关于向量< T>的问题 [英] question on vector<T>

问题描述

考虑以下计划：


#include< iostream>

#include< vector>


使用命名空间std;


class测试

{

public：

Test（） ;

朋友ostream& operator<<（ostream& os，const Test& t）;

private：

static int counter;

int value;

};


int Test :: counter = 0;


测试::测试（）

{

value = ++ counter;

cout<< 来自默认的ctor <<值<<结束;

}


ostream& operator<<（ostream& os，const Test& t）

{

os<< t.value;

返回os;

}


int main（）

{

vector< Testtvec（10）;


for（vector< Test> :: iterator iter = tvec.begin（）;

iter！= tvec.end（）;

++ iter）

cout<< * iter<< endl;


返回0;

}


当我在VC ++ 2005 Express Edition下编译并运行该程序时，

它打印以下


默认ctor 1

1

1
1

1

1

1

1

1

1

1


默认ctor只调用一次。我认为默认ctor应该是

调用了10次，因为该语句包含tvec（10），如

vector< Testtvec（10）;

我哪里错了。


tvec中的每个Test对象都有相同的值1.为什么会这样？


请解释一下。


谢谢

V.Subramanian

解决方案

su ************** @ yahoo.com ，印度写道：

>

默认的ctor只被调用一次。我是你的ht默认ctor应该是

调用10次，因为该语句包含tvec（10），如

vector< Testtvec（10）;

Where我错了。


tvec中的每个Test对象都有相同的值1.为什么会这样？

这是因为默认构造函数只调用一次。您还需要

来设置复制构造函数。


-


- Pete

Roundhouse Consulting，Ltd。（ www.versatilecoding.com ）

标准C ++库扩展：教程和
参考的作者。 （ www.petebecker.com/tr1book ）

6月19日凌晨4:16，subramanian10 ... @ yahoo.com，India

< subramanian10 ... @ yahoo.comwrote：

考虑以下程序：


#include< iostream>

#include< vector>


使用命名空间std;


class测试

{

public：

测试（）;

朋友ostream& operator<<（ostream& os，const Test& t）;

private ：

static int counter;

int value;


};


int Test :: counter = 0;


Test :: Test（）

{

value = ++ counter;

cout<< 来自默认的ctor <<值<<结束;


}


ostream& operator<<（ostream& os，const Test& t）

{

os<< t.value;

返回os;


}


int main（）

{

vector< Testtvec（10）;


for（vector< Test> :: iterator iter = tvec.begin（）;

iter！= tvec.end（）;

++ iter）

cout<< * iter<<结束;


返回0;


}


当我编译并运行此程序时VC ++ 2005 Express Edition，

它打印以下


默认ctor 1

1

1

1

1

1

1

1

1

1

1


默认的ctor只被调用一次。我认为默认的ctor应该是

调用10次，因为该语句包含tvec（10），如

vector< Testtvec（10）;

我哪里错了。


tvec中的每个Test对象都有相同的值1.为什么会这样？


请解释一下。

谢谢

V.Subramanian

试试这个


测试::测试（）

{

value = counter;

cout<< 来自默认的ctor <<值<<结束;


}


测试::测试（const测试& t）

{

value = ++ counter;

cout<< 来自默认复制ctor <<值<<结束;


}


现在当你运行它时，它将从默认的ctor 0输出以下



来自默认副本ctor 1

来自默认副本ctor 2

来自默认副本ctor 3

来自默认副本ctor 4

来自默认复制ctor 5

来自默认复制ctor 6

来自默认复制ctor 7

来自默认复制ctor 8

来自默认复制ctor 9

来自默认复制ctor 10

1

2

3

4

5

6

7

8

9

10

HTH

< blockquote> 6月19日晚上8:45，Pete Becker< p ... @ versatilecoding.comwrote：

subramanian10 ... @ yahoo.com，India写道：

默认ctor只被调用一次。我认为默认ctor应该是

调用10次因为st atement包含tvec（10），如

vector< Testtvec（10）;

