STL的迭代器--Iterator

STL中的迭代器类似指针，但不仅仅是指针。迭代器和指针很像，功能很像指针，但是实际上，迭代器是通过重载一元的”*”和”->”来从容器
中间接地返回一个值。将这些值存储在容器中并不是一个好主意，因为每当一个新值添加到容器中或者有一个值从容器中删除，这些值就会失效。在某种程度上，迭
代器可以看作是句柄（handle）。通常情况下迭代器（iterator）的类型可以有所变化，这样容器也会有几种不同方式的转变：
iterator——对于除了vector以外的其他任何容器，你可以通过这种迭代器在一次操作中在容器中朝向前的方向走一步。这意味着对于这种迭代器你
只能使用“++”操作符。而不能使用“--”或“+=”操作符。而对于vector这一种容器，你可以使用“+=”、“—”、“++”、“-=”中的任何
一种操作符和“<”、“<=”、“>”、“>=”、“==”、“!=”等比较运算符。 reverse_iterator
——如果你想用向后的方向而不是向前的方向的迭代器来遍历除vector之外的容器中的元素，你可以使用reverse_iterator
来反转遍历的方向，你还可以用rbegin()来代替begin()，用rend()代替end()，而此时的“++”操作符会朝向后的方向遍历。
const_iterator ——一个向前方向的迭代器，它返回一个常数值。你可以使用这种类型的迭代器来指向一个只读的值。
const_reverse_iterator ——一个朝反方向遍历的迭代器，它返回一个常数值。

混合迭代器函数

在涉及到容器和算法的操作中，还有两个迭代器函数非常有用：

· advance() 按指定的数目来增减迭代器。
template<class InIt, class Dist>
void advance(InIt& it, Dist n);
n为正时，递增第一个参数所传递的迭代器；n为负时，递减所传递的迭代器。

· distance() 返回到达一个迭代器所需（递增）操作的数目。
template<class Init, class Dist>
Dist distance(InIt first, InIt last, Dist& n);

Distance is an overloaded name; there are actually two distance functions.

template <class InputIterator>
inline iterator_traits<InputIterator>::difference_type distance(InputIterator first, InputIterator last);

template <class InputIterator, class Distance>
void distance(InputIterator first, InputIterator last, Distance& n);

比较前后两个迭代器之间的数目。

例如：

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list iList;

list::iterator p = find(iList.begin(), iList.end(), 2);

cout << "before: p == "
<< *p << endl;

advance(p, 2); // same as p = p + 2;

cout << "after : p == "
<< *p << endl;

int
k = 0;

distance(p, iList.end(), k);

cout << "k == "
<< k << endl;
// 返回两者之间的数目

list iList;

list::iterator p = find(iList.begin(), iList.end(), 2);

cout << "before: p == " << *p << endl;

advance(p, 2); // same as p = p + 2;

cout << "after : p == " << *p << endl;

int k = 0;

distance(p, iList.end(), k);

cout << "k == " << k << endl; // 返回两者之间的数目

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advance()函数接受两个参数。第二个参数是向前推进的数目。对于前推迭代器，该值必须为正，而对于双向迭代器和随机访问迭代器，该值可以为负。

advance()函数接受两个参数。第二个参数是向前推进的数目。对于前推迭代器，该值必须为正，而对于双向迭代器和随机访问迭代器，该值可以为负。

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使用 distance()函数来返回到达另一个迭代器所需要的步骤。 注意 distance()函数是迭代的，也就是说，它递增第三个参数。因此，你必须初始化该参数。未初始化该参数几乎注定要失败。

使用 distance()函数来返回到达另一个迭代器所需要的步骤。 注意 distance()函数是迭代的，也就是说，它递增第三个参数。因此，你必须初始化该参数。未初始化该参数几乎注定要失败。

distance ()
Sample Code:

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<pre
class
=
"csharp"
name=
"code"
>
// distance.cpp

// compile with: /EHsc

#pragma warning (disable:4786)

#include <iostream>

#include <vector>

#include <iterator>

#include <string>

using

namespace
std;

typedef vector<string
> VTRLIST;

int
main() {

VTRLIST Vector;

