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：
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#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：
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#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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原文链接：https://blog.csdn.net/lybhit/article/details/122799780
