C++ Tutorial/Pointer/auto ptr
Содержание
class auto_ptr: improved standard conforming implementation
<source lang="cpp">/* The following code example is taken from the book
* "The C++ Standard Library - A Tutorial and Reference" * by Nicolai M. Josuttis, Addison-Wesley, 1999 * * (C) Copyright Nicolai M. Josuttis 1999. * Permission to copy, use, modify, sell and distribute this software * is granted provided this copyright notice appears in all copies. * This software is provided "as is" without express or implied * warranty, and with no claim as to its suitability for any purpose. */
/* class auto_ptr
* - improved standard conforming implementation */
namespace std {
// auxiliary type to enable copies and assignments (now global)
template<class Y>
struct auto_ptr_ref {
Y* yp;
auto_ptr_ref (Y* rhs)
: yp(rhs) {
}
};
template<class T>
class auto_ptr {
private:
T* ap; // refers to the actual owned object (if any)
public:
typedef T element_type;
// constructor
explicit auto_ptr (T* ptr = 0) throw()
: ap(ptr) {
}
// copy constructors (with implicit conversion)
// - note: nonconstant parameter
auto_ptr (auto_ptr& rhs) throw()
: ap(rhs.release()) {
}
template<class Y>
auto_ptr (auto_ptr<Y>& rhs) throw()
: ap(rhs.release()) {
}
// assignments (with implicit conversion)
// - note: nonconstant parameter
auto_ptr& operator= (auto_ptr& rhs) throw() {
reset(rhs.release());
return *this;
}
template<class Y>
auto_ptr& operator= (auto_ptr<Y>& rhs) throw() {
reset(rhs.release());
return *this;
}
// destructor
~auto_ptr() throw() {
delete ap;
}
// value access
T* get() const throw() {
return ap;
}
T& operator*() const throw() {
return *ap;
}
T* operator->() const throw() {
return ap;
}
// release ownership
T* release() throw() {
T* tmp(ap);
ap = 0;
return tmp;
}
// reset value
void reset (T* ptr=0) throw() {
if (ap != ptr) {
delete ap;
ap = ptr;
}
}
/* special conversions with auxiliary type to enable copies and assignments
*/
auto_ptr(auto_ptr_ref<T> rhs) throw()
: ap(rhs.yp) {
}
auto_ptr& operator= (auto_ptr_ref<T> rhs) throw() { // new
reset(rhs.yp);
return *this;
}
template<class Y>
operator auto_ptr_ref<Y>() throw() {
return auto_ptr_ref<Y>(release());
}
template<class Y>
operator auto_ptr<Y>() throw() {
return auto_ptr<Y>(release());
}
};
}</source>
const auto_ptr
<source lang="cpp">/* The following code example is taken from the book
* "The C++ Standard Library - A Tutorial and Reference" * by Nicolai M. Josuttis, Addison-Wesley, 1999 * * (C) Copyright Nicolai M. Josuttis 1999. * Permission to copy, use, modify, sell and distribute this software * is granted provided this copyright notice appears in all copies. * This software is provided "as is" without express or implied * warranty, and with no claim as to its suitability for any purpose. */
- include <iostream>
- include <memory>
using namespace std; /* define output operator for auto_ptr
* - print object value or NULL */
template <class T> ostream& operator<< (ostream& strm, const auto_ptr<T>& p) {
// does p own an object ?
if (p.get() == NULL) {
strm << "NULL"; // NO: print NULL
}
else {
strm << *p; // YES: print the object
}
return strm;
} int main() {
const auto_ptr<int> p(new int(42)); const auto_ptr<int> q(new int(0)); const auto_ptr<int> r; cout << "after initialization:" << endl; cout << " p: " << p << endl; cout << " q: " << q << endl; cout << " r: " << r << endl; *q = *p;
// *r = *p; // ERROR: undefined behavior
*p = -77; cout << "after assigning values:" << endl; cout << " p: " << p << endl; cout << " q: " << q << endl; cout << " r: " << r << endl;
// q = p; // ERROR at compile time // r = p; // ERROR at compile time }</source>
after initialization: p: 42 q: 0 r: NULL after assigning values: p: -77 q: 42 r: NULL
Define output operator for auto_ptr print object value or NULL
<source lang="cpp">/* The following code example is taken from the book
* "The C++ Standard Library - A Tutorial and Reference" * by Nicolai M. Josuttis, Addison-Wesley, 1999 * * (C) Copyright Nicolai M. Josuttis 1999. * Permission to copy, use, modify, sell and distribute this software * is granted provided this copyright notice appears in all copies. * This software is provided "as is" without express or implied * warranty, and with no claim as to its suitability for any purpose. */
- include <iostream>
- include <memory>
using namespace std; /* define output operator for auto_ptr
* - print object value or NULL */
template <class T> ostream& operator<< (ostream& strm, const auto_ptr<T>& p) {
// does p own an object ?
if (p.get() == NULL) {
strm << "NULL"; // NO: print NULL
}
else {
strm << *p; // YES: print the object
}
return strm;
} int main() {
auto_ptr<int> p(new int(42)); auto_ptr<int> q; cout << "after initialization:" << endl; cout << " p: " << p << endl; cout << " q: " << q << endl; q = p; cout << "after assigning auto pointers:" << endl; cout << " p: " << p << endl; cout << " q: " << q << endl; *q += 13; // change value of the object q owns p = q; cout << "after change and reassignment:" << endl; cout << " p: " << p << endl; cout << " q: " << q << endl;
}</source>
after initialization: p: 42 q: NULL after assigning auto pointers: p: NULL q: 42 after change and reassignment: p: 55 q: NULL
Demonstrate an auto_ptr
<source lang="cpp">#include <iostream>
- include <memory>
using namespace std; class MyClass { public:
MyClass() {
cout << "constructing\n";
}
~MyClass() {
cout << "destructing\n";
}
void f() {
cout << "f()\n";
}
}; int main() {
auto_ptr<MyClass> p1(new MyClass), p2; p2 = p1; // transfer ownership p2->f(); // can assign to a normal pointer MyClass *ptr = p2.get(); ptr->f(); return 0;
}</source>
constructing f() f() destructing