VTRLIST::iterator iVector;

VTRLIST::difference_type dTheDiff;

Vector.push_back("A1"
);

Vector.push_back("B2"
);

Vector.push_back("C3"
);

Vector.push_back("D4"
);

Vector.push_back("E5"
);

Vector.push_back("F6"
);

Vector.push_back("G7"
);

// Print out the list

iVector=Vector.begin();

cout << "The list is: "
;

for
(
int
i = 0; i < 7 ; i++, iVector++)

cout << *iVector << " "
;

// Initialize the iterator the first element"

iVector=Vector.begin();

cout << "/n/nAdvance to the 3rd element."
<< endl;

advance( iVector, 2);

cout << "The element is "
<< *iVector << endl;

dTheDiff = distance( Vector.begin(), iVector);

cout << "The distance from the beginning is "
<< dTheDiff << endl;

cout << "Calculate it in reverse order "
<< endl;

dTheDiff = distance( iVector, Vector.begin());

cout << "The distance is "
<< dTheDiff << endl;

cout << "/nUse distance() to count from the 3rd element to the end."

<< endl;

dTheDiff = distance( iVector, Vector.end());

// Note that end() returns one past the end of the sequence

cout << "The distance is "
<< dTheDiff << endl;

cout <<"/nUse distance() to count the total length."
<< endl;

dTheDiff = distance( Vector.begin(), Vector.end() );

cout << "The total distance is "
<< dTheDiff << endl;

}

</pre>

<pre class
=
"csharp"
name=
"code"
><div
class
=
"MTPS_CollapsibleSection"
id=
"ctl00_rs1_mainContentContainer_cpe63954_c"
style=
"display: block; overflow: visible; width: auto; height: auto;"
><div
class
=
"MTPS_CollapsibleSection"
id=
""
style=
"border: medium none; display: block;"
><pre><pre
class
=
"csharp"
name=
"code"
>
/*

Output

The list is: A1 B2 C3 D4 E5 F6 G7

Advance to the 3rd element.

The element is C3

The distance from the beginning is 2

Calculate it in reverse order

The distance is -2

Use distance() to count from the 3rd element to the end.

The distance is 5

Use distance() to count the total length.

The total distance is 7

*/

</pre>

</pre>

</div>

</div>

</pre>

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// distance.cpp

// compile with: /EHsc

#pragma warning (disable:4786)

#include <iostream>

#include <vector>

#include <iterator>

#include <string>

using

namespace
std;

typedef vector<string
> VTRLIST;

int
main() {

VTRLIST Vector;

VTRLIST::iterator iVector;

VTRLIST::difference_type dTheDiff;

Vector.push_back("A1"
);

Vector.push_back("B2"
);

Vector.push_back("C3"
);

Vector.push_back("D4"
);

Vector.push_back("E5"
);

Vector.push_back("F6"
);

Vector.push_back("G7"
);

// Print out the list

iVector=Vector.begin();

cout << "The list is: "
;

for
(
int
i = 0; i < 7 ; i++, iVector++)

cout << *iVector  << "  "
;

// Initialize the iterator the first element"

iVector=Vector.begin();

cout << "/n/nAdvance to the 3rd element."
<< endl;

advance( iVector, 2);

cout << "The element is "
<< *iVector << endl;

dTheDiff = distance( Vector.begin(), iVector);

cout << "The distance from the beginning is "
<< dTheDiff << endl;

cout << "Calculate it in reverse order "
<< endl;

dTheDiff = distance( iVector, Vector.begin());

cout << "The distance is "
<< dTheDiff << endl;

cout << "/nUse distance() to count from the 3rd element to the end."

