GeekBand笔记: STL与泛型编程（容器）

stl

• stl 6 components
  • allocator
  • container
  • algorithm
  • iterator
  • functor
  • adaptor

sequential container 顺序容器

• elements are stored and accessed sequentially by their position in the container

  • the sequence of elements is corresponding with the position while they adding to the container, not their value.

• category of sequential container

  • vector
  • deque (double-ended queue)
  • list
  • forward_list (singly linked list)
  • array
  • string

• sequential container adaptor 顺序容器适配器

  • stack
  • queue
  • priority_queue

• performance trade-offs(性能折中) of container

  • the costs to add/remove elements in container
  • the costs to random access the elements

• contiguous-memory container (vector, string, array)

  • support fast random access
  • In exchange, add/remove element in the middle of container is inefficient

• list storage container (list, forward_list)

  • fast to add/remove an element anywhere.
  • In exchange, not support random access
  • Moreover, memory overhead is often substantial. If your program has lots of small elements and space overhead matters, don’t use list or forward_list

• deque
  • supports fast random access
  • fast insert/delete at front or back
  • 存储方式待定

associative container 关联容器

• Elements in an associative container are stored and retrieved by a key

  • In contrast, elements in a sequential container are stored and accessed sequentially by their position in the container.

• two primary types

  • map

    • The elements in a map are key–value pairs
    • use as dictionary
    • often refer to as an associative array. an associative array is like a normal array except that its subscript don't have to be int.

  • set

    • A set element contains only a key
    • a set is most useful when we simply want to know whether a value is present. for example: white-list
    • use when to keep elements sorted and unique

// elements ordered by key
map // key-value pairs, unique key
set // key is the value, unique key
multimap // allow multiple key
multiset

// unordered key
unordered_map
unordered_set
unordered_multimap
unordered_multiset

• pair: key-value
  • the data members of pair are public. these members are named first and second
  • In a map, the elements are key-value pairs. That is each element is a pair object.
    • map<T1,T2>::value_type => pair<const T1, T2> //the value type of map is a pair and we can change the value but not the key of pair!
    • map<T1,T2>::key_type => T1
    • map<T1,T2>::mapped_type => T2

pair<T1, T2> p(v1, v2)
pair<T1, T2> p = {v1, v2}
make_pair(v1, v2)

p.first
p.second

• set<T>::iterator are const

  • the keys in a set are const!, we can use a set iterator to read, but not write.

• associative container and algorithm

  • In general, we do not use the generic algorithm with associative container. the fact that the keys are const means that we can not pass the associative container iterators to algorithm that write or reorder container elements.
  • associative container can be used with read-only algorithm. however, it is much faster to use the find member(map.find()) than to call generic version.
  • In practice, if we can use an associative container with the algorithm either as source sequence or as destination.

  set<int> c;
  vector<int> v;
  copy(c.begin(), c.end(), back_inserter(v, v.end())) // as a source sequence
  copy(v.begin(), v.end(), inserter(c, c.end())) // as a destination
  

container common operation

typedef in container

common

iterator
const_iterator
size_type
difference_type
value_type
reference
const_reference

associative container

key_type    // type of the key
mapped_type // type associated with each key, map only
value_type // for set, same as key_type; for map, pair<const key_type, mapped_type>

constructor and assignment

common

C c;
C c1(c2); // container type and element type must match
C c(b, e);  // when pass iterators. no requirement that container types are identical, Moreover element type can differ as long as compatible

C c{a,b,c...}; // list initialization.
C c={a,b,c...};

c1 = c2
c1 = {a,b,c...} // replace with the initializer list
a.swap(b) // swap is usually much faster than copy b to a
swap(a,b) //

only for seq container, not valid for associative containers or array

seq.assign(b, e)
seq.assign(il)
seq.assign(n,t)

size

c.size() // forward_list not support
c.max_size()
c.empty()

relational operator

the container type and element type of operands must be identical.

common
==, !=

all except unordered associative container
>, >=, <, <=

add/remove

common, but not support array

c.insert(args)
c.emplace(inits) //???

sequential container add

// add in front or back
c.push_back(t)
c.push_front(t) // only list and deque

// add in anywhere(before the element denoted by iterator p)
c.insert(p, t)  
c.insert(p, n, t)
c.insert(p, b, e) // return an iterator to the first element inserted
c.insert(p, il)

c.emplace_back(args) // args pass to element type's constructor
c.emplace_front(args)
c.emplace(p, args)

associative container add

c.insert(b, e) // iterator range
c.insert(il) // map.insert({word, 1})
.... TODO

remove

common

c.erase(args)
c.clear

sequential container remove

c.pop_back() // return void
c.pop_back()
c.erase(p)
c.erase(b, e) // return an iterator to the element after the last deleted one
c.clear() // return void

access element

sequential container access

c.back() // not for forward_list
c.front()

// ranom access: at() and subscript(operator[]) valid only for contiguous-memory containers
c[n] // no bounds checked, if overrun, undefined error happen. more efficient than at
c.at(n) // bounds checked, if overrun, throw out_of_range exception

notes:

• make sure the container is not empty, or undefined error happen
• all access operation return a reference.
• safe random access
  • operator[] must be "in range [0, size)", or undefined happen.
    • it's up to the programmer to ensure that the index is valid
    • compiler will not check whether the index is out_of_range
  • use at() to ensure the index is invalid
    • if the index is invalid, throw an out_of_range exception

resize a container

c.resize(n) // if n<c.size(), the excess elements are discarded
c.resize(n, t)

capacity

c.shrink_to_fit()
c.capacity()
c.reserve(n)

container operation may invalidate iterator

• container operations add, remove, or resize, may invalidate pointers, references, or iterators to container elements.
  • it's an serious run-time error to use an invalidated pointers, references, or iterators, as using uninitialized pointer
• ensure pointers, references, or iterators is refreshed when container is changed by add/remove/resize

选择合适的容器

• list : anytime when a doubly-linked list is required.
• vector : anytime when a dynamic array is required.
• deque :
  • Why do we need Deque data structures in the real world?
• map : dictionary.
• set : when to keep elements sorted and unique.