关键字：static_cast，dynamic_cast，fast_dynamic_cast，VS 2015。
OS：Window 10。
C++类之间类型转换有：static_cast、dynamic_cast、reinterpret_cast、和const_cast。
static_cast - 编译时类型检查。如果没有继承关系的类之间转换编译不通过。优点是快，缺点是从父类转换到子类不安全的。
dynamic_cast - 运行时类型检查。可以父子之间转换，也可以兄弟之间转换。优点是安全，缺点是运行时效率低。
reinterpret_cast - 强制转换。最不安全。只有特定场合才能使用。
const_cast - const类型和非const类型互转。
一般从父类转换到子类，或兄弟类之间转换使用dynamic_cast是正确的选择。但是对于大型应用程序的底层开发，dynamic_cast的性能问题就暴露出来了，使用static_cast又不能保证安全，这时就需要自己实现一套高效安全的运行时动态类型转换。
基于虚函数+类型检查的类型转换
1. 为每个类实现classType和queryObject方法。运行时，通过虚函数queryObject调用以及在queryObject里面检查classType来找到合适的对象。具体实现如下：
class A { public: static const char* classType(); virtual void* queryObject(const char* classType) const; }; const char* A::classType() { const char* s_classType = "A"; return s_classType; } void* A::queryObject(const char* classType) const { if (classType == A::classType()) return const_cast<A*>(this); return nullptr; } class B { public: static const char* classType(); virtual void* queryObject(const char* classType) const; }; const char* B::classType() { const char* s_classType = "B"; return s_classType; } void* B::queryObject(const char* classType) const { if (classType == B::classType()) return const_cast<B*>(this); return nullptr; } class C : public A, public B { public: static const char* classType(); void* queryObject(const char* classType) const override; }; const char* C::classType() { const char* s_classType = "C"; return s_classType; } void* C::queryObject(const char* classType) const { if (classType == C::classType()) return const_cast<C*>(this); if (void* res = A::queryObject(classType)) return res; if (void* res = B::queryObject(classType)) return res; return nullptr; } class D : public A { public: static const char* classType(); void* queryObject(const char* classType) const override; }; const char* D::classType() { const char* s_classType = "D"; return s_classType; } void* D::queryObject(const char* classType) const { if (classType == D::classType()) return const_cast<D*>(this); return A::queryObject(classType); } template <typename To, typename From> To* fast_dynamic_cast(const From* from) { if (!from) return nullptr; return static_cast<To*>(from->queryObject(To::classType())); }
2. new对象C，用指针A指向C对象。C继承于A和B。
A* a = new C(); cout << "A* a = new C();" << endl;
3. 测试类型转换从A到C，计时和检验转换结果。从测试结果看，dyanmic_cast、static_cast、fast_dynamic_cast结果都正确，static_cast效率最高，fast_dynamic_cast其次，dyanmic_cast效率最差。
cout << "===== cast from pointer A to pointer C, should be not null =====" << endl; C* c = nullptr; start = clock(); for (int i = 0; i < count; i++) { c = dynamic_cast<C*>(a); } stop = clock(); cout << "dynamic_cast from A to C: " << c << ", " << "Time: " << stop - start << endl; c = nullptr; start = clock(); for (int i = 0; i < count; i++) { c = static_cast<C*>(a); } stop = clock(); cout << "static_cast from A to C: " << c << ", " << "Time: " << stop - start << endl; c = nullptr; start = clock(); for (int i = 0; i < count; i++) { c = fast_dynamic_cast<C>(a); } stop = clock(); cout << "fast_dynamic_cast from A to C: " << c << ", " << "Time: " << stop - start << endl;
测试结果：
===== cast from pointer A to pointer C, should be not null ===== dynamic_cast from A to C: 00000202D48C9FE0, Time: 2227 static_cast from A to C: 00000202D48C9FE0, Time: 0 fast_dynamic_cast from A to C: 00000202D48C9FE0, Time: 199
4. 测试类型转换从A到B，计时和检验转换结果。从测试结果看，static_cast编译不通过，dyanmic_cast、fast_dynamic_cast结果都正确，fast_dynamic_cast效率比dyanmic_cast高。
cout << "\n===== cast from pointer A to pointer B, should be not null =====" << endl; B* b = nullptr; start = clock(); for (int i = 0; i < count; i++) { b = dynamic_cast<B*>(a); } stop = clock(); cout << "dynamic_cast from A to B: " << b << ", " << "Time: " << stop - start << endl; b = nullptr; start = clock(); for (int i = 0; i < count; i++) { //b = static_cast<B*>(a); //compiler error } stop = clock(); cout << "static_cast from A to B: " << "compiler error" << endl; b = nullptr; start = clock(); for (int i = 0; i < count; i++) { b = fast_dynamic_cast<B>(a); } stop = clock(); cout << "fast_dynamic_cast from A to B: " << b << ", " << "Time: " << stop - start << endl;
测试结果：
===== cast from pointer A to pointer B, should be not null ===== dynamic_cast from A to B: 000001D65F2FA308, Time: 2927 static_cast from A to B: compiler error fast_dynamic_cast from A to B: 000001D65F2FA308, Time: 208
5. 测试类型转换从A到D，计时和检验转换结果。从测试结果看，static_cast结果不正确，应为空指针，dyanmic_cast、fast_dynamic_cast结果都正确，fast_dynamic_cast效率比dyanmic_cast高。
