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keyws — Sun, 13 Aug 2006 11:52:00 GMT

www.cppreference.com

assign

Syntax:

														
																1 #include <vector>
2 void assign( size_type num, const TYPE& val );
3 void assign( input_iterator start, input_iterator end );

The assign() function either gives the current vector the values from start to end, or gives it num copies of val.

This function will destroy the previous contents of the vector.

For example, the following code uses assign() to put 10 copies of the integer 42 into a vector:

 vector v;
 v.assign( 10, 42 );
 for( int i = 0; i < v.size(); i++ ) {
   cout << v[i] << " ";
 }
 cout << endl;

The above code displays the following output:

 42 42 42 42 42 42 42 42 42 42

The next example shows how assign() can be used to copy one vector to another:

 vector v1;
 for( int i = 0; i < 10; i++ ) {
   v1.push_back( i );
 }              

 vector v2;
 v2.assign( v1.begin(), v1.end() );             

 for( int i = 0; i < v2.size(); i++ ) {
   cout << v2[i] << " ";
 }
 cout << endl;

When run, the above code displays the following output:

 0 1 2 3 4 5 6 7 8 9

back

Syntax:

				
						1 #include <vector>
2 TYPE& back();
3 const TYPE& back() const;

The back() function returns a reference to the last element in the vector.

For example:

 vector v;
 for( int i = 0; i < 5; i++ ) {
   v.push_back(i);
 }
 cout << "The first element is " << v.front()
      << " and the last element is " << v.back() << endl;

This code produces the following output:

 The first element is 0 and the last element is 4

Syntax:

  #include 
  TYPE& at( size_type loc );
  const TYPE& at( size_type loc ) const;

The at() function returns a reference to the element in the vector at index loc. The at() function is safer than the [] operator, because it won't let you reference items outside the bounds of the vector.

For example, consider the following code:

 vector v( 5, 1 );
 for( int i = 0; i < 10; i++ ) {
   cout << "Element " << i << " is " << v[i] << endl;
 }

This code overrunns the end of the vector, producing potentially dangerous results. The following code would be much safer:

 vector v( 5, 1 );
 for( int i = 0; i < 10; i++ ) {
   cout << "Element " << i << " is " << v.at(i) << endl;
 }

Instead of attempting to read garbage values from memory, the at() function will realize that it is about to overrun the vector and will throw an exception.

capacity

Syntax:

  #include 
  size_type capacity() const;

The capacity() function returns the number of elements that the vector can hold before it will need to allocate more space.

For example, the following code uses two different methods to set the capacity of two vectors. One method passes an argument to the constructor that suggests an initial size, the other method calls the reserve function to achieve a similar goal:

 vector v1(10);
 cout << "The capacity of v1 is " << v1.capacity() << endl;
 vector v2;
 v2.reserve(20);
 cout << "The capacity of v2 is " << v2.capacity() << endl;

When run, the above code produces the following output:

 The capacity of v1 is 10
 The capacity of v2 is 20

C++ containers are designed to grow in size dynamically. This frees the programmer from having to worry about storing an arbitrary number of elements in a container. However, sometimes the programmer can improve the performance of her program by giving hints to the compiler about the size of the containers that the program will use. These hints come in the form of the reserve() function and the constructor used in the above example, which tell the compiler how large the container is expected to get.

The capacity() function runs in constant time.

begin

Syntax:

  #include 
  iterator begin();
  const_iterator begin() const;

The function begin() returns an iterator to the first element of the vector. begin() should run in constant time.

