#include <iostream>

using namespace std;
enum BREED {GOLDEN, CAIRN, DANDIE, SHETLAND, DOBERMAN, LAB};

class Mammal
{
public:
    Mammal():itsAge(2),itsWeight(5){}//initialize itsAge=2,itsWeight=5
    ~Mammal(){}

    int GetAge()const{return itsAge;}
    void SetAge(int age){itsAge= age;}
    int GetWeight()const{return itsWeight;}
    void SetWeight(int weight){itsWeight = weight;}

    void Speek()const{cout<<"Mammal sound!\n";}
    void Sleep()const{cout<<"shhh. I'm sleeping.\n";}

protected:
    int itsAge;
    int itsWeight;
};

class Dog:public Mammal
{
public:
    Dog():itsBreed(GOLDEN){}//what it means?
    ~Dog(){}

    BREED GetBreed()const{return itsBreed;}
    void SetBreed(BREED breed){itsBreed = breed;}

    void WagTail()const{cout<<"Tail wagging...\n";}
    void BegForFood()const{cout<<"Begging for food...\n";}

protected:
    BREED itsBreed;
};

int main()
{
    Dog Fido;
    Fido.Speek();
    Fido.WagTail();
    cout<<"Fido is "<<Fido.GetAge()<<" years old."<<endl;
    return 0;
}

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帮忙说一下Dog():itsBreed(GOLDEN){}到底有什么用啊。我就知道这是构造函数，用来初始化类对象的。itsBreed(GOLDEN)就是初始化，相当于itsBreed = GOLDEN;但是没发觉这个在程序中的用处啊。。。

Dog():itsBreed(GOLDEN){}

这个纯属为了维护一个无参的构造函数。作者想初始化成员变量，防止对象生成后，直接访问成员变量的误操作。又不相暴露一个带参的构造函数。
所以就用缺省枚举值GOLDEN初始化了itsBreed成员变量。