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C語(yǔ)言?智能指針?shared_ptr?和?weak_ptr_C 語(yǔ)言

作者：哈利馬其 ? 更新時(shí)間： 2022-06-12 編程語(yǔ)言

weak_ptr引入可以解決shared_ptr交叉引用時(shí)無(wú)法釋放資源的問(wèn)題。

示例代碼：

#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;
}

運(yùn)行代碼得到以下輸出：

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!

從結(jié)果可以看出，由于交叉引用導(dǎo)致申請(qǐng)的內(nèi)存A，B無(wú)法正常釋放。
為什么會(huì)這樣呢？這個(gè)應(yīng)該從析構(gòu)原理進(jìn)行考慮，shared_ptr引用計(jì)數(shù)需要為0才會(huì)進(jìn)行析構(gòu)！但是ptr_a離開(kāi)作用域會(huì)導(dǎo)致A引用計(jì)數(shù)減少1，但是A的引用計(jì)數(shù)此時(shí)為1，那么 pb不會(huì)釋放；同理，ptr_b離開(kāi)作用域會(huì)導(dǎo)致B引用計(jì)數(shù)減少1，但是B的引用計(jì)數(shù)為此時(shí)為1，那么pa不會(huì)釋放。如此導(dǎo)致了資源無(wú)法釋放掉。
由于weak_ptr并不會(huì)改變shared_ptr的引用計(jì)數(shù)，所以修改類(lèi)A，和類(lèi)B中的shared_ptr對(duì)象為weak_ptr對(duì)象即可釋放資源。

修改后的代碼如下：

#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;
}

運(yùn)行結(jié)果如下，可以正常釋放資源。

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!

原文鏈接：https://blog.csdn.net/lybhit/article/details/122799780

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