typeid(*e) == typeid(BinExp) )
{
    BinExp* be = e->IsBinaryExp(); // or dynamic_cast<BinExp*>(e);
    printf("Yes, `e` is a binary expression\n");
}

但是我想知道当我需要在性能很重要的循环中频繁执行检查时，哪种方式(或任何其他方式)会更高效和有效.任何形式的建议我都会很感激.

But I want to know which of those ways ( or any other ) will be more efficient and effective when I need to perform the checking frequently inside a loop where performance is a matter. Any kind of suggestion I will appreciate.

推荐答案

最快的方法是保留一个成员变量，比如一个 enum ，然后在基类中定义一个内联 getter，然后你可以比较结果是否是你期望什么.

The fastest way would be to keep a member variable , say an enum , then define in the base class a inline getter, then you can compare if the result is what you expect.

示例(未编译，可能会出现一些错误):

Sample (uncompiled, some errors might occur) :

  enum eExpTypes {
        ET_UNDEFINED,
        ET_BINARY
  }
  class Expresion
  {
      protected:
            eExpTypes myType;
      public:
            Expresion(): myType(ET_UNDEFINED){};
            inline eExpTypes getType(){return myType;};
  }

  class BinExpresion : public Expresion
  {
     public:
            BinExpresion():myType(ET_BINARY){};
  }

性能提升:

• 你将取出两个指示:从指针到 vfptable ，从 vfptable 到函数
• 如果类型函数是唯一的虚函数，你的类大小会更小

动态转换通常比创建您自己的类型检查机制要慢，因此对于您的 3 个示例，第一个应该是最快的.

Dynamic cast is usually slower then making your own type check mechanism, so in case of your 3 examples the first one should be the fastest.

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