C++中的智能指针实际上是代理模式与RAII的结合。
自定义unique_ptr，主要是release()和reset()。代码如下。
#include <iostream>
using namespace std;
template<typename T>
class my_unique_ptr {
public:
my_unique_ptr(T *p = 0): pointee(p) {};
~my_unique_ptr() {
delete pointee;
}
T &operator * () const {
return *pointee;
}
T *operator -> () const {
return pointee;
}
T *get() {
return pointee;
}
T *release() {
T *oldPointee = pointee;
pointee = 0;
return oldPointee;
}
void reset(T *p = 0) {
if(pointee != p) {
delete pointee;
pointee = p;
}
}
private:
my_unique_ptr(const my_unique_ptr<T> &rhs);
my_unique_ptr<T> &operator = (const my_unique_ptr<T> &rhs);
T *pointee;
};
struct C {
~C() {
cout << "destructor" << endl;
};
void print() {
cout << "print" << endl;
};
};
int main() {
my_unique_ptr<C> u1;
cout << u1.get() << endl; //0
my_unique_ptr<C> u2(new C());
u2->print(); //print
my_unique_ptr<C> u3(u2.release());
cout << u2.get() << endl; //0
u3->print(); //print
my_unique_ptr<C> u4;
u4.reset(u3.release());
cout << u3.get() << endl; //0
u4->print(); //print
return 0; //destructor
}
自定义shared_ptr，主要是引用计数，代码如下。
#include <iostream>
using namespace std;
template<typename T>
class my_shared_ptr {
public:
my_shared_ptr(T *p = 0): pointee(p), count(new size_t(0)) {
if(p) ++*count;
};
my_shared_ptr(const my_shared_ptr<T> &rhs): pointee(rhs.pointee), count(rhs.count) {
++*count;
}
my_shared_ptr<T> &operator = (const my_shared_ptr<T> &rhs) {
if(&rhs != this) {
decrease_count();
pointee = rhs.pointee;
count = rhs.count;
++*count;
}
return *this;
}
~my_shared_ptr() {
decrease_count();
}
size_t use_count() {
return *count;
}
T &operator * () const {
return *pointee;
}
T *operator -> () const {
return pointee;
}
private:
void decrease_count() {
if(--*count == 0) {
delete pointee;
delete count;
}
}
T *pointee;
size_t *count;
};
struct C {
~C() {
cout << "destructor" << endl;
};
void print() {
cout << "print" << endl;
};
};
int main() {
my_shared_ptr<C> p1;
cout << "p1: " << p1.use_count() << endl; //p1: 0
my_shared_ptr<C> p2(new C());
cout << "p2: " << p2.use_count() << endl; //p2: 1
my_shared_ptr<C> p3(p2);
cout << "p3: " << p3.use_count() << endl; //p3: 2
my_shared_ptr<C> p4 = p3;
cout << "p4: " << p4.use_count() << endl; //p4: 3
{
my_shared_ptr<C> p5(p4);
cout << "p5: " << p5.use_count() << endl; //p5: 4
}
cout << "p2: " << p2.use_count() << endl; //p2: 3
cout << "p3: " << p3.use_count() << endl; //p3: 3
cout << "p4: " << p4.use_count() << endl; //p4: 3
(*p4).print(); //print
p4->print(); //print
return 0; //destructor
}