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plain-ptr.cpp
资源名称:CppPrimer.rar [点击查看]
上传用户:qdkongtiao
上传日期:2022-06-29
资源大小:356k
文件大小:3k
源码类别:
书籍源码
开发平台:
Visual C++
- #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;
- }