我想使用智能指针实现一个简单的引用计数.变量指针表示存储对象的指针,reference_count表示对象的副本总数.
>如果我们使用NULL初始化对象：reference_count = -1 else reference_count = 1
> copy ctor和operator = increment reference_count
>析构函数递减reference_count并且如果没有对指向对象的其他引用则执行其删除.
这是我的代码：
#ifndef smart_pointer_H
#define smart_pointer_H
template < typename T > class smart_pointer
{
private:
T* pointer;
int reference_count;
public:
smart_pointer() : pointer(0),reference_count(-1) {}
smart_pointer(T* p) : pointer(p)
{
if (p != NULL)
{
this->reference_count = 1;
}
else
{
this->reference_count = -1;
}
}
smart_pointer(const smart_pointer <T> & p) : pointer(p.pointer),reference_count(p.reference_count + 1) {}
bool operator == (const smart_pointer <T>& p) { return pointer == p.pointer; }
bool operator != (const smart_pointer <T>& p) { return pointer != p.pointer; }
~ smart_pointer()
{
if(-- reference_count == 0)
{
std::cout << "Destructing: " << '\n';
delete pointer;
}
}
T& operator * () { return *pointer; }
T* operator -> () { return pointer; }
smart_pointer <T> & operator = (const smart_pointer <T> & p)
{
if (this != &p)
{
if( -- reference_count == 0)
{
delete pointer;
}
pointer = p.pointer;
reference_count = p.reference_count + 1;
}
return *this;
}
};
这是我的测试代码,类样本存储2D点和两个指向任何其他2D点的指针.
template < typename T >
class smart_pointer;
class Point
{
private:
double x,y;
smart_pointer <Point> p1;
smart_pointer <Point> p2;
public:
Point(double xx,double yy): x(xx),y(yy) {this-> p1 = NULL; this->p2 = NULL;}
Point(double xx,double yy,smart_pointer <Point> p1,smart_pointer <Point> p2): x(xx),y(yy) {this-> p1 = p1,this->p2 = p2; }
double getX(){ return x;}
double getY(){ return y;}
void setX(double xx) {this->x = xx;}
void setY(double yy) {this->y = yy;}
void setP1(smart_pointer <Point> p1) {this->p1 = p1;}
void setP2(smart_pointer <Point> p2) {this->p2 = p2;}
void print()
{
std::cout << "x= " << x << " y= " << y << '\n';
std::cout << "p1" << '\n';
if (p1 != NULL)
{
p1->print();
}
std::cout << "p2" << '\n';
if (p2 != NULL)
{
p2->print();
}
std::cout << '\n';
}
};
2D点列表：
#include "Point.h"
class PointsList
{
private:
std::vector <smart_pointer <Point> > points;
public:
smart_pointer <Point> & operator [] ( int index ) {return points[index];}
public:
void push_back(smart_pointer <Point> p) {points.push_back(p);}
void erase(unsigned int index) {points.erase(points.begin() += index );}
void printPoints()
{
std::cout << "List of points" << '\n';
for (unsigned int i = 0; i < points.size(); i++)
{
points[i]->print();
}
}
};
测试代码：
#include "Point.h"
#include "PointsList.h"
int main()
{
smart_pointer <Point> pb = NULL;
pb = (new Point(0,0));
smart_pointer <Point> p0(new Point(0,0));
p0->print();
smart_pointer <Point> p1(new Point(10,10));
p1->print();
smart_pointer <Point> p2(new Point(20,20));
p2->print();
smart_pointer <Point> p3(new Point(30,30));
p3->print();
smart_pointer <Point> pa(p3);
p0->setP1(p2);
p0->setP2(p3);
p0->print();
p0 = p1;
p0->print();
p0->print();
PointsList pl1;
pl1.push_back(p0);
pl1.push_back(p1);
PointsList pl2;
pl2.push_back(p2);
pl2.push_back(p3);
pl1.erase(0);
pl1.printPoints();
pl2.printPoints();
return 0;
}
这种解决方案的优势或不足之处在哪里？如何为大量数据运行速度,转换,可能的继承问题等等.Thanx为您提供帮助.
