www.gusucode.com > C++ Primer 书籍源代码源码程序 > C++ Primer 书籍源代码源码程序/code/13/old/plain-ptr.cpp
//Download by http://www.NewXing.com #include <iostream> using std::ostream; using std::cout; using std::endl; #include <string> // class that has a pointer member that behaves like a plain pointer class HasPtr { public: friend ostream& operator<<(ostream&, const HasPtr&); // copy of the values we're given HasPtr(int *p, int i): ptr(p), val(i) { } // const members to return the value of the indicated data member int *get_ptr() const { return ptr; } int get_int() const { return val; } // nonconst members to change the indicated data member void set_ptr(int *p) { ptr = p; } void set_int(int i) { val = i; } // return or change the value pointed to, so ok for const objects int get_ptr_val() const { return *ptr; } void set_ptr_val(int val) const { *ptr = val; } private: int *ptr; int val; }; void f3() { int i = 42; HasPtr p1(&i, 42); HasPtr p2 = p1; cout << p2.get_ptr_val() << endl; p1.set_ptr_val(0); cout << p2.get_ptr_val() << endl; } int main() { int obj = 0; HasPtr ptr1(&obj, 42); // int* member points to obj, val is 42 HasPtr ptr2(ptr1); // int* member points to obj, val is 42 cout << "(1) ptr1: " << ptr1 << endl << "ptr2: " << ptr2 << endl; ptr1.set_ptr_val(42); // sets object to which both ptr1 and ptr2 point ptr2.get_ptr_val(); // returns 42 cout << "(2) ptr1: " << ptr1 << endl << "ptr2: " << ptr2 << endl; ptr1.set_int(0); // changes val member only in ptr1 ptr2.get_int(); // returns 42 ptr1.get_int(); // returns 0 cout << "(3) ptr1: " << ptr1 << endl << "ptr2: " << ptr2 << endl; int *ip = new int(42); // dynamically allocated int initialized to 42 HasPtr ptr(ip, 10); // HasPtr points to same object as ip does delete ip; // object pointed to by ip is freed ptr.set_ptr_val(0); // disaster: The object to which HasPtr points was freed! cout << "(4) ptr: " << ptr << endl; f3(); return 0; } void f(int *p) { // new socpe // allocates new int to hold a copy of the object to which p points HasPtr local_copy(p, 0); // . . . } // local_copy goes out of scope void f2() { int obj= 42; HasPtr local1(&obj, 0); // allocates a new int to hold a copy of obj if (obj) { // new scope HasPtr local2(local1); // local1 and local2 hold same pointer // . . . } // local2 goes out of scope, object to which it points is freed local1.set_ptr_val(0); // disaster -- the object to which local1 points was freed! } ostream& operator<<(ostream &os, const HasPtr &hp) { cout << "*ptr: " << hp.get_ptr_val() << "\tval: " << hp.get_int() << endl; return os; }