cout << "\n===== cast from pointer A to pointer D, should be null =====" << endl; D* d = nullptr; start = clock(); for (int i = 0; i < count; i++) { d = dynamic_cast<D*>(a); } stop = clock(); cout << "dynamic_cast from A to D: " << d << ", " << "Time: " << stop - start << endl; d = nullptr; start = clock(); for (int i = 0; i < count; i++) { d = static_cast<D*>(a); } stop = clock(); cout << "static_cast from A to D: " << d << ", " << "Time: " << stop - start << endl; d = nullptr; start = clock(); for (int i = 0; i < count; i++) { d = fast_dynamic_cast<D>(a); } stop = clock(); cout << "fast_dynamic_cast from A to D: " << d << ", " << "Time: " << stop - start << endl;
测试结果：
===== cast from pointer A to pointer D, should be null ===== dynamic_cast from A to D: 0000000000000000, Time: 3534 static_cast from A to D: 0000026050C6D310, Time: 0 fast_dynamic_cast from A to D: 0000000000000000, Time: 227
总结：根据以上测试结果，fast_dynamic_cast和dynamic_cast行为完全一致，并且效率上fast_dynamic_cast比dynamic_cast快10倍以上，在追求性能的底层开发完全可以用fast_dynamic_cast代替dynamic_cast。
附完整代码：
#include <string> #include <time.h> #include <iostream> #include <unordered_map> using namespace std; namespace { class A { public: static const char* classType(); virtual void* queryObject(const char* classType) const; }; const char* A::classType() { const char* s_classType = "A"; return s_classType; } void* A::queryObject(const char* classType) const { if (classType == A::classType()) return const_cast<A*>(this); return nullptr; } class B { public: static const char* classType(); virtual void* queryObject(const char* classType) const; }; const char* B::classType() { const char* s_classType = "B"; return s_classType; } void* B::queryObject(const char* classType) const { if (classType == B::classType()) return const_cast<B*>(this); return nullptr; } class C : public A, public B { public: static const char* classType(); void* queryObject(const char* classType) const override; }; const char* C::classType() { const char* s_classType = "C"; return s_classType; } void* C::queryObject(const char* classType) const { if (classType == C::classType()) return const_cast<C*>(this); if (void* res = A::queryObject(classType)) return res; if (void* res = B::queryObject(classType)) return res; return nullptr; } class D : public A { public: static const char* classType(); void* queryObject(const char* classType) const override; }; const char* D::classType() { const char* s_classType = "D"; return s_classType; } void* D::queryObject(const char* classType) const { if (classType == D::classType()) return const_cast<D*>(this); return A::queryObject(classType); } template <typename To, typename From> To* fast_dynamic_cast(const From* from) { if (!from) return nullptr; return static_cast<To*>(from->queryObject(To::classType())); } } int _tmain(int argc, _TCHAR* argv[]) { A* a = new C(); cout << "A* a = new C();" << endl; clock_t start, stop; const int count = 100000000; cout << "===== cast from pointer A to pointer C, should be not null =====" << endl; C* c = nullptr; start = clock(); for (int i = 0; i < count; i++) { c = dynamic_cast<C*>(a); } stop = clock(); cout << "dynamic_cast from A to C: " << c << ", " << "Time: " << stop - start << endl; c = nullptr; start = clock(); for (int i = 0; i < count; i++) { c = static_cast<C*>(a); } stop = clock(); cout << "static_cast from A to C: " << c << ", " << "Time: " << stop - start << endl; c = nullptr; start = clock(); for (int i = 0; i < count; i++) { c = fast_dynamic_cast<C>(a); } stop = clock(); cout << "fast_dynamic_cast from A to C: " << c << ", " << "Time: " << stop - start << endl; cout << "\n===== cast from pointer A to pointer B, should be not null =====" << endl; B* b = nullptr; start = clock(); for (int i = 0; i < count; i++) { b = dynamic_cast<B*>(a); } stop = clock(); cout << "dynamic_cast from A to B: " << b << ", " << "Time: " << stop - start << endl; b = nullptr; start = clock(); for (int i = 0; i < count; i++) { //b = static_cast<B*>(a); //compiler error } stop = clock(); cout << "static_cast from A to B: " << "compiler error" << endl; b = nullptr; start = clock(); for (int i = 0; i < count; i++) { b = fast_dynamic_cast<B>(a); } stop = clock(); cout << "fast_dynamic_cast from A to B: " << b << ", " << "Time: " << stop - start << endl; cout << "\n===== cast from pointer A to pointer D, should be null =====" << endl; D* d = nullptr; start = clock(); for (int i = 0; i < count; i++) { d = dynamic_cast<D*>(a); } stop = clock(); cout << "dynamic_cast from A to D: " << d << ", " << "Time: " << stop - start << endl; d = nullptr; start = clock(); for (int i = 0; i < count; i++) { d = static_cast<D*>(a); } stop = clock(); cout << "static_cast from A to D: " << d << ", " << "Time: " << stop - start << endl; d = nullptr; start = clock(); for (int i = 0; i < count; i++) { d = fast_dynamic_cast<D>(a); } stop = clock(); cout << "fast_dynamic_cast from A to D: " << d << ", " << "Time: " << stop - start << endl; delete a; a = nullptr; return 0; }