我对这个例子还有一个问题：哪种类型的智能指针(共享,作用域)最适合这样的数据结构：
//Class with cross-references to points p1,p2
class PointTopo
{
private:
double x,y;
PointTopo * p1;
Point * p2;
public:
PointTopo(double xx,y(yy) {this-> p1 = NULL; this->p2 = NULL;}
...
};
//Class with cross references: topological model for delaunay triangulation class Edge { private: Point2D * start; Edge *next; Edge *prevIoUs; Edge *twin; ... };
谢谢你的帮助…
解决方法
您的引用计数不起作用.
如果您将两个智能指针复制或分配在一起,则需要使用相同的位置来执行计数.
目前,每个对象都保留自己的计数,因此它们可能会变得不同步.
smart_pointer<int> x(new x); // x.pointer: <good> x.reference_count: 1
{
smart_pointer<int> y; // y.pointer: NULL y.reference_count: -1
y = x; // x.pointer: <good> x.reference_count: 1
// y.pointer: <good> y.reference_count: 2
smart_pointer<int> z;
x = z; // x.pointer: NULL x.reference_count: 0 (BAD)
// z.pointer: NULL z.reference_count: -1
// y.pointer: <bad> (it was deleted by x) y.reference_count: 2
}
编辑：
按照评论中的要求说明问题.
在这一点上.我们刚刚创建了z.但尚未完成x = z;
x { pointer: 0xabcd1234 reference_count: 1 }
y { pointer: 0xabcd1234 reference_count: 2 }
z { pointer: NULL reference_count: -1 }
// So here is your assignment operator.
// Doing the `x = z` we will walk through the following code.
//
smart_pointer <T> & operator = (const smart_pointer <T> & p)
{
if (this != &p)
{
// We get here.
// Left hand side is 'x' so this condition will be true.
if( -- reference_count == 0)
{
// Now we are deleting a pointer.
// That is held by 'x'
delete pointer;
// But 'y' is holding a pointer with the same value.
// Now y is holding a pointer to a deleted variable.
}
// Here we copy 'z' into 'x'
// copy the pointer. That happens to be NULL.
pointer = p.pointer;
// Now we copy and increment the reference count.
// So 'x' has a value of 0 while 'z' has a value of -1.
// This also breaks the invariant on 'x' that NULL values should
// have a reference count of -1 (as X is NULL and ref-count is 0)
reference_count = p.reference_count + 1;
}
return *this;
}
如果有人试图使用’y’,我们现在有未定义的行为,因为它包含一个指向已经解除分配的内存的指针.
编辑经典(但过于简单的智能指针：
#include <vector>
template<typename T>
class SP
{
T* object;
size_t* count;
public:
SP(T* data)
try
// Use weird try around initializer list to catch new throwing.
// If it does we delete data to stop it from leaking.
:object(data),count(data ? new int(1) : NULL)
{ /* This is the constructor */}
catch(...)
{delete data;}
SP(): object(NULL),count(NULL) {}
//SP(T* data): object(data),count(new int(1)) {} // Lined up here so it look neat but implemented above to use weird try catch
SP(SP<T> const& rhs): object(rhs.object),count(rhs.count) {if (count) {++(*count);}}
SP<T>& operator=(SP<T> rhs) // Note implicit copy construction in rhs
{
// Using copy swap idiom for assignment.
// The copy is hidden because the parameter is pass by value.
this->swap(rhs);
return *this;
}
void swap(SP<T>& rhs) throw()
{
std::swap(object,rhs.object);
std::swap(count,rhs.count);
}
~SP()
{
if ((count) && (--(*count) == 0))
{
delete count;
delete object;
}
}
};
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