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()
    {
        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;
}

原文地址：https://www.cnblogs.com/ldlchina/p/8698346.html