weak_ptr引入可以解决shared_ptr交叉引用时无法释放资源的问题。
示例代码：
#include <iostream>
#include <memory>
using namespace std;
class B;
class A{
public:
A(){cout << "A constructor ... "<< endl;}
~A(){cout << "A destructor ..." << endl;}
std::shared_ptr<B> pb;
};
class B{
public:
B(){cout << "B constructor ... "<< endl;}
~B(){cout << "B destructor ..." << endl;}
std::shared_ptr<A> pa;
};
int main(int argc, char **argv) {
std::shared_ptr<int> a = std::make_shared<int>(3);
std::shared_ptr<char> b = std::make_shared<char>('a');
std::cout << "shared_ptr object(int) size = " << sizeof(a) << std::endl;
std::cout << "shared_ptr object(char) size = " << sizeof(b) << std::endl;
std::weak_ptr<A> shadow_a;
std::weak_ptr<B> shadow_b;
{
std::shared_ptr<A> ptr_a = std::make_shared<A>();
std::shared_ptr<B> ptr_b = std::make_shared<B>();
shadow_a = ptr_a;
shadow_b = ptr_b;
ptr_a->pb = ptr_b;
ptr_b->pa = ptr_a;
cout << "reference count of A = " << shadow_a.use_count() << endl;
cout << "reference count of B = " << shadow_b.use_count() << endl;
cout << endl;
}
cout << "reference count of A = " << shadow_a.use_count() << endl;
cout << "reference count of B = " << shadow_b.use_count() << endl;
std::cout << "Hello, world!" << std::endl;
return 0;
}运行代码得到以下输出：
shared_ptr object(int) size = 16
shared_ptr object(char) size = 16
A constructor ...
B constructor ...
reference count of A = 2
reference count of B = 2reference count of A = 1
reference count of B = 1
Hello, world!
从结果可以看出，由于交叉引用导致申请的内存A，B无法正常释放。
为什么会这样呢？这个应该从析构原理进行考虑，shared_ptr引用计数需要为0才会进行析构！但是ptr_a离开作用域会导致A引用计数减少1，但是A的引用计数此时为1，那么 pb不会释放；同理，ptr_b离开作用域会导致B引用计数减少1，但是B的引用计数为此时为1，那么pa不会释放。如此导致了资源无法释放掉。
由于weak_ptr并不会改变shared_ptr的引用计数，所以修改类A，和类B中的shared_ptr对象为weak_ptr对象即可释放资源。
修改后的代码如下：
#include <iostream>
#include <memory>
using namespace std;
class B;
class A{
public:
A(){cout << "A constructor ... "<< endl;}
~A(){cout << "A destructor ..." << endl;}
//std::shared_ptr<B> pb;
std::weak_ptr<B> pb;
};
class B{
public:
B(){cout << "B constructor ... "<< endl;}
~B(){cout << "B destructor ..." << endl;}
//std::shared_ptr<A> pa;
std::weak_ptr<A> pa;
};
int main(int argc, char **argv) {
std::shared_ptr<int> a = std::make_shared<int>(3);
std::shared_ptr<char> b = std::make_shared<char>('a');
std::cout << "shared_ptr object(int) size = " << sizeof(a) << std::endl;
std::cout << "shared_ptr object(char) size = " << sizeof(b) << std::endl;
std::weak_ptr<A> shadow_a;
std::weak_ptr<B> shadow_b;
{
std::shared_ptr<A> ptr_a = std::make_shared<A>();
std::shared_ptr<B> ptr_b = std::make_shared<B>();
shadow_a = ptr_a;
shadow_b = ptr_b;
ptr_a->pb = ptr_b;
ptr_b->pa = ptr_a;
cout << "reference count of A = " << shadow_a.use_count() << endl;
cout << "reference count of B = " << shadow_b.use_count() << endl;
cout << endl;
}
cout << "reference count of A = " << shadow_a.use_count() << endl;
cout << "reference count of B = " << shadow_b.use_count() << endl;
std::cout << "Hello, world!" << std::endl;
return 0;
}
运行结果如下，可以正常释放资源。
shared_ptr object(int) size = 16
shared_ptr object(char) size = 16
A constructor ...
B constructor ...
reference count of A = 1
reference count of B = 1B destructor ...
A destructor ...
reference count of A = 0
reference count of B = 0
Hello, world!
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