For example, the following code uses begin() to initialize an iterator that is used to traverse a list:

   // Create a list of characters
   list charList;
   for( int i=0; i < 10; i++ ) {
     charList.push_front( i + 65 );
   }
   // Display the list
   list::iterator theIterator;
   for( theIterator = charList.begin(); theIterator != charList.end(); theIterator++ ) {
     cout << *theIterator;
   }

max_size

Syntax:

  #include 
  size_type max_size() const;

The max_size() function returns the maximum number of elements that the vector can hold. The max_size() function should not be confused with the size() or capacity() functions, which return the number of elements currently in the vector and the the number of elements that the vector will be able to hold before more memory will have to be allocated, respectively.

clear

Syntax:

  #include 
  void clear();

The function clear() deletes all of the elements in the vector. clear() runs in linear time.

empty

Syntax:

  #include 
  bool empty() const;

The empty() function returns true if the vector has no elements, false otherwise.

For example, the following code uses empty() as the stopping condition on a (C/C++ Keywords) while loop to clear a vector and display its contents in reverse order:

 vector v;
 for( int i = 0; i < 5; i++ ) {
   v.push_back(i);
 }
 while( !v.empty() ) {
   cout << v.back() << endl;
   v.pop_back();
 }

end

Syntax:

  #include 
  iterator end();
  const_iterator end() const;

The end() function returns an iterator just past the end of the vector.

Note that before you can access the last element of the vector using an iterator that you get from a call to end(), you'll have to decrement the iterator first. This is because end() doesn't point to the end of the vector; it points just past the end of the vector.

For example, in the following code, the first "cout" statement will display garbage, whereas the second statement will actually display the last element of the vector:

  vector v1;
  v1.push_back( 0 );
  v1.push_back( 1 );
  v1.push_back( 2 );
  v1.push_back( 3 );

  int bad_val = *(v1.end());
  cout << "bad_val is " << bad_val << endl;

  int good_val = *(v1.end() - 1);
  cout << "good_val is " << good_val << endl;

The next example shows how begin() and end() can be used to iterate through all of the members of a vector:

 vector v1( 5, 789
  ); vector::iterator it; for( it = v1.begin(); it !=
  v1.end(); it++ ) { cout << *it << endl; }

The iterator is initialized with a call to begin(). After the body of the loop has been executed, the iterator is incremented and tested to see if it is equal to the result of calling end(). Since end() returns an iterator pointing to an element just after the last element of the vector, the loop will only stop once all of the elements of the vector have been displayed.

end() runs in constant time.

erase

Syntax:

  #include 
  iterator erase( iterator loc );
  iterator erase( iterator start, iterator end );

The erase() function either deletes the element at location loc, or deletes the elements between start and end (including start but not including end). The return value is the element after the last element erased.

The first version of erase (the version that deletes a single element at location loc) runs in constant time for lists and linear time for vectors, dequeues, and strings. The multiple-element version of erase always takes linear time.

For example:

 // Create a vector, load it with the first ten characters of the alphabet
 vector alphaVector;
 for( int i=0; i < 10; i++ ) {
   alphaVector.push_back( i + 65 );
 }
 int size = alphaVector.size();
 vector::iterator startIterator;
 vector::iterator tempIterator;
 for( int i=0; i < size; i++ ) {
   startIterator = alphaVector.begin();
   alphaVector.erase( startIterator );
   // Display the vector
   for( tempIterator = alphaVector.begin(); tempIterator != alphaVector.end(); tempIterator++ ) {
     cout << *tempIterator;
   }
   cout << endl;
 }

That code would display the following output:

 BCDEFGHIJ
 CDEFGHIJ
 DEFGHIJ
 EFGHIJ
 FGHIJ
 GHIJ
 HIJ
 IJ
 J

In the next example, erase() is called with two iterators to delete a range of elements from a vector:

 // create a vector, load it with the first ten characters of the alphabet
 vector alphaVector;
 for( int i=0; i < 10; i++ ) {
   alphaVector.push_back( i + 65 );
 }
 // display the complete vector
 for( int i = 0; i < alphaVector.size(); i++ ) {
   cout << alphaVector[i];
 }
 cout << endl;            

 // use erase to remove all but the first two and last three elements
 // of the vector
 alphaVector.erase( alphaVector.begin()+2, alphaVector.end()-3 );
 // display the modified vector
 for( int i = 0; i < alphaVector.size(); i++ ) {
   cout << alphaVector[i];
 }
 cout << endl;