<< endl;

dTheDiff = distance( iVector, Vector.end());

// Note that end() returns one past the end of the sequence

cout << "The distance is "
<< dTheDiff << endl;

cout <<"/nUse distance() to count the total length."
<< endl;

dTheDiff = distance( Vector.begin(), Vector.end() );

cout << "The total distance is "
<< dTheDiff << endl;

}

// distance.cpp

// compile with: /EHsc

#pragma warning (disable:4786)

#include <iostream>

#include <vector>

#include <iterator>

#include <string>

using namespace std;

typedef vector<string > VTRLIST;

int main() {

VTRLIST Vector;

VTRLIST::iterator iVector;

VTRLIST::difference_type dTheDiff;

Vector.push_back("A1");

Vector.push_back("B2");

Vector.push_back("C3");

Vector.push_back("D4");

Vector.push_back("E5");

Vector.push_back("F6");

Vector.push_back("G7");

// Print out the list

iVector=Vector.begin();

cout << "The list is: ";

for (int i = 0; i < 7 ; i++, iVector++)

cout << *iVector  << "  ";

// Initialize the iterator the first element"

iVector=Vector.begin();

cout << "/n/nAdvance to the 3rd element." << endl;

advance( iVector, 2);

cout << "The element is " << *iVector << endl;

dTheDiff = distance( Vector.begin(), iVector);

cout << "The distance from the beginning is " << dTheDiff << endl;

cout << "Calculate it in reverse order " << endl;

dTheDiff = distance( iVector, Vector.begin());

cout << "The distance is " << dTheDiff << endl;

cout << "/nUse distance() to count from the 3rd element to the end."

<< endl;

dTheDiff = distance( iVector, Vector.end());

// Note that end() returns one past the end of the sequence

cout << "The distance is " << dTheDiff << endl;

cout <<"/nUse distance() to count the total length." << endl;

dTheDiff = distance( Vector.begin(), Vector.end() );

cout << "The total distance is " << dTheDiff << endl;

}

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?

<div
class
=
"MTPS_CollapsibleSection"
id=
"ctl00_rs1_mainContentContainer_cpe63954_c"
style=
"display: block; overflow: visible; width: auto; height: auto;"
><div
class
=
"MTPS_CollapsibleSection"
id=
""
style=
"border: medium none; display: block;"
><pre><pre
class
=
"csharp"
name=
"code"
>
/*

Output

The list is: A1 B2 C3 D4 E5 F6 G7

Advance to the 3rd element.

The element is C3

The distance from the beginning is 2

Calculate it in reverse order

The distance is -2

Use distance() to count from the 3rd element to the end.

The distance is 5

Use distance() to count the total length.

The total distance is 7

*/

</pre>

</pre>

</div>

</div>

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copy to clipboard
print
?

/*

Output

The list is: A1  B2  C3  D4  E5  F6  G7

Advance to the 3rd element.

The element is C3

The distance from the beginning is 2

Calculate it in reverse order

The distance is -2

Use distance() to count from the 3rd element to the end.

The distance is 5

Use distance() to count the total length.

The total distance is 7

*/

/*

Output

The list is: A1  B2  C3  D4  E5  F6  G7

Advance to the 3rd element.

The element is C3

The distance from the beginning is 2

Calculate it in reverse order

The distance is -2

Use distance() to count from the 3rd element to the end.

The distance is 5

Use distance() to count the total length.

The total distance is 7

*/

[/code]

advance()  Sample Code: 

// Advance.cpp

// compile with: /EHsc

#pragma warning (disable:4786)

#include <iostream>

#include <string>

#include <list>

using namespace std ;

typedef list<string> STRLIST;

int main() {

STRLIST List;

STRLIST::iterator iList;

STRLIST::difference_type dTheDiff;

List.push_back("A1");

List.push_back("B2");

List.push_back("C3");

List.push_back("D4");

List.push_back("E5");

List.push_back("F6");

List.push_back("G7");

// Print out the list

iList=List.begin();

cout << "The list is: ";

for (int i = 0; i < 7 ; i++, iList++)

cout << *iList  << "  ";

// Initialize to the first element"

iList=List.begin();

cout << "/n/nAdvance to the 3rd element." << endl;

advance(iList,2);

cout << "The element is " << *iList << endl;

}

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/*

Output

The list is: A1  B2  C3  D4  E5  F6  G7

Advance to the 3rd element.

The element is C3

*/

[code]/*

Output

The list is: A1  B2  C3  D4  E5  F6  G7

Advance to the 3rd element.

The element is C3

*/

[/code]
当然advance() 和distance() 还可以用来对const_iterator转换为iterator.