Table of Contents

1 Functional

1.1 hash

unordered_set require to hash the items, but std::pair does not have a hash to apply.

struct my_hash {
  inline std::size_t operator() (const std::pair<int, int> &p) const {
    std::hash<int> hasher;
    return hasher(p.first) ^ hasher(p.second);
std::unordered_set<std::pair<int, int>, my_hash> s;

2 Containers

2.1 Comparison

container time complexity iterator validity
vector constant at beginning, linear in middle and end Memory allocate; Insert at beginning and middle
deque constant at beginning and end, linear in the middle All insertions; Erase in middle.
set   erase invalidates the element removed

2.1.1 convert set to vector

std::copy doesn't add elements to the container into which you are inserting: it can't; it only has an iterator into the container. Because of this, if you pass an output iterator directly to std::copy, you must make sure it points to a range that is at least large enough to hold the input range.

std::back_inserter creates an output iterator that calls pushback on a container for each element, so each element is inserted into the container. Alternatively, you could have created a sufficient number of elements in the std::vector to hold the range being copied.

// method 1
std::copy(input.begin(), input.end(), std::back_inserter(output));
// method 2
// note that std::copy will NOT allocate memory
// so make sure the vector is large enough before copy
std::vector<double> output(input.size());
std::copy(input.begin(), input.end(), output.begin());
// method 3
std::vector<double> output(input.begin(), input.end());


2.2 vector

Time complexity: constant time insertion or removal at the end, linear at the beginning or middle.

Iterator invalidity: Memory will be allocated automatically, which, when happens, invalidates all iterators. Reserve() causes a reallocation manually. inserting or deleting an element in the middle of a vector invalidates all iterators that point to elements following the insertion or deletion point.

The initiation of vector can have the following forms:

// ONLY WITH C++11, compile with flag =-std=c++11=
// 1
vector<int> v {1,2,3};
// 2
int row,col;
vector< vector<int> > heights(row, vector<int>(col));
// 3
char init[] = "1111";
vector<char> v(init, end(init)-1); // remove '\0'

Reserve() causes a reallocation manually. The main reason for using reserve() is efficiency: if you know the capacity to which your vector must eventually grow, then it is usually more efficient to allocate that memory all at once rather than relying on the automatic reallocation scheme. The other reason for using reserve() is so that you can control the invalidation of iterators

Do not use vector<bool>. It actually not store bool, but proxy object, as the design to save space.

2.3 set

Set has the important property that inserting a new element into a set does not invalidate iterators that point to existing elements. Erasing an element from a set also does not invalidate any iterators, except, of course, for iterators that actually point to the element that is being erased.

2.3.1 unorderedset

  • Use hash, Constant complexity.
  • no specific order guarantee

2.4 map

The map is usually used as the following example:

map<int,int> m;
for(map<int,int>::iterator it=m.begin;it!=m.end();it++) {

In a map, when using [] operator, the item will be inserted and init to default value for that type, e.g. 0 for a int.

2.4.1 multimap

multimap do not have [] operator! The following is an example for the usage of multimap:

std::multimap<char,int> mymm;


std::cout << "mymm contains:\n";
for (char ch='a'; ch<='d'; ch++)
  std::pair <std::multimap<char,int>::iterator, std::multimap<char,int>::iterator> ret;
  ret = mymm.equal_range(ch);
  std::cout << ch << " =>";
  for (std::multimap<char,int>::iterator it=ret.first; it!=ret.second; ++it) {
    std::cout << ' ' << it->second;

// maybe it is helpful to just document some usage example
multimap<int, int> mm;
mm.emplace(3, 8); // using std::pair constructor
for (auto elem : mm) {
  mm.first; // 3
  mm.second; // 8
auto range = mm.equal_range(3);
for (auto it=mm.begin();it!=mm.end();++it) {
  it->first; // 3
  it->second; // 8

2.4.2 unorderedmap

  • Use hash, constant complexity
  • no specific order guarantee

2.5 deque

deque refers to Double Ended Queue. It differs from vector in the sense that the insertion at the front is constant time. Like vector, insertion in the end is constant, and insertion in the middle is n.

Iterator Validity:

  • Insert (including pushfront and pushback) invalidates all iterators that refer to a deque.
  • Erase in the middle of a deque invalidates all iterators that refer to the deque.
  • Erase at the beginning or end of a deque (including popfront and popback) invalidates an iterator only if it points to the erased element.