When run, the above code displays:

 ABCDEFGHIJ
 ABHIJ

front

Syntax:

  #include 
  TYPE& front();
  const TYPE& front() const;

The front() function returns a reference to the first element of the vector, and runs in constant time.

insert

Syntax:

  #include 
  iterator insert( iterator loc, const TYPE& val );
  void insert( iterator loc, size_type num, const TYPE& val );
  template<TYPE> void insert( iterator loc, input_iterator start, input_iterator end );

The insert() function either:

inserts val before loc, returning an iterator to the element inserted,
inserts num copies of val before loc, or
inserts the elements from start to end before loc.

Note that inserting elements into a vector can be relatively time-intensive, since the underlying data structure for a vector is an array. In order to insert data into an array, you might need to displace a lot of the elements of that array, and this can take linear time. If you are planning on doing a lot of insertions into your vector and you care about speed, you might be better off using a container that has a linked list as its underlying data structure (such as a List or a Deque).

For example, the following code uses the insert() function to splice four copies of the character 'C' into a vector of characters:

 // Create a vector, load it with the first 10 characters of the alphabet
 vector alphaVector;
 for( int i=0; i < 10; i++ ) {
   alphaVector.push_back( i + 65 );
 }              

 // Insert four C's into the vector
 vector::iterator theIterator = alphaVector.begin();
 alphaVector.insert( theIterator, 4, 'C' );             

 // Display the vector
 for( theIterator = alphaVector.begin(); theIterator != alphaVector.end(); theIterator++ )    {
   cout << *theIterator;
 }

This code would display:

 CCCCABCDEFGHIJ

Here is another example of the insert() function. In this code, insert() is used to append the contents of one vector onto the end of another:

  vector v1;
  v1.push_back( 0 );
  v1.push_back( 1 );
  v1.push_back( 2 );
  v1.push_back( 3 );

  vector v2;
  v2.push_back( 5 );
  v2.push_back( 6 );
  v2.push_back( 7 );
  v2.push_back( 8 );

  cout << "Before, v2 is: ";
  for( int i = 0; i < v2.size(); i++ ) {
    cout << v2[i] << " ";
  }
  cout << endl;

  v2.insert( v2.end(), v1.begin(), v1.end() );

  cout << "After, v2 is: ";
  for( int i = 0; i < v2.size(); i++ ) {
    cout << v2[i] << " ";
  }
  cout << endl;

When run, this code displays:

  Before, v2 is: 5 6 7 8
  After, v2 is: 5 6 7 8 0 1 2 3

Vector constructors

Syntax:

  #include 
  vector();
  vector( const vector& c );
  vector( size_type num, const TYPE& val = TYPE() );
  vector( input_iterator start, input_iterator end );
  ~vector();

The default vector constructor takes no arguments, creates a new instance of that vector.

The second constructor is a default copy constructor that can be used to create a new vector that is a copy of the given vector c.

The third constructor creates a vector with space for num objects. If val is specified, each of those objects will be given that value. For example, the following code creates a vector consisting of five copies of the integer 42:

 vector v1( 5, 42 );

The last constructor creates a vector that is initialized to contain the elements between start and end. For example:

 // create a vector of random integers
 cout << "original vector: ";
 vector v;
 for( int i = 0; i < 10; i++ ) {
   int num = (int) rand() % 10;
   cout << num << " ";
   v.push_back( num );
 }
 cout << endl;            

 // find the first element of v that is even
 vector::iterator iter1 = v.begin();
 while( iter1 != v.end() && *iter1 % 2 != 0 ) {
   iter1++;
 }              

 // find the last element of v that is even
 vector::iterator iter2 = v.end();
 do {
   iter2--;
 } while( iter2 != v.begin() && *iter2 % 2 != 0 );              

 cout << "first even number: " << *iter1 << ", last even number: " << *iter2 << endl;         

 cout << "new vector: ";
 vector v2( iter1, iter2 );
 for( int i = 0; i < v2.size(); i++ ) {
   cout << v2[i] << " ";
 }
 cout << endl;

When run, this code displays the following output:

 original vector: 1 9 7 9 2 7 2 1 9 8
 first even number: 2, last even number: 8
 new vector: 2 7 2 1 9

All of these constructors run in linear time except the first, which runs in constant time.

