C++高效安全的运行时动态类型转换
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()
{
static 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()
{
static 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()
{
static 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()
{
static 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 = ; 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 = ; 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 = ; 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:
static_cast from A to C: 00000202D48C9FE0, Time:
fast_dynamic_cast from A to C: 00000202D48C9FE0, Time:
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 = ; 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 = ; 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 = ; 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:
static_cast from A to B: compiler error
fast_dynamic_cast from A to B: 000001D65F2FA308, Time:
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 = ; 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 = ; 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 = ; 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: , Time:
static_cast from A to D: 0000026050C6D310, Time:
fast_dynamic_cast from A to D: , Time:
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 "stdafx.h" #include <string>
#include <time.h>
#include <iostream> using namespace std; namespace
{
class A
{
public:
static const char* classType();
virtual void* queryObject(const char* classType) const;
template <typename From>
static A* queryObject(const From* from)
{
if (!from)
return nullptr; return static_cast<A*>(from->queryObject(A::classType()));
}
}; const char* A::classType()
{
static 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;
template <typename From>
static B* queryObject(const From* from)
{
if (!from)
return nullptr; return static_cast<B*>(from->queryObject(B::classType()));
}
}; const char* B::classType()
{
static 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;
template <typename From>
static C* queryObject(const From* from)
{
if (!from)
return nullptr; return static_cast<C*>(from->queryObject(C::classType()));
}
}; const char* C::classType()
{
static 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;
template <typename From>
static D* queryObject(const From* from)
{
if (!from)
return nullptr; return static_cast<D*>(from->queryObject(D::classType()));
}
}; const char* D::classType()
{
static 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)
{
return To::queryObject(from);
}
} int main()
{
A* a = new C();
cout << "A* a = new C();" << endl; clock_t start, stop;
const int count = ; cout << "===== cast from pointer A to pointer C, should be not null =====" << endl; C* c = nullptr;
start = clock();
for (int i = ; 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 = ; 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 = ; 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 = ; 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 = ; 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 = ; 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 = ; 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 = ; 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 = ; 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 ;
}
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