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我上一篇博客C++的静态分发(CRTP)和动态分发(虚函数多态)的比较介绍了如何用CRTP(Curiously Recurring Template Pattern)实现静态分发，今天再讲另外一个CRTP的例子。在C++中静态函数是不能设成virtual的，但是用CRTP可以实现静态函数的多态。

先看代码:

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template<typename Derived>  class Parent 
{
public:
    void SayHi()
    {
        static_cast<Derived*>(this)->SayHiImpl();
    }
    void static SayHiStatic()
    {
        Derived::SayHiStatic();
    }
private:
    void SayHiImpl()
    {
        cout << "hi, i'm default!" << endl;
    }
};

class ChildA :public Parent<ChildA>
{
public:
    void SayHiImpl()
    {
        cout << "hi, i'm child A!" << endl;
    }
    void static SayHiStatic()
    {
        cout << "Static hi from Child A" << endl;
    }
};

class ChildB :public Parent<ChildB>
{
public:
    void static SayHiStatic()
    {
        cout << "Static hi from Child B" << endl;
    }
};

和上次的代码类似，这次父类有一个静态的interface，两个子类分别实现了自己的静态实现。

再看调用静态interface的函数和main函数。

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template<typename Derived> void CRTP(Parent<Derived>& p)
{
    p.SayHiStatic();
}

int _tmain(int argc, TCHAR* argv[])
{
    ChildA a;
    CRTP(a);
    cout << "size of ChildA: " << sizeof(a) << endl;

    ChildB b;
    CRTP(b);
    cout << "size of ChildB: " << sizeof(b) << endl;

    return 0;
}

运行可以得到如下输出：

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Static hi from Child A
size of ChildA: 1
Static hi from Child B
size of ChildB: 1
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