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CS253 STL Containers

Overview

The most popular containers:
- vector: variable-length array
- array: a fixed-length array
- string: vector<char> and more methods
- list: doubly-linked list
- forward_list: singly-linked list
- set: an ordered binary tree
- multiset: a set where duplicates are permitted
- map: a tree of key,value (or a translation)
- multimap: a map where duplicate keys are permitted
- deque: a vector with push/pop at either end
- unordered_set: hash version of set
- unordered_multiset: hash version of multiset
- unordered_map: hash version of map
- unordered_multimap: hash version of multimap

Attributes

Name	Key/Value	Insert	Append	Search	Order
string, vector	value	O(n)	O(1)	O(n)	no
deque	value	O(n)	O(1)	O(n)	no
list, forward_list	value	O(1)	O(1)	O(n)	no
[multi]set	value	O(log n)		O(log n)	yes
[multi]map	key/value	O(log n)		O(log n)	yes
unordered_[multi]set	value	O(1)		O(1)	hashed
unordered_[multi]map	key/value	O(1)		O(1)	hashed

Popular methods

Universal container methods:
- default ctor
- copy ctor
- operator=
- ctor(iter, iter)
- empty()
- size() / max_size()
- clear()
- begin() / end() / cbegin() / cend()
Popular container methods:
- ctor(n)
- rbegin() / rend() / crbegin() / crend()
- front() / back()
- insert() / erase()
- find() / count()

Popular types

Universal types:
- value_type
- iterator
- const_iterator
- size_type
Popular types:
- key_type
- reverse_iterator
- const_reverse_iterator

string

string con = "The quick brown fox jumps over a lazy dog.";
cout << "size=" << con.size() << ' ';
for (auto c : con)
    cout << c;

size=42 The quick brown fox jumps over a lazy dog.

This displays a std::string.
After this, the same code will be shown, but using different containers.
Sure, we could have just said cout << con; since we’re using a std::string, but but that won’t work for other container types.

vector

string s = "The quick brown fox jumps over a lazy dog.";
vector<char> con(s.begin(), s.end());
cout << "size=" << con.size() << ' ';
for (auto c : con)
    cout << c;

size=42 The quick brown fox jumps over a lazy dog.

We can’t initialize a vector directly from a C-style string, so we put it into a C++ string, and initialize the vector from that, using the two-iterator constructor.

array

string s = "The quick brown fox jumps over a lazy dog.";
array<char, 42> con;
for (size_t i=0; i<s.size(); i++)
    con[i] = s[i];
cout << "size=" << con.size() << ' ';
for (auto c : con)
    cout << c;

size=42 The quick brown fox jumps over a lazy dog.

A std::array is not a C array, but its size is fixed at compile time.
Its storage is all on the stack, with no dynamic memory allocation, so it’s as effecient as a C array. It has typical STL container methods such as .size(), .begin(), .end(), etc.
It lacks the two-iterator ctor, perhaps due to its fixed size.

list

string s = "The quick brown fox jumps over a lazy dog.";
list<char> con(s.begin(), s.end());
cout << "size=" << con.size() << ' ';
for (auto c : con)
    cout << c;

size=42 The quick brown fox jumps over a lazy dog.

A list is a doubly-linked list.
It has a large storage overhead, but insertion is cheap, and you can easily go forward & backward.
Unlike a vector, .push_back() never causes reallocation.

forward_list

string s = "The quick brown fox jumps over a lazy dog.";
forward_list<char> con(s.begin(), s.end());
for (auto c : con)
    cout << c;

The quick brown fox jumps over a lazy dog.

A forward_list (formerly slist) is a singly-linked list.
It has a large storage overhead, but insertion is cheap, and you can easily go forward.
Curiously, there is no forward_list::size().

deque

string s = "The quick brown fox jumps over a lazy dog.";
deque<char> con(s.begin(), s.end());
cout << "size=" << con.size() << ' ';
for (auto c : con)
    cout << c;

size=42 The quick brown fox jumps over a lazy dog.

A deque is a double-ended queue. It’s like a vector, but you can easily push or pop from the front or the back.

set

string s = "The quick brown fox jumps over a lazy dog.";
set<char> con(s.begin(), s.end());
cout << "size=" << con.size() << ' ';
for (auto c : con)
    cout << c;

size=28  .Tabcdefghijklmnopqrsuvwxyz

A set is generally implemented as a binary tree. Hence, it is sorted, has fairly large storage overhead, and has O(log n) find/insertion/deletion times.

multiset

string s = "The quick brown fox jumps over a lazy dog.";
multiset<char> con(s.begin(), s.end());
cout << "size=" << con.size() << ' ';
for (auto c : con)
    cout << c;

size=42         .Taabcdeefghijklmnoooopqrrsuuvwxyz

A multiset is like a set, but allows copies.

unordered_set

string s = "The quick brown fox jumps over a lazy dog.";
unordered_set<char> con(s.begin(), s.end());
cout << "size=" << con.size() << ' ';
for (auto c : con)
    cout << c;

size=28 .gdyzlaspmjxfnworbkciuqv ehT

A unordered_set (formerly hash_set), is a hash table implementation like a set.

unordered_multiset

string s = "The quick brown fox jumps over a lazy dog.";
unordered_multiset<char> con(s.begin(), s.end());
cout << "size=" << con.size() << ' ';
for (auto c : con)
    cout << c;

size=42 .gdyzlaaspmjxfnwoooorrbkciuuqv        eehT

A unordered_multiset (formerly hash_multiset), is a hash table implementation like a multiset.

map, multimap, unordered_map, unordered_multimap

map, multimap, unordered_map, and unordered_multimap are like
set, multiset, unordered_set, and unordered_multiset, except that they store pairs.
You can think of them as look-up tables.
You can think of them as mapping functions.

map example

map<string, double> gpa = {
    { "Jack",  3.998 },
    { "Russ",  4.20  },
    { "Wim",   3.92  },
    { "Craig", 2.0   }
};

for (auto p : gpa)
    cout << p.first << " has a GPA of " << p.second << '\n';
cout << "Jack’s GPA is: " << gpa["Jack"] << '\n';

Craig has a GPA of 2
Jack has a GPA of 3.998
Russ has a GPA of 4.2
Wim has a GPA of 3.92
Jack’s GPA is: 3.998

A map<A,B> is like a set<pair<const A,B>>, ordered by A.
pair<A,B> has public data members (😲) .first and .second.

CS253: Software Development with C++

Spring 2021

STL Containers