相比C语言，C++中通过class/struct来定义既包含数据，又包含行为的结构，从而支持了“对象”。现实世界中，一个人（一个对象）通常 拥有一些资产（数据），并且掌握某些技能（行为），并且这些资产和技能通常可以分为三类：
- 可以与任何人分享的
- 有限分享的，比如留给子孙后代的财产或本领
- 除了自己之外谁也不能用的，比如给自己留的棺材^_^
为了表达类似的概念，在C++中使用public、protected以及private，分别代表可任意分享的、有限分享的以及独享的。比现实世界稍微复杂些，在C++中这三个关键字不仅可以修饰类成员，还可以修饰类的继承关系。
当这三个关键字用在类成员时：
class Base { public: void publicMethod() { cout << "Begin of " << __FUNCTION__ << endl; cout << "End of " << __FUNCTION__ << endl; } static void staticPublicMethod(Base* obj) { // just call this class‘s static private method staticPrivateMethod(obj); } int getProperty() { return property_; } protected: void protectedMethod() { cout << "Begin of " << __FUNCTION__ << endl; cout << "call publicMethod from protected method: " << endl; publicMethod(); cout << "End of " << __FUNCTION__ << endl; } private: void privateMethod() { cout << "Begin of " << __FUNCTION__ << endl; cout << "call publicMethod from private method: " << endl; publicMethod(); cout << "call protected method from private method: " << endl; protectedMethod(); cout << "End of " << __FUNCTION__ << endl; } static void staticPrivateMethod(Base* obj) { // ok obj->publicMethod(); // ok obj->protectedMethod(); // ok obj->privateMethod(); } int property_; }; class Derived : public Base { public: void accessBase() { cout << "In Derived::" << __FUNCTION__ << ": " << endl; cout << "we can access Base‘s public method:" << endl; publicMethod(); cout << "we can access Base‘s protected method too: " << endl; protectedMethod(); cout << "but we can‘t access Base‘s private method!" << endl; // we will get a compile error if we try to call privateMethod: ‘privateMethod‘ is a private member of ‘Base‘ // privateMethod(); } static void staticDerivedMethod(Derived* obj) { // ok obj->protectedMethod(); // compile error: ‘privateMethod‘ is a private member of ‘Base‘ // obj->privateMethod(); } }; int main() { Base obj; // ok obj.publicMethod(); // compile error: ‘protectedMethod‘ is a protected member of ‘Base‘ // obj.protectedMethod(); // compile error: ‘privateMethod‘ is a private member of ‘Base‘ // obj.privateMethod(); // ok Base::staticPublicMethod(&obj); Derived dobj; // ok dobj.accessBase(); // ok Derived::staticDerivedMethod(&dobj); return 0; }
从上面的程序中，可以看出：
- 用public修饰的成员，既可以在其类内部使用（在privateMethod中调用），也可以在类外部通过类的实例来访问（main函数中调用obj.publicMethod()）
- 用protected修饰的成员，能在该类内部或者其派生类（严格上说是非私有继承的派生类，下面会详细介绍）中使用（见privateMethod、accessBase以及staticDerivedMethod）
- 用private修饰的成员，仅能在该类内部使用，在类外部或者派生类中都不能使用。
注意：以上描述中的“使用”，指的是直接引用（如不能在Derived中直接调用privateMethod）。尽管不能直接引用，但是仍然可以通过其他方法来访问（虽然在 Derived中无法直接访问Base的property_，但通过getProperty()，我们仍然可以在需要的时候获取其值）。
除了可以修饰类成员，这三个关键字还可以修饰类的继承关系：
class Base { public: void publicBaseMethod() { cout << __FUNCTION__ << endl; } protected: void protectedBaseMethod() { cout << __FUNCTION__ << endl; } private: void privateBaseMethod() { cout << __FUNCTION__ << endl; } }; // public inheritance, all Base‘s members remain the same scope class PublicDerived : public Base { public: void accessBase() { // ok publicBaseMethod(); // ok protectedBaseMethod(); // compile error: ‘privateBaseMethod‘ is a private member of ‘Base‘ // privateBaseMethod(); } }; // protected inheritance, Base‘s public members will become protected in ProtectedDerived class ProtectedDerived : protected Base { public: void accessBase() { // ok publicBaseMethod(); // ok