dynamic_cast效率问题

最新推荐文章于 2025-07-13 21:22:34 发布

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本文通过实验对比了dynamic_cast与虚函数方法在C++中的性能表现。测试结果显示，在百万级别的调用中，dynamic_cast的开销相对较小，表明其在实际应用中的效率是可接受的。

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

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

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

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。