2.6 pass to legacy API

2.6.1 vector

vector<int> v;
void func(const int* pi, size_t num);
// wrong, the size of v may be 0
func(&v[0], v.size());
if (!v.empty()) {
  func(&v[0], v.size());

do not use v.begin() instead of &v[0], because:

  • v.begin() is a iterator, not always a pointer
  • &*v.begin() is same as &v[0], but …

Note, the legacy API should not add/remove items, because no way for the container to know the size.

2.6.2 String

Only vectors are guarnteed to have the same underlying memory layout as arrays. String not.

  • data for strings is not guaranteed to be stored in contiguous memory.
  • is not guaranteed to be null terminated

so put the data into a vector<char> first, and use vector trick.

3 Algorithms

3.1 comparison function

Always have comparison functions return false for equal values.

set<int, less_equal<int> > s;

check equivalence:

!(10A<=10B) && !(10B<=10A);
!true && !true

a easy-to-made error:

bool operator()(const string* ps1, const string* ps2) onst {
  return !(* ps1<* ps2); // always pay attention to negative

3.2 Algorithm

3.2.1 count & count_if

template< class InputIt, class T >
typename iterator_traits<InputIt>::difference_type
  InputIt first,
  InputIt last,
  const T &value
template< class InputIt, class UnaryPredicate >
typename iterator_traits<InputIt>::difference_type
  InputIt first,
  InputIt last,
  UnaryPredicate p

3.2.2 std::find

Returns an iterator to the first element in the range [first,last) that compares equal to val. If no such element is found, the function returns last.

template<class InputIterator, class T>
InputIterator find (InputIterator first, InputIterator last, const T& val)
  while (first!=last) {
    if (*first==val) return first; //*
  return last;

3.2.3 std::find_if

Returns an iterator to the first element in the range [first,last) for which pred returns true. If no such element is found, the function returns last.

template<class InputIterator, class UnaryPredicate>
InputIterator find_if (InputIterator first, InputIterator last, UnaryPredicate pred)
  while (first!=last) {
    if (pred(* first)) return first;
  return last;

for example

template <typename T>
class is_greater_than
  is_greater_than (const T & n)
  : value(n)

  bool operator() (const T & element) const
  return element > value;

  T value;

use here:

find_if (values.begin(), values.end(), is_greater_than<int> (5)) != values.end();

3.2.4 for

vector<int> v;
for (auto i : v) {
  // do with i

here auto means vector<int>::value_type.

is equal-valent to:

vector<int> v;
for (std::vector<int>::const_iterator it=v.begin();it!=v.end();it++) {
  auto i=*it; //*
  // do with i

3.2.5 std::for_each

void myfunction (int i) {  // function:
  std::cout << ' ' << i;

struct myclass {           // function object type:
  void operator() (int i) {std::cout << ' ' << i;}
} myobject;

std::vector<int> myvector;

for_each (myvector.begin(), myvector.end(), myfunction);
for_each (myvector.begin(), myvector.end(), myobject);
for_each (v.begin(),v.end(),[](int i) { cout<<i; });

3.2.6 mem_fun

list<Widget*> lpw;
  // because the test is the member function of Widget
  // and we want to call it on all for_each item.
  // If no mem_fun, it can not compile


  • ptrfun
  • memfunref

3.2.7 std::move

#include <utility>

transfer ownership of the assets and properties of an object directly without having to copy them when the argument is an rvalue.

moved-from object is left in a valid but unspecified state

std::string foo = "foo-string";
std::string bar = "bar-string";
std::vector<std::string> myvector;
myvector.push_back (foo);                    // copies. foo remain.
myvector.push_back (std::move(bar));         // moves. bar contain unspecified value.
int main()
  std::string str = "Hello";
  std::vector<std::string> v;

  // uses the push_back(const T&) overload, which means
  // we'll incur the cost of copying str
  std::cout << "After copy, str is \"" << str << "\"\n";

  // uses the rvalue reference push_back(T&&) overload,
  // which means no strings will be copied; instead, the contents
  // of str will be moved into the vector.  This is less
  // expensive, but also means str might now be empty.
  std::cout << "After move, str is \"" << str << "\"\n";

  std::cout << "The contents of the vector are \"" << v[0]
  << "\", \"" << v[1] << "\"\n";

