Setting limits
◆ to keep the client programmer's hands off members they shouldn't touch.
◆ to allow the library designer to change the internal workings of the structure without worrying about how it will affect the client programmer.
C++ access control
◆ The mumbers of a class can be cataloged, marked as:
- public //公开的
- private //只有类自己的成员函数可以访问
- protected //只有类自己的成员函数或子类可以访问
public
◆ public means all member declarations that follow are available to everyone
private
◆ The private keyword means that no one can access that member except inside function members of that type.
【例】
#include <iostream> using namespace std; class A { private: int i; public: void set(int i); void g(A* p); }; void A::set(int i) { this->i = i; } void A::g(A* p) { cout << p->i << endl; } int main(int argc, const char* argv[]) { A a; A b; b.set(100); a.g(&b); return 0; }这说明，private的限制仅仅在编译时刻，同一类的不同对象可以借助指针互相访问其私有的成员
Friends
◆ to explicitly grant access to a function that isn't a member of the structure
◆ The class itself controls which code has access to its members.
◆ Can declare a global function as a friend, as well as a member function of another class, or even an entire class, as a friend.
【例】
struct X; //前向声明 struct Y { void f(X*); }; struct X { private: int i; public: void initialize(); friend void g(X*, int); //Global friend friend void Y::f(X*); //Struct member friend friend struct Z; //Entire struct is a friend friend void h(); }; void X::initialize() { i = 0; } void g(X* x, int i) { x->i = i; } void Y::f(X* x) { x->i = 47; } struct Z { private: int j; public: void ff(X*); }; void Z::ff(X* x) { x->i = 10; }friend的授权也是在编译时刻检查的
class vs. struct
◆ class defaults to private
◆ struct defaults to public