The default destructor is called when the vector should be destroyed.

pop_back

Syntax:

  #include 
  void pop_back();

The pop_back() function removes the last element of the vector.

pop_back() runs in constant time.

push_back

Syntax:

  #include 
  void push_back( const TYPE& val );

The push_back() function appends val to the end of the vector.

For example, the following code puts 10 integers into a list:

   list the_list;
   for( int i = 0; i < 10; i++ )
     the_list.push_back( i );

When displayed, the resulting list would look like this:

 0 1 2 3 4 5 6 7 8 9

push_back() runs in constant time.

rbegin

Syntax:

  #include 
  reverse_iterator rbegin();
  const_reverse_iterator rbegin() const;

The rbegin() function returns a reverse_iterator to the end of the current vector.

rbegin() runs in constant time.

rend

Syntax:

  #include 
  reverse_iterator rend();
  const_reverse_iterator rend() const;

The function rend() returns a reverse_iterator to the beginning of the current vector.

rend() runs in constant time.

reserve

Syntax:

  #include 
  void reserve( size_type size );

The reserve() function sets the capacity of the vector to at least size.

reserve() runs in linear time.

resize

Syntax:

  #include 
  void resize( size_type num, const TYPE& val = TYPE() );

The function resize() changes the size of the vector to size. If val is specified then any newly-created elements will be initialized to have a value of val.

This function runs in linear time.

size

Syntax:

  #include 
  size_type size() const;

The size() function returns the number of elements in the current vector.

swap

Syntax:

  #include 
  void swap( const container& from );

The swap() function exchanges the elements of the current vector with those of from. This function operates in constant time.

For example, the following code uses the swap() function to exchange the values of two strings:

   string first( "This comes first" );
   string second( "And this is second" );
   first.swap( second );
   cout << first << endl;
   cout << second << endl;

The above code displays:

   And this is second
   This comes first

Vector operators

Syntax:

  #include 
  TYPE& operator[]( size_type index );
  const TYPE& operator[]( size_type index ) const;
  vector operator=(const vector& c2);
  bool operator==(const vector& c1, const vector& c2);
  bool operator!=(const vector& c1, const vector& c2);
  bool operator<(const vector& c1, const vector& c2);
  bool operator>(const vector& c1, const vector& c2);
  bool operator<=(const vector& c1, const vector& c2);
  bool operator>=(const vector& c1, const vector& c2);

All of the C++ containers can be compared and assigned with the standard comparison operators: ==, !=, <=, >=, <, >, and =. Individual elements of a vector can be examined with the [] operator.

Performing a comparison or assigning one vector to another takes linear time. The [] operator runs in constant time.

Two vectors are equal if:

Their size is the same, and
Each member in location i in one vector is equal to the the member in location i in the other vector.

Comparisons among vectors are done lexicographically.

For example, the following code uses the [] operator to access all of the elements of a vector:

 vector v( 5, 1 );
 for( int i = 0; i < v.size(); i++ ) {
   cout << "Element " << i << " is " << v[i] << endl;
 }

keyws 2006-08-13 19:52 发表评论

keyws — Sun, 13 Aug 2006 11:36:00 GMT

(一)

One of the basic classes implemented by the Standard Template Library is the vector class. A vector is, essentially, a resizeable array; the vector class allows random access via the [] operator, but adding an element anywhere but to the end of a vector causes some overhead as all of the elements are shuffled around to fit them correctly into memory. Fortunately, the memory requirements are equivalent to those of a normal array. The header file for the STL vector library is vector. (Note that when using C++, header files drop the .h; for C header files - e.g. stdlib.h - you should still include the .h.) Moreover, the vector class is part of the std namespace, so you must either prefix all references to the vector template with std:: or include "using namespace std;" at the top of your program.

