value-ptr.cpp

C++ primer 4 书中源码

#include <iostream>
using std::ostream; using std::cout; using std::endl;
#include <string>

class HasPtr {
public:
    // no point to passing a pointer if we're going to copy it anyway
    // store pointer to a copy of the object we're given
    HasPtr(const int &p, int i): ptr(new int(p)), val(i) {}

    // copy members and increment the use count
    HasPtr(const HasPtr &orig):
       ptr(new int (*orig.ptr)), val(orig.val) { }

    HasPtr& operator=(const HasPtr&);
    // wrong: don't define a destructor without also defining copy and assign
    ~HasPtr() { delete ptr; } 
    friend ostream& operator<<(ostream&, const HasPtr&);

    // accessors must change to fetch value from Ptr object
    int get_ptr_val() const { return *ptr; } 
    int get_int() const { return val; }

    // change the appropriate 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() const { return ptr; } 
    void set_ptr_val(int p) const { *ptr = p; }
private:
    int *ptr;        // points to an int
    int val;
};

HasPtr& HasPtr::operator=(const HasPtr &rhs)
{
    // Note: Every HasPtr is guaranteed to point at an actual int;
    //       We know that ptr cannot be a zero pointer
    *ptr = *rhs.ptr;      // copy the value pointed to
    val = rhs.val;        // copy the int 
    return *this;
}

ostream& operator<<(ostream &os, const HasPtr &hp)
{
    os << "*ptr: " << hp.get_ptr_val() << "\tval: " << hp.get_int() << endl;
    return os;
}

int main()
{
int obj = 0;

HasPtr ptr1(obj, 42);  // int* member points copy of obj, val is 42
HasPtr ptr2(ptr1);     // int* member points new copy obj, val is 42

cout << "(1) ptr1: " << ptr1 << endl << "ptr2: " << ptr2 << endl;

ptr1.set_ptr_val(42); // sets copy in ptr1; value in ptr2 unchanged
ptr2.get_ptr_val();   // returns 0

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); // ok: ptr has its own copy

cout << "(4) ptr: " << ptr << endl;
}