学步园

    都说dynamic_cast等动态类型识别效率不高。现编写一段代码，实际测一下。剖分代码源于网络

#include <iostream><br /> #include <ctime><br /> #include <typeinfo.h><br /> using namespace std;<br /> class Fo<br /> {<br /> virtual void show() {};<br /> };<br /> class Root : Fo<br /> {<br /> virtual void show() {};<br /> };<br /> class Base : public Root<br /> {<br /> public:<br /> virtual void show() {};<br /> };</p> <p>class Derived : public Base<br /> {<br /> public:<br /> virtual void show() {};<br /> };</p> <p>int main()<br /> {<br /> Base* pBase = new Derived();</p> <p> Derived* pDerived = NULL;</p> <p> time_t begin, end;</p> <p> begin = time(NULL);<br /> for(int i=0; i < 100000; ++i) // 10W次<br /> {<br /> pDerived = dynamic_cast<Derived * >(pBase);<br /> }<br /> end = time(NULL);</p> <p> cout << "seconds: " << end-begin << endl;<br /> cout << typeid( *pDerived ).name() << endl;<br /> cout << typeid(pDerived ).name() << endl;<br /> system("pause");</p> <p> return 0;<br /> }

seconds: 0
seconds: 5
class Derived
class Derived *

看了一下dynamic_cast和typeid的汇编指令，

每个都有至少几百条指令。毕竟要”traverse the class derivation lattice of its argument at runtime“

我又测了虚函数方法，代码结果如下

#include <iostream><br /> #include <ctime><br /> #include <typeinfo.h><br /> using namespace std;<br /> class Fo<br /> {<br /> public:<br /> virtual int GetType();<br /> virtual void show() {};<br /> };<br /> int Fo::GetType()<br /> {<br /> return 1;<br /> }<br /> class Root : public Fo<br /> {<br /> public:<br /> virtual void show() {};<br /> };<br /> class Base : public Root<br /> {<br /> public:<br /> virtual void show() {};<br /> };</p> <p>class Derived : public Base<br /> {<br /> public:<br /> int GetType();<br /> virtual void show() {};<br /> };<br /> int Derived::GetType()<br /> {<br /> return 9;<br /> }<br /> int main()<br /> {<br /> Base* pBase = new Derived();</p> <p> time_t begin, end;<br /> begin = time(NULL);<br /> Derived * p;<br /> for (int i=0; i<100000000; ++i)<br /> {<br /> if (pBase->GetType() == 9)<br /> {<br /> p = (Derived *)(pBase);<br /> }<br /> }<br /> end = time(NULL);<br /> cout << "seconds: " << end-begin << endl;</p> <p> system("pause");<br /> return 0;<br /> }

seconds: 3

处理亿数量级的问题，无论采用什么动态类型方法，都会慢的。因为要调用亿次函数。

如下代码，调用了Add方法1亿次。

#include <iostream><br /> #include <ctime><br /> #include <typeinfo.h><br /> using namespace std;<br /> class Fo<br /> {<br /> public:<br /> virtual int GetType();<br /> virtual void show() {};<br /> };<br /> int Fo::GetType()<br /> {<br /> return 1;<br /> }<br /> class Root : public Fo<br /> {<br /> public:<br /> virtual void show() {};<br /> };<br /> class Base : public Root<br /> {<br /> public:<br /> virtual void show() {};<br /> };</p> <p>class Derived : public Base<br /> {<br /> public:<br /> int GetType();<br /> virtual void show() {};<br /> };<br /> int Derived::GetType()<br /> {<br /> return 9;<br /> }<br /> int Add(int i, int j)<br /> {<br /> return i;<br /> }<br /> int main()<br /> {<br /> Base* pBase = new Derived();</p> <p> time_t begin, end;<br /> begin = time(NULL);<br /> Derived * p;<br /> for (int i=0; i<100000000; ++i)<br /> {<br /> //if (pBase->GetType() == 9)<br /> //{<br /> //p = (Derived *)(pBase);<br /> //}<br /> Add(1,2);<br /> }<br /> end = time(NULL);<br /> cout << "seconds: " << end-begin << endl;</p> <p> system("pause");<br /> return 0;<br /> }

seconds: 3

综合对比来看，dynamic_cast效率还是不错的。可见，估算效率不能只“想想”，要实际测试一下。