Vectors are more powerful than arrays because the number of functions that are available for accessing and modifying vectors. Unfortunately, the [] operator still does not provide bounds checking. There is an alternative way of accessing the vector, using the function at, which does provide bounds checking at an additional cost. Let's take a look at several functions provided by the vector class:

1 unsigned int size(); // Returns the number of elements in a vector
2 push_back(type element); // Adds an element to the end of a vector
3 bool empty(); // Returns true if the vector is empty
4 void clear(); // Erases all elements of the vector
5 type at(int n); //Returns the element at index n, with bounds checking

also, there are several basic operators defined for the vector class:

=           Assignment replaces a vector's contents with the contents of another
==          An element by element comparison of two vectors
[]          Random access to an element of a vector (usage is similar to that
            of the operator with arrays.) Keep in mind that it does not provide
            bounds checking.

Let's take a look at an example program using the vector class:

				
						 1 #include <iostream>
 2 #include <vector>
 3 using namespace std;
 4 
 5 int main()
 6 {
 7     vector <int> example;         //Vector to store integers
 8     example.push_back(3);         //Add 3  onto the vector
 9     example.push_back(10);        //Add 10 to the end
10     example.push_back(33);        //Add 33 to the end

11     for(int x=0; x<example.size(); x++) 
12     {
13         cout<<example[x]<<" ";    //Should output: 3 10 33
14     }

15     if(!example.empty())          //Checks if empty
16         example.clear();          //Clears vector
17     vector <int> another_vector;  //Creates another vector to store integers
18     another_vector.push_back(10); //Adds to end of vector
19     example.push_back(10);        //Same
20     if(example==another_vector)   //To show testing equality
21     {
22         example.push_back(20); 
23     }
24     for(int y=0; y<example.size(); y++)
25     {
26         cout<<example[y]<<" ";    //Should output 10 20
27     }
28     return 0;
29 }

Summary of Vector Benefits

Vectors are somewhat easier to use than regular arrays. At the very least, they get around having to be resized constantly using new and delete. Furthermore, their immense flexibility - support for any datatype and support for automatic resizing when adding elements - and the other helpful included functions give them clear advantages to arrays.

Another argument for using vectors are that they help avoid memory leaks--you don't have to remember to free vectors, or worry about how to handle freeing a vector in the case of an exception. This simplifies program flow and helps you write tighter code. Finally, if you use the at() function to access the vector, you get bounds checking at the cost of a slight performance penalty.

(�?

C++ vector is a container template available with Standard Template Library pack. This C++ vector can be used to store similar typed objects sequentially, which can be accessed at a later point of time. As this is a template, all types of data including user defined data types like struct and class can be stored in this container.

This article explains briefly about how to insert, delete, access the data with respect to the C++ vector. The type stl::string is used for the purposes of explaining the sample code. Using stl::string is only for sample purposes. Any other type can be used with the C++ vector.

The values can be added to the c++ vector at the end of the sequence. The function push_back should be used for this purpose. The header file should be included in all the header files in order to access the C++ vector and its functions.

1 #include <vector>
2 #include <string>
3 #include <iostream.h>
4
5 void main()
6 {
7     //Declaration for the string data
8     std::string strData = "One";
9     //Declaration for C++ vector
10     std:: vector <std::string> str_Vector;
11     str_Vector.push_back(strData);
12     strData = "Two";
13     str_Vector.push_back(strData);
14     strData = "Three";
15     str_Vector.push_back(strData);
16     strData = "Four";
17     str_Vector.push_back(strData);
18 }

The above code adds 4 strings of std::string type to the str_Vector. This uses std:: vector.push_back function. This function takes 1 parameter of the designated type and adds it to the end of the c++ vector.

