Container | 容器

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STL

Abstract

在 C++ 中，容器是 STL 中的模板提供的

• STL = Standard Template Library | 标准模板库
• Part of the ISO Standard C++ Library
• Data Structures and algorithms for C++

Why use STL

• Reduce development time
• Code readability
• Robustness
• Portable code
• Maintainable code
• Easy

• Library includes:
  • A Pair class (pairs of anything, int/int, int/char, etc)
  • Containers
    • vector (expandable array)
    • deque (expandable array, expands at both ends)
    • list (double-linked)
    • sets and maps
  • Basic Algorithms (sort, search, etc)
• All identifiers in library are in std namespace (using namespace std;)
• The three parts of STL
  • Containers
  • Algorithms
  • Iterators
• Top 3 data structures
  • map
    • Any key type, any value type
    • sorted
  • vector
    • like c array, but auto-extending
  • list
    • doubly-linked list
• All Sequential Containers
  • vector: variable array
  • deque: dual-end queue
  • list: double-linked-list
  • forward_list: as it
  • array: as “array”
  • string: char.array

Vector Class

Example

#include <iostream>
using namespace std;
// Use namespace so that you can refer to vectors in C++ library
#include <vector>

int main()
{
    vector<int> x;  //Just declare a vector of ints(no need to worry about size)
    for(int a = 0; a < 1000; a++)
    {
        x.push_back(a); //Add elements
    }
    /* Have a pre-defined iterator for vector class, can use it to print out the items in vector */
    vector<int>::iterator p;    
    for(p = x.begin(); p < x.end(); p++)
    {
        cout << *p << " ";
    }
    return 0;
}

• vector<int> x; 中 x 是变量的名字，<int> 表示要在 vector 里面放 int类型的数据
• ::iterator 表示 vector 中的类 iterator，可以对 vector of int 枚举, 拿出来的每一个都应该是 int
• begin 会返回一个枚举器，用这个枚举器可以枚举容器中的每一个元素

• It is able to increase its internal capacity as required: as more items are added, it simply makes enough room for them
• It keeps its own private count of how many items it is currently storing. Its size method returns the number of objects currently in it
• In maintains the order of items you insert into it. You can later retrieve them in the same order.

Basic Vector Operations

• Constructors
  • vector<ElementType> c;
  • vector<ElementType> c1(c2); //c1 可以得到 c2 里的所有内容
• Simple Methods
  • V.size(); //num items
  • V.empty(); //Judge if empty
  • ==, !=, <, >, <=, >= //比较两个 vector 的大小，依次拿对应元素比较
  • V.swap(v2); //swap 两个 vector 的全部内容
• Iterators
  • I.begin(); //first positiion
  • I.end(); //last position
• Element access
  • V.at(index)
  • V[index]
  • V.front(); //first item
  • V.back(); //last item
• Add/Remove/Find
  • V.push_back(e);
  • V.pop_back(e);
  • V.insert(position, e); // position 是一个 iterator，不能使用 1，2，3
  • V.erase(position); // size will change after "erase"
  • V.clear();
  • V.find(first, last, item);

Pitfalls

WRONGRIGHT

• Accessing an invalid vector<> element
  • vector<int> v;
  • v[100] = 1; //Whooops!

• use push_back()
• Preallocate with constructor
• Reallocation with reserve()
• Check capacity()

List Class

双向链表

• Same basic concepts as vector
  • Constructors
  • Ability to compare lists (==, !=, <, >, <=, >=)
  • Ability to access front and back of list
    • x.front(), x.back()
  • Ability to assign items to a list, remove items
    • x.push_back(item), x.push_front(item)
    • x.pop_back(), x.pop_front()
    • x.remove(item)

code

Sample List Application

#include <iostream>
#include <list>
#include <string>
using namespace std;
int main()
{
    list<string> s;
    s.push_back("hello");
    s.push_back("world");
    s.push_front("tide");
    s.push_front("crimson");
    list<string>::iterator p;
    for(p = s.begin(); p != s.end(); p++)
    {
        cout << *p << " ";
    }
    cout << endl;
}

• Note the termination condition for the for loop: p != s.end()

Pitfalls

WRONGRIGHT

• Using invalid iterator
  • list<int> L; list<int>::iterator li;
  • li = L.begin(); L.erase(li);
  • ++li; // WRONG - 此时 li 已经被 erase

• Use return value of erase to advance
  • li = L.erase(li); //RIGHT
  • 此时 li 指向的是原来的下一个

Maps

• Maps are collections that contain pairs of values.
• Pairs consist of a key and a value (故声明要指定两个类型)
• Lookup works by supplying a key, and retrieving a value
• An example: a telephone book

Example

#include <iostream>
#include <string>
#include <map>
using namespace std;

int main()
{
    map<string, float> price;
    price["apple"] = 0.75;
}

Pitfalls

WRONGRIGHT

• Inadvertently inserting into map<>
  • if(foo["bob"] == 1) //silently created entry "bob"

• Use count() to check for a key without creating a new entry
  • if(foo.count("bob"))

Iterator

use list as example list<int> L;

• Declaring
  • list<int>::iterator li;
• Front of container
  • li = L.begin();
• Past the end
  • li = L.end();
• Can increment
  • li = L.begin();
  • ++li; // second item
• Can be dereferenced
  • *li = 10;

Self-defined classes in STL Containers

• Might need:
  • Assignment Operator -> operator=()
  • Default Constructor
• For sorted types, like map<>
  • Need less-than operator -> operator<()
    • Some types have this by default (int, char, string)
    • Some do not (char *)

Example

···
class Student{
public:
    int x;
    Student(int k) : x(k) {}
};
int main()
{
    vector<Student>  ss;   
    vector<Student&> sl;   
    vector<Student*> sp;    
    ···
}

• vector<Student*> sp; 表示 vector 中存的是指针

Other data structures

• set, multiset, multimap
• queue, priority_queue
• stack, deque
• slist, bitset, valarray

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