C++ 補充 & C++ 11 - C++智能指針weak_ptr 使用 (自從C++11)

weak_ptr 使用 (自從C++11)

weak_ptr 設計的目的是爲配合 shared_ptr 而引入的一種智能指針來協助 shared_ptr 工作, 它只可以從一個 shared_ptr 或另一個 weak_ptr 對象構造, 它的構造和析構不會引起引用記數的增加或減少. 同時weak_ptr 沒有重載*和->但可以使用 lock 獲得一個可用的 shared_ptr 對象。

測試代碼:

#include <stdio.h>
#include <iostream>
#include <string>
#include <memory>
#include <vector>

using namespace std;

class girl;

class boy
{
public:
	boy()
	{
		cout << "boy construct!" << endl;
	}

	~boy()
	{
		cout << "boy destruct!" << endl;
	}

	void set_girl_friend(shared_ptr<girl>& g)
	{
		this->girl_friend = g;
	}

private:
	shared_ptr<girl> girl_friend;
};

class girl
{
public:
	girl()
	{
		cout << "girl construct !" << endl;
	}

	~girl()
	{
		cout << "girl destruct!" << endl;
	}

	void set_boy_friend(shared_ptr<boy>& b)
	{
		this->boy_friend = b;
	}

private:
	shared_ptr<boy> boy_friend;
};

void use_trap()
{
	shared_ptr<girl> sp_girl(new girl()); /* 白娘子 */
	shared_ptr<boy> sp_boy(new boy()); /* 許仙 */

	sp_girl->set_boy_friend(sp_boy);
	sp_boy->set_girl_friend(sp_girl);

}


int main()
{
	use_trap();

	system("pause");
	return 0;
}

執行:

修改測試代碼:

#include <stdio.h>
#include <iostream>
#include <string>
#include <memory>
#include <vector>

using namespace std;

class girl;

class boy
{
public:
	boy()
	{
		cout << "boy construct!" << endl;
	}

	~boy()
	{
		cout << "boy destruct!" << endl;
	}

	void set_girl_friend(shared_ptr<girl>& g)
	{
		this->girl_friend = g;
	}

private:
	shared_ptr<girl> girl_friend;
};

class girl
{
public:
	girl()
	{
		cout << "girl construct !" << endl;
	}

	~girl()
	{
		cout << "girl destruct!" << endl;
	}

	void set_boy_friend(shared_ptr<boy>& b)
	{
		this->boy_friend = b;
	}

private:
	shared_ptr<boy> boy_friend;
};

void use_trap()
{
	shared_ptr<girl> sp_girl(new girl()); /* 白娘子 */
	shared_ptr<boy> sp_boy(new boy()); /* 許仙 */

	sp_girl->set_boy_friend(sp_boy);
	//sp_boy->set_girl_friend(sp_girl);

}


int main()
{
	use_trap();

	system("pause");
	return 0;
}

畫圖理解:

老師的講解:

執行修改的測試代碼

demo

#include <stdio.h>
#include <iostream>
#include <string>
#include <memory>
#include <vector>

using namespace std;

class girl;

class boy
{
public:
	boy()
	{
		cout << "boy construct!" << endl;
	}

	~boy()
	{
		cout << "boy destruct!" << endl;
	}

	void set_girl_friend(shared_ptr<girl>& g)
	{
		this->girl_friend = g;
	}

private:
	shared_ptr<girl> girl_friend;
};

class girl
{
public:
	girl()
	{
		cout << "girl construct !" << endl;
	}

	~girl()
	{
		cout << "girl destruct!" << endl;
	}

	void set_boy_friend(shared_ptr<boy>& b)
	{
		this->boy_friend = b;
	}

private:
	shared_ptr<boy> boy_friend;
};

void use_trap()
{
	shared_ptr<girl> sp_girl(new girl()); /* 白娘子 */
	shared_ptr<boy> sp_boy(new boy()); /* 許仙 */

	/* 弱指針的使用 */
	weak_ptr<girl> wp_gir1; /* 定義空的弱指針 */
	weak_ptr<girl> wp_gir2(sp_girl); /* 使用共享構造 */
	wp_gir1 = sp_girl; /* 允許共享指針賦值給弱指針 */

	cout << "sp_girl ref_count: " << sp_girl.use_count() << endl;
	cout << "wp_girl.use_count: " << wp_gir1.use_count() << endl;

	//(*sp_girl).set_boy_friend(sp_boy); 弱指針不支持* 和 -> 對指針訪問
	//sp_girl->set_boy_friend(sp_boy);

	// 在必要的時候可以轉成共享指針
	shared_ptr<girl> sp_girl1;
	sp_girl1 = wp_gir1.lock();
	cout << " after lock wp_girl.use_count: " << wp_gir1.use_count() << endl;

	sp_girl->set_boy_friend(sp_boy);
	//sp_boy->set_girl_friend(sp_girl);

}


int main()
{
	use_trap();

	system("pause");
	return 0;
}

執行:

結語:

弱指針主要是爲了解決, 交叉引用

時間: 2020-07-04