C语言 智能指针 shared_ptr 和 weak_ptr

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 
#include 

using namespace std;

class B;

class A{
public:
    A(){cout << "A constructor ... "<< endl;}
    ~A(){cout << "A destructor ..." << endl;}
    std::shared_ptr pb;
};

class B{
public:
    B(){cout << "B constructor ... "<< endl;}
    ~B(){cout << "B destructor ..." << endl;}
    std::shared_ptr pa;
};

int main(int argc, char **argv) {
    
    std::shared_ptr a = std::make_shared(3);
    std::shared_ptr b = std::make_shared('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 shadow_a;
    std::weak_ptr shadow_b;
    
    {
    std::shared_ptr ptr_a = std::make_shared();
    std::shared_ptr ptr_b = std::make_shared();
    
    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 = 2

reference 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 
#include 

using namespace std;

class B;

class A{
public:
    A(){cout << "A constructor ... "<< endl;}
    ~A(){cout << "A destructor ..." << endl;}
    //std::shared_ptr pb;
    std::weak_ptr pb;
};

class B{
public:
    B(){cout << "B constructor ... "<< endl;}
    ~B(){cout << "B destructor ..." << endl;}
    //std::shared_ptr pa;
    std::weak_ptr pa;
};

int main(int argc, char **argv) {
    
    std::shared_ptr a = std::make_shared(3);
    std::shared_ptr b = std::make_shared('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 shadow_a;
    std::weak_ptr shadow_b;
    
    {
    std::shared_ptr ptr_a = std::make_shared();
    std::shared_ptr ptr_b = std::make_shared();
    
    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 = 1

B destructor ...
A destructor ...
reference count of A = 0
reference count of B = 0
Hello, world!

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