  // string move assignment operator is often implemented as swap,
  // in this case, the moved-from object is NOT empty
  std::string str2 = "Good-bye";
  std::cout << "Before move from str2, str2 = '" << str2 << "'\n";
  v[0] = std::move(str2);
  std::cout << "After move from str2, str2 = '" << str2 << "'\n";

Possible output:

After copy, str is "Hello"
After move, str is ""
The contents of the vector are "Hello", "Hello"
Before move from str2, str2 = 'Good-bye'
After move from str2, str2 = 'Hello'

3.2.8 not1

template< class Predicate >
std::unary_negate<Predicate> not1(const Predicate& pred);
template< class Predicate >
constexpr std::unary_negate<Predicate> not1(const Predicate& pred);

not1 is a helper function to create a function object that returns the complement of the unary predicate function passed.


struct LessThan7 : std::unary_function<int, bool>
  bool operator()(int i) const { return i < 7; }
std::function<int(int)> less_than_9 = [](int x){ return x < 9; };

3.2.9 reverse

reverse(v.begin(), v.end());

3.2.10 sort

Do not use qsort for some unknown reason..

faster to slower:

  1. partition
  2. stablepartition
  3. nthelement
  4. partialsort
  5. sort
  6. stablesort

stable means the order of equal element is guaranteed to maintain. partition

The returned iterator is middle. From first to middle, the predicate is true. From middle to last, the predicate is false.


template< class BidirIt, class UnaryPredicate >
BidirIt partition(
  BidirIt first,
  BidirIt last,
  UnaryPredicate p
template< class ForwardIt, class UnaryPredicate >
ForwardIt partition(
  ForwardIt first,
  ForwardIt last,
  UnaryPredicate p
); nth_element

The first n elements in the container are best, but not sorted.


template< class RandomIt >
void nth_element(
  RandomIt first,
  RandomIt nth,
  RandomIt last
template< class RandomIt, class Compare >
void nth_element(
  RandomIt first,
  RandomIt nth,
  RandomIt last,
  Compare comp
); partial_sort

The first n elements in the container are the best, and in order.


template< class RandomIt >
void partial_sort(
  RandomIt first,
  RandomIt middle,
  RandomIt last
template< class RandomIt, class Compare >
void partial_sort(
  RandomIt first,
  RandomIt middle,
  RandomIt last,
  Compare comp



3.2.11 transform

template< class InputIt, class OutputIt, class UnaryOperation >
OutputIt transform(
  InputIt first1,
  InputIt last1,
  OutputIt d_first,
  UnaryOperation unary_op
template< class InputIt1, class InputIt2, class OutputIt, class BinaryOperation >
OutputIt transform(
  InputIt1 first1,
  InputIt1 last1,
  InputIt2 first2,
  OutputIt d_first,
  BinaryOperation binary_op

std::transform applies the given function to a range and stores the result in another range, beginning at dfirst.

4 Idioms

4.1 erase-remove idiom

To erase certain elements in a container, the remove and remove_if is provided in <algorithm>. Algorithms operate on a range of elements denoted by two forward iterators, they have no knowledge of the underlying container or collection. Thus, no elements are actually removed from the container. Rather, all elements which don't fit the remove criteria are brought together to the front of the range, in the same relative order. The remaining elements are left in a valid, but unspecified, state. So, after using remove, the size() of the container is unchanged. To actually remove those, it should be used together with the erase() member function of the container.

When the remove function is done, remove returns an iterator pointing one element past the last unremoved element. So the erase is used as follows:

v.erase( std::remove( v.begin(), v.end(), 5 ), v.end() );

However, this is not the case for map and set.

In simple associative containers, where the elements are the keys, the elements are completely immutable; the nested types iterator and constiterator are therefore the same.

That means the iterator and const_iterator are actually the same for set and map. The erase-remove idiom cannot be used here. Rather, it should use the following loop:

typedef std::set::iterator set_iter; 
for( set_iter it = s.begin(); it != s.end(); /* blank */ ) {
  if( some_condition() ) {
    // s.erase( it++ );       // Note the subtlety here
    // I think this is better, erase return iterator to the next element
    it = s.erase(it);
  else {

Author: Hebi Li

Created: 2017-03-27 Mon 14:36