Accessing Elements of C++ Vector:

The elements after being added can be accessed in two ways. One way is our legacy way of accessing the data with vector.at(position_in_integer) function. The position of the data element is passed as the single parameter to access the data.

Using our normal logic for accessing elements stored in C++ Vector:

If we want to access all the data, we can use a for loop and display them as follows.

1 for(int i=0;i < str_Vector.size(); i++)
2 {
3 std::string strd = str_Vector.at(i);
4 cout<<strd.c_str()<<endl;
5 }

The std:: vector .size() function returns the number of elements stored in the vector.

Using C++ vector iterator provided by STL:

The next way is to use the iterator object provided by the STL. These iterators are general purpose pointers allowing c++ programmers to forget the intricacies of object types and access the data of any type.

1 std::vector<std::string>::iterator itVectorData;
2 for(itVectorData = str_Vector.begin(); itVectorData != str_Vector.end(); itVectorData++)
3 {
4 std::string strD = *(itVectorData);
5 }

Removing elements from C++ vector:

Removing elements one by one from specified positions in c++ vector is achieved with the erase function.

The following code demonstrates how to use the erase function to remove an element from position 2 in the str_Vector from our sample.

1 str_Vector.erase(str_Vector.begin() + 1 ,str_Vector.begin() + 2 );

The following sample demonstrates how to use the erase() function for removing elements 2,3 in the str_Vector used in the above sample.

1 str_Vector.erase(str_Vector.begin() + 1 ,str_Vector.begin() + 3 );

If one wants to remove all the elements at once from the c++ vector, the vector.clear() function can be used.

keyws 2006-08-13 19:36 发表评论

【�{】iterators

keyws — Sun, 13 Aug 2006 11:04:00 GMT

http://www.cprogramming.com/tutorial/stl/iterators.html

The concept of an iterator is fundamental to understanding the C++ Standard Template Library (STL) because iterators provide a means for accessing data stored in container classes such a vector, map, list, etc.

You can think of an iterator as pointing to an item that is part of a larger container of items. For intance, all containers support a function called begin, which will return an iterator pointing to the beginning of the container (the first element) and function, end, that returns an iterator corresponding to having reached the end of the container. In fact, you can access the element by "dereferencing" the iterator with a *, just as you would dereference a pointer.

To request an iterator appropriate for a particular STL templated class, you use the syntax

1 std::class_name<template_parameters>::iterator name

where name is the name of the iterator variable you wish to create and the class_name is the name of the STL container you are using, and the template_paramters are the parameters to the template used to declare objects that will work with this iterator. Note that because the STL classes are part of the std namespace, you will need to either prefix every container class type with "std::", as in the example, or include "using namespace std;" at the top of your program.

For instance, if you had an STL vector storing integers, you could create an iterator for it as follows:

1std::vector<int> myIntVector;
2std::vector<int>::iterator myIntVectorIterator;

Different operations and containers support different types of iterator behavior. In fact, there are several different classes of iterators, each with slightly different properties. First, iterators are distinguished by whether you can use them for reading or writing data in the container. Some types of iterators allow for both reading and writing behavior, though not necessarily at the same time.

Some of the most important are the forward, backward and the bidirectional iterators. Both of these iterators can be used as either input or output iterators, meaning you can use them for either writing or reading. The forward iterator only allows movement one way -- from the front of the container to the back. To move from one element to the next, the increment operator, ++, can be used.

For instance, if you want to access the elements of an STL vector, it's best to use an iterator instead of the traditional C-style code. The strategy is fairly straightforward: call the container's begin function to get an iterator, use ++ to step through the objects in the container, access each object with the * operator ("*iterator") similar to the way you would access an object by dereferencing a pointer, and stop iterating when the iterator equals the container's end iterator. You can compare iterators using != to check for inequality, == to check for equality. (This only works for one twhen the iterators are operating on the same container!)

