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 <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 = 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 <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 = 1

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

到此这篇关于C语言 智能指针 shared_ptr 和 weak_ptr的文章就介绍到这了,更多相关 shared_ptr 和 weak_ptr内容请搜索我们以前的文章或继续浏览下面的相关文章希望大家以后多多支持我们！