protectedBaseMethod(); // compile error: ‘privateBaseMethod‘ is a private member of ‘Base‘ // privateBaseMethod(); } }; // private inheritance, Base‘s public and protected members will become private in PrivateDerived class PrivateDerived : private Base { public: void accessBase() { // ok publicBaseMethod(); // ok protectedBaseMethod(); // compile error: ‘privateBaseMethod‘ is a private member of ‘Base‘ // privateBaseMethod(); } }; class SonOfPrivateDerived : public PrivateDerived { public: void accessParent() { // compile error: ‘publicBaseMethod‘ is a private member of ‘Base‘ // publicBaseMethod(); // compile error: ‘protectedBaseMethod‘ is a private member of ‘Base‘ // protectedBaseMethod(); // IMPORTANTE NOTE: why PrivateDerived can access publicBaseMethod and protectedBaseMethod, // although they are both private members of ‘Base‘??? // compile error: ‘privateBaseMethod‘ is a private member of ‘Base‘ // privateBaseMethod(); } }; int main() { PublicDerived pubd; // ok pubd.publicBaseMethod(); // compile error: ‘protectedBaseMethod‘ is a protected member of ‘Base‘ // pubd.protectedBaseMethod(); // compile error: ‘privateBaseMethod‘ is a private member of ‘Base‘ // pubd.privateBaseMethod(); pubd.accessBase(); ProtectedDerived prod; // compile error: ‘publicBaseMethod‘ is a protected member of ‘Base‘, // NOTE: the Base‘s public members now become protected in ProtectedDerived‘s perspective // prod.publicBaseMethod(); // compile error: ‘protectedBaseMethod‘ is a protected member of ‘Base‘ // NOTE: The Base‘s protected members remain protected in ProtectedDerived‘s perspective // prod.protectedBaseMethod(); // compile error: ‘privateBaseMethod‘ is a private member of ‘Base‘ // NOTE: The Base‘s private members remain private in ProtectedDerived‘s perspective // prod.privateBaseMethod(); prod.accessBase(); PrivateDerived prid; // compile error: ‘publicBaseMethod‘ is a private member of ‘Base‘ // NOTE: publicBaseMethod now become private in PrivateDerived‘s perspective // prid.publicBaseMethod(); // compile error: ‘protectedBaseMethod‘ is a private member of ‘Base‘ // NOTE: protectedBaseMethod also become private in PrivateDerived‘s perspective // prid.protectedBaseMethod(); // compile error: ‘privateBaseMethod‘ is a private member of ‘Base‘ // prid.privateBaseMethod(); prid.accessBase(); SonOfPrivateDerived pubprid; // NOTE: check the comments in accessParent to get the point pubprid.accessParent(); return 0; }
根据以上程序，可以得出public、protected和private在修饰继承关系中的作用为：
- 当使用公有继承（public Base）时，派生类中看到的基类成员属性与基类中定义的一致，如示例程序中publicBaseMethod、protectedBaseMethod和privateBaseMethod仍然分别为public、protected和private
- 当使用保护继承（protected Base）时，派生类中看到的基类的成员属性与基类定义不一致，原来基类中的public成员在派生类中被认为是基类的protected成员了，如示例中无法通过ProtectedDerived类的实例在类外部调用publicBaseMethod
- 当使用私有继承（private Base）时，派生类中看到的基类成员属性同样与基类定义不一致，即原来基类中的public和protected成员在派生类中都被当成了private成员了，如示例中只能在PrivateDerived类内部调用publicBaseMethod和protectedBaseMethod
- 在私有继承中，尽管当在外部调用public或protected成员时，其出错信息（通过clang++编译得到的）都是“‘xxx‘ is a private member of ‘Base‘”，事实上并非如此：在私有继承关系中，基类的public和protected成员并非被当做基类的私有成员了，而是被当做派生类的私有成员了。如果是被当做是基类的私有成员，那么PrivateDerived中是无法访问publicBaseMethod和protectedBaseMethod的；但实际上，从PrivateDerived中可以访问publicBaseMethod和protectedBaseMethod，而SonOfPrivateDerived中无法访问它们，因此publicBaseMethod和protectedBaseMethod应该是PrivateDerived的私有成员而非Base的私有成员。