The old approach (avoid)

 1 using namespace std;
 2 
 3 vector<int> myIntVector;
 4 
 5 // Add some elements to myIntVector
 6 myIntVector.push_back(1);
 7 myIntVector.push_back(4);
 8 myIntVector.push_back(8);
 9 
10 for(int y=0; y<myIntVector.size(); y++)
11 {
12     cout<<myIntVector[y]<<" ";  //Should output 1 4 8
13 }

The STL approach (use this)

 1 using namespace std;
 2 
 3 vector<int> myIntVector;
 4 vector<int>::iterator myIntVectorIterator;
 5 
 6 // Add some elements to myIntVector
 7 myIntVector.push_back(1);
 8 myIntVector.push_back(4);
 9 myIntVector.push_back(8);
10 
11 for(myIntVectorIterator = myIntVector.begin(); 
12         myIntVectorIterator != myIntVector.end();
13         myIntVectorIterator++)
14 {
15     cout<<*myIntVectorIterator<<" ";    //Should output 1 4 8
16 }
17

As you might imagine, you can use the decrement operator, --, when working with a bidirectional iterator or a backward operator.

Iterators are often handy for specifying a particular range of things to operate on. For instance, the range item.begin(), item.end() is the entire container, but smaller slices can be used. This is particularly easy with one other, extremely general class of iterator, the random access iterator, which is functionally equivalent to a pointer in C or C++ in the sense that you can not only increment or decrement but also move an arbitrary distance in constant time (for instance, jump multiple elements down a vector).

For instance, the iterators associated with vectors are random access iterators so you could use arithmetic of the form

iterator + n

where n is an integer. The result will be the element corresponding to the nth item after the item pointed to be the current iterator. This can be a problem if you happen to exceed the bounds of your iterator by stepping forward (or backward) by too many elements.

The following code demonstrates both the use of random access iterators and exceeding the bounds of the array (don't run it!):

1 vector<int> myIntVector;
2 vector<int>::iterator myIntVectorIterator;
3 myIntVectorIterator = myIntVector.begin() + 2;

You can also use the standard arithmetic shortcuts for addition and subtraction, += and -=, with random access iterators. Moreover, with random access iterators you can use <, >, <=, and >= to compare iterator positions within the container.

Iterators are also useful for some functions that belong to container classes that require operating on a range of values. A simple but useful example is the erase function. The vector template supports this function, which takes a range as specified by two iterators -- every element in the range is erased. For instance, to erase an entire vector:

1 vector<int>::iterator myIntVectorIterator;
2 myIntVector.erase(myIntVectorIterator.begin(), myIntVectorIterator.end());

which would delete all elements in the vector. If you only wanted to delete the first two elements, you could use

1myIntVector.erase(myIntVectorIterator.begin(), myIntVectorIterator.begin()+2);

Note that various container class support different types of iterators -- the vector class, which has served as our model for iterators, supports a random access iterator, the most general kind. Another container, the list container (to be discussed later), only supports bidirectional iterators.

So why use iterators? First, they're a flexible way to access the data in containers that don't have obvious means of accessing all of the data (for instance, maps [to be discussed later]). They're also quite flexible -- if you change the underlying container, it's easy to change the associated iterator so long as you only use features associated with the iterator supported by both classes. Finally, the STL algorithms defined in (to be discussed later) use iterators.

Summary

The Good

The STL provides iterators as a convenient abstraction for accessing many different types of containers.
Iterators for templated classes are generated inside the class scope with the syntax
```
class_name<parameters>::iterator
```
Iterators can be thought of as limited pointers (or, in the case of random access iterators, as nearly equivalent to pointers)

The Gotchas

Iterators do not provide bounds checking; it is possible to overstep the bounds of a container, resulting in segmentation faults
Different containers support different iterators, so it is not always possible to change the underlying container type without making changes to your code
Iterators can be invalidated if the underlying container (the container being iterated over) is changed significantly

keyws 2006-08-13 19:04 发表评论