vector resize的时候调用类的构造函数和析构函数问题

[清水大湿]

代码很简单，如下：

#include <iostream>
#include <vector>
 
using namespace std;

class TestA
{
public:
	TestA()
	{
		cout << "Create TestA" << endl;
	}
	~TestA()
	{
		cout << "Destory TestA" << endl;
	}
};

int main()
{
	vector<TestA> mytest;
	mytest.resize(4);
	mytest.resize(3);
	return 0;
}

测试环境：vs2008，输出如下：
Create TestA
Destory TestA
Destory TestA
Create TestA
Destory TestA
Destory TestA
Destory TestA
Destory TestA
Destory TestA
Destory TestA
请按任意键继续. . .

http://www.cplusplus.com/reference/vector/vector/resize/ 对resize的解释如下：

Change size
Resizes the container so that it contains n elements.

If n is smaller than the current container size, the content is reduced to its first n elements, removing those beyond (and destroying them).

If n is greater than the current container size, the content is expanded by inserting at the end as many elements as needed to reach a size of n. If val is specified, the new elements are initialized as copies of val, otherwise, they are value-initialized.

If n is also greater than the current container capacity, an automatic reallocation of the allocated storage space takes place.

Notice that this function changes the actual content of the container by inserting or erasing elements from it.

我mytest.resize(4); n既大于当前容器大小又大于当前容量，create一次，为什么destory两次？
还有总得destory次数，为什么会有8次？总是比我两次resize的数字之和大1，求大神解答

[小鬼小鬼小鬼啊]

vs2012测试没发现楼主说的情况: #include <iostream> using namespace std; #include <vector> class TestA { public: TestA(){ printf("create\n"); } ~TestA(){ printf("destroy\n"); } TestA( const TestA& test ){printf("copy\n");} }; int main(){ vector< TestA > test; test.resize(4); test.resize(3); system("pause"); return 0; } create create create create destroy 请按任意键继续. . . destroy destroy destroy 请按任意键继续. . .

[赵4老师]

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[yangyunzhao]

#include <iostream>
#include <vector>

using namespace std;

class TestA
{
public:
	TestA()
	{
		cout << "Create TestA1" << endl;
	}
	TestA(const TestA & a)
	{
		cout << "Create TestA2" << endl;
	}
	~TestA()
	{
		cout << "Destory TestA" << endl;
	}
};

int main()
{
	vector<TestA> mytest;
	mytest.resize(4);
	mytest.resize(3);
	return 0;
}

[yangyunzhao]

再增加几行代码，更能说明问题：

#include <iostream>
#include <vector>

using namespace std;

class TestA
{
public:
	TestA()
	{
		cout << "Create TestA1" << endl;
	}
	TestA(const TestA & a)
	{
		cout << "Create TestA2" << endl;
	}
	~TestA()
	{
		cout << "Destory TestA" << endl;
	}
};

int main()
{
	vector<TestA> mytest;
	printf("%d\n", mytest.capacity());
	mytest.resize(4);
	printf("%d\n", mytest.capacity());
	mytest.resize(3);
	printf("%d\n", mytest.capacity());
	return 0;
}

执行结果：

0
Create TestA1
Create TestA2
Create TestA2
Create TestA2
Create TestA2
Destory TestA
4
Create TestA1
Destory TestA
Destory TestA
4
Destory TestA
Destory TestA
Destory TestA
请按任意键继续. . .

[yangyunzhao]

引用 4 楼 yangyunzhao 的回复:

再增加几行代码，更能说明问题：

#include <iostream>
#include <vector>

using namespace std;

class TestA
{
public:
	TestA()
	{
		cout << "Create TestA1" << endl;
	}
	TestA(const TestA & a)
	{
		cout << "Create TestA2" << endl;
	}
	~TestA()
	{
		cout << "Destory TestA" << endl;
	}
};

int main()
{
	vector<TestA> mytest;
	printf("%d\n", mytest.capacity());
	mytest.resize(4);
	printf("%d\n", mytest.capacity());
	mytest.resize(3);
	printf("%d\n", mytest.capacity());
	return 0;
}

执行结果：

0
Create TestA1
Create TestA2
Create TestA2
Create TestA2
Create TestA2
Destory TestA
4
Create TestA1
Destory TestA
Destory TestA
4
Destory TestA
Destory TestA
Destory TestA
请按任意键继续. . .

说错了，应该这样才对

int main()
{
	vector<TestA> mytest;
	printf("%d %d\n", mytest.capacity(), mytest.size());
	mytest.resize(4);
	printf("%d %d\n", mytest.capacity(), mytest.size());
	mytest.resize(3);
	printf("%d %d\n", mytest.capacity(), mytest.size());
	return 0;
}

[清水大湿]

引用 5 楼 yangyunzhao 的回复:

[quote=引用 4 楼 yangyunzhao 的回复:] 再增加几行代码，更能说明问题：

#include <iostream>
#include <vector>

using namespace std;

class TestA
{
public:
	TestA()
	{
		cout << "Create TestA1" << endl;
	}
	TestA(const TestA & a)
	{
		cout << "Create TestA2" << endl;
	}
	~TestA()
	{
		cout << "Destory TestA" << endl;
	}
};

int main()
{
	vector<TestA> mytest;
	printf("%d\n", mytest.capacity());
	mytest.resize(4);
	printf("%d\n", mytest.capacity());
	mytest.resize(3);
	printf("%d\n", mytest.capacity());
	return 0;
}

执行结果：

0
Create TestA1
Create TestA2
Create TestA2
Create TestA2
Create TestA2
Destory TestA
4
Create TestA1
Destory TestA
Destory TestA
4
Destory TestA
Destory TestA
Destory TestA
请按任意键继续. . .

说错了，应该这样才对

int main()
{
	vector<TestA> mytest;
	printf("%d %d\n", mytest.capacity(), mytest.size());
	mytest.resize(4);
	printf("%d %d\n", mytest.capacity(), mytest.size());
	mytest.resize(3);
	printf("%d %d\n", mytest.capacity(), mytest.size());
	return 0;
}

[/quote] soga，只有第一次是调用默认构造函数，然后后面都是调用拷贝构造函数，谢谢