我哪里错了。

tvec中的每个Test对象都具有相同的值1.为什么会这样？

这是因为默认构造函数只调用一次。您还需要

来设置复制构造函数。


-


- Pete

Roundhouse Consulting，Ltd。（ www.versatilecoding.com ）

标准C ++库扩展：教程和
参考的作者。 （ www.petebecker.com/tr1book ）

为什么复制构造函数只被调用一次？


问候，

Sarath

Consider the following program:

#include <iostream>
#include <vector>

using namespace std;

class Test
{
public:
Test();
friend ostream &operator <<(ostream &os, const Test &t);
private:
static int counter;
int value;
};

int Test::counter = 0;

Test::Test()
{
value = ++counter;
cout << "from default ctor " << value << endl;
}

ostream &operator <<(ostream &os, const Test &t)
{
os << t.value;
return os;
}

int main()
{
vector<Testtvec(10);

for (vector<Test>::iterator iter = tvec.begin();
iter != tvec.end();
++iter)
cout << *iter << endl;

return 0;
}

When I compile and run this program under VC++ 2005 Express Edition,
it prints the following

from default ctor 1
1
1
1
1
1
1
1
1
1
1

The default ctor is called only once.I thought default ctor should be
called 10 times because the statement contains tvec(10) as in
vector<Testtvec(10);
Where am I going wrong.

Also every Test object in tvec has the same value 1. Why is it so ?

Kindly explain.

Thanks
V.Subramanian

解决方案

su**************@yahoo.com, India wrote:

>
The default ctor is called only once.I thought default ctor should be
called 10 times because the statement contains tvec(10) as in
vector<Testtvec(10);
Where am I going wrong.

Also every Test object in tvec has the same value 1. Why is it so ?

It''s because the default constructor is only called once. You also need
to instrument the copy constructor.

--

-- Pete
Roundhouse Consulting, Ltd. (www.versatilecoding.com)
Author of "The Standard C++ Library Extensions: a Tutorial and
Reference." (www.petebecker.com/tr1book)

On Jun 19, 4:16 am, "[e-mail], India"
<[e-mail]:

Consider the following program:

#include <iostream>
#include <vector>

using namespace std;

class Test
{
public:
Test();
friend ostream &operator <<(ostream &os, const Test &t);
private:
static int counter;
int value;

};

int Test::counter = 0;

Test::Test()
{
value = ++counter;
cout << "from default ctor " << value << endl;

}

ostream &operator <<(ostream &os, const Test &t)
{
os << t.value;
return os;

}

int main()
{
vector<Testtvec(10);

for (vector<Test>::iterator iter = tvec.begin();
iter != tvec.end();
++iter)
cout << *iter << endl;

return 0;

}

When I compile and run this program under VC++ 2005 Express Edition,
it prints the following

from default ctor 1
1
1
1
1
1
1
1
1
1
1

The default ctor is called only once.I thought default ctor should be
called 10 times because the statement contains tvec(10) as in
vector<Testtvec(10);
Where am I going wrong.

Also every Test object in tvec has the same value 1. Why is it so ?

Kindly explain.

Thanks
V.Subramanian

Try this

Test::Test()
{
value = counter;
cout << "from default ctor " << value << endl;

}

Test::Test(const Test& t)
{
value = ++counter;
cout << "from default copy ctor " << value << endl;

}

Now when you run it, it would output the followng
from default ctor 0
from default copy ctor 1
from default copy ctor 2
from default copy ctor 3
from default copy ctor 4
from default copy ctor 5
from default copy ctor 6
from default copy ctor 7
from default copy ctor 8
from default copy ctor 9
from default copy ctor 10
1
2
3
4
5
6
7
8
9
10

HTH

On Jun 19, 8:45 pm, Pete Becker <[e-mail]:

[e-mail], India wrote:

The default ctor is called only once.I thought default ctor should be
called 10 times because the statement contains tvec(10) as in
vector<Testtvec(10);
Where am I going wrong.

Also every Test object in tvec has the same value 1. Why is it so ?

It''s because the default constructor is only called once. You also need
to instrument the copy constructor.

--

-- Pete
Roundhouse Consulting, Ltd. (www.versatilecoding.com)
Author of "The Standard C++ Library Extensions: a Tutorial and
Reference." (www.petebecker.com/tr1book)

Why the copy constructor being called only once?

Regards,
Sarath

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