在设计class时,该class可能会被用作智能指针,若其成员变量使用了指向自己的shared_ptr,这样就形成了shared_ptr的嵌套,导致资源无法回收,造成内存泄漏。
官方解释:
因为实现使用引用计数,所以shared_ptr实例的循环将不会被回收。例如,如果main()将shared_ptr保存到a,而后者直接或间接地将shared_ptr保存回a,则a的使用计数将为2。对原始的’ shared_ptr的破坏将会留下一个使用计数为1的悬空值。使用weak_ptr来“中断循环”。
Because the implementation uses reference counting, cycles of shared_ptr instances will not be reclaimed. For example, if main() holds a shared_ptr to A, which directly or indirectly holds a shared_ptr back to A, A’s use count will be 2. Destruction of the original `shared_ptr will leave A dangling with a use count of 1. Use weak_ptr to “break cycles.”
解决方案:
通过weak_ptr防止shared_ptr循环计数
test result:
#include "boost/smart_ptr.hpp"
#include <iostream>
using namespace boost;
using namespace std;
class B;
class A
{
public:
A(){ cout << "A()\n";}
~A(){ cout << "~A()\n";}
void setptr(boost::shared_ptr<B> b)
{
m_pb = b;
}
void setptrref(boost::shared_ptr<B> &b)
{
m_pb = b;
}
boost::shared_ptr<B> m_pb;
boost::weak_ptr<B> m_wb;
};
class B
{
public:
B(){ cout << "B()\n";}
~B(){ cout << "~B()\n";}
boost::shared_ptr<A> m_pa;
boost::weak_ptr<A> m_wa;
};
class C
{
public:
C(){ cout << "c()\n";}
~C(){ cout << "~c()\n";}
boost::shared_ptr<C> m_pc;
};
// 间接嵌套
void t1()
{
boost::shared_ptr<A> pa = boost::make_shared<A>();
boost::shared_ptr<B> pb = boost::make_shared<B>();
pa->m_pb = pb;
pb->m_pa = pa;
std::cout << __FUNCTION__ << " pa count: " << pa.use_count() << std::endl;
std::cout << __FUNCTION__ << " pb count: " << pb.use_count() << std::endl;
}
// copy construct
void t2()
{
boost::shared_ptr<B> pb = boost::make_shared<B>();
boost::shared_ptr<B> pb1 = pb;
std::cout << __FUNCTION__ << " pb count: " << pb.use_count() << std::endl;
std::cout << __FUNCTION__ << " pb1 count: " << pb1.use_count() << std::endl;
}
// assign
void t3()
{
boost::shared_ptr<B> pb = boost::make_shared<B>();
boost::shared_ptr<B> pb1;
pb1 = pb;
std::cout << __FUNCTION__ << " pb count: " << pb.use_count() << std::endl;
std::cout << __FUNCTION__ << " pb1 count: " << pb1.use_count() << std::endl;
}
// 直接嵌套
void t4()
{
boost::shared_ptr<C> pc = boost::make_shared<C>();
pc->m_pc = pc;
std::cout << __FUNCTION__ << " pc count: " << pc.use_count() << std::endl;
}
// weak_ptr
void t5()
{
boost::shared_ptr<A> pa = boost::make_shared<A>();
boost::shared_ptr<B> pb = boost::make_shared<B>();
pa->m_wb = pb;
pb->m_wa = pa;
std::cout << __FUNCTION__ << " pa count: " << pa.use_count() << std::endl;
std::cout << __FUNCTION__ << " pb count: " << pb.use_count() << std::endl;
}
void t6()
{
boost::shared_ptr<A> pa = boost::make_shared<A>();
boost::shared_ptr<B> pb = boost::make_shared<B>();
pa->setptr(pb);
std::cout << __FUNCTION__ << " pa count: " << pa.use_count() << std::endl;
std::cout << __FUNCTION__ << " pb count: " << pb.use_count() << std::endl;
}
void t7()
{
boost::shared_ptr<A> pa = boost::make_shared<A>();
boost::shared_ptr<B> pb = boost::make_shared<B>();
pa->setptrref(pb);
std::cout << __FUNCTION__ << " pa count: " << pa.use_count() << std::endl;
std::cout << __FUNCTION__ << " pb count: " << pb.use_count() << std::endl;
}
void t8()
{
boost::shared_ptr<A> pa = boost::make_shared<A>();
boost::shared_ptr<B> pb = boost::make_shared<B>();
pa->m_pb = pb;
std::cout << __FUNCTION__ << " pa count: " << pa.use_count() << std::endl;
std::cout << __FUNCTION__ << " pb count: " << pb.use_count() << std::endl;
}
void testboostptr()
{
t1();
t2();
t3();
t4();
t5();
t6();
t7();
t8();
}
A()
B()
t1 pa count: 2
t1 pb count: 2
B()
t2 pb count: 2
t2 pb1 count: 2
~B()
B()
t3 pb count: 2
t3 pb1 count: 2
~B()
c()
t4 pc count: 2
A()
B()
t5 pa count: 1
t5 pb count: 1
~B()
~A()
A()
B()
t6 pa count: 1
t6 pb count: 2
~A()
~B()
A()
B()
t7 pa count: 1
t7 pb count: 2
~A()
~B()
A()
B()
t8 pa count: 1
t8 pb count: 2
~A()
~B()
从结果可以看出,t1()和t4()使用了shared_ptr嵌套,导致资源没有被回收。t5()中使用了weak_ptr,则资源能够正常回收。
总结归纳为两类问题:
- 自我嵌套,如t4()用例
- 相互依赖,你中有我我中有你,如t1()用例