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Posted 16 Dec 2014

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Top 5 Beautiful C++ std Algorithms Examples

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16 Dec 2014CPOL4 min read
Several examples of beautiful code made up of algorithms from the C++ standard library. Heavily uses modern C++.

Image 1

Some time ago, I saw an inspiring talk from CppCon 2013: "C++ Seasoning" by Sean Parent. One of the main points of this presentation was not to use raw loops. Instead, prefer to use existing algorithms or write functions that 'wraps' such loops. I was curious about this idea and searched for nice code examples. Here is my short list of usage of algorithms from the C++ std library that might help in writing better code.

Of course. I could not skip two prominent examples from the original "C++ Seasoning" talk: slide and gather.

The Plan

Insertion Sort

In just two lines of code!

for (auto i = start; i != end; ++i)
    std::rotate(std::upper_bound(start, i, *i), i, std::next(i));

How It Works?

Rotate(first, middle, last)- takes a range [first, last) and rotates it so that the middle element becomes the first in that range.

upper_bound - Returns an iterator pointing to the first element in the range [first,last) which compares greater than val. The range should be already sorted (or at least partitioned).

How do those two elements combine into Insertion sort?

std::upper_bound(start, i, *i) returns position of the first element greater than *i. Then, the range is shifted, so that i-th element becomes first.

Let's look at one example:

Image 2

Pretty nice!

Quick Sort

Found on Stack Overflow:

template<class FwdIt, class Compare = std::less<>>
void quickSort(FwdIt first, FwdIt last, Compare cmp = Compare{})
    auto const N = std::distance(first, last);
    if (N <= 1) return; 
    auto const pivot = std::next(first, N / 2);
    std::nth_element(first, pivot, last, cmp);
    quickSort(first, pivot, cmp); 
    quickSort(pivot, last, cmp); 

How It Works?

I will not describe quick sort algorithm... you should know how it works already! In this implementation, std::nth_element is used to do most of the job. This function partially sorts the range so that given n-th elements is placed in proper position. All of the elements before n-th element are less than or equal to the elements after the n-th element.


Example from the Sean Parent's talk:

template <typename It> 
auto slide(It first, It last, It pos) -> std::pair<It, It>
    if (pos < first) return { pos, std::rotate(pos, frist, last) };
    if (last < pos) return { std::rotate(first, last, pos), pos };
    return { first, llast };

How It Works?

As an example, you can imagine a list of items on a UI dialog. User selects a continuous range and then algorithm takes this range and moves it into some other place of the list.

Image 3

  • This function uses std::rotate: to move elements forward or backward.
  • It returns two iterators - the start and the end of the new sequence. In C++11 std::rotate got new version and now can return iterator to the new position of p element.
  • If you are not interested in returning of this iterator pair, you can simplify this code much more.

Implementation note:

  • In GCC 4.9 (and previous versions) std::rotate does not return an iterator, but only void. So currently, this code will not work there.


Another example from Sean Parent's talk:

template <typename BiIt, typename UnPred> 
auto gather(BiIt f, BiIt l, BiIt p, UnPred s) -> std::pair <BiIt, BiIt>
    return { stable_partition(f, p, not1(s)), 
             stable_partition(p, l, s) };

How It Works?

Its use case can be similar to slide: select elements - using a predicate s (so this time continuous range is not needed), then gather those elements into a range and move this range to position around p. It returns the start and the end of the selected range.

UnPred is a predicate that returns if a given element is selected or not.

Image 4

std::stable_partition: from cppreference

Reorders the elements in a given range in such a way that all elements for which the predicate returns true precede the elements for which predicate returns false. Relative order of the elements is preserved.

std::stable_partition is used twice:

Image 5

Implementation note:

  • std::not1 does not work with the code correctly, so there is a proposal to use simple lambda. Read more here in Sean's comment.

String trim

Found on Stack Overflow

std::string trim(const std::string &s) {
    return trimLeft(trimRight(s));

std::string trimLeft(const std::string &s) {
    auto temp = s;
                std::find_if(std::begin(temp), std::end(temp), 
                    [](char c){return !std::isspace(c, std::locale()); 
    return temp;

std::string trimRight(const std::string &s) {
    auto temp = s;
    temp.erase(std::find_if(std::rbegin(temp), std::rend(temp), 
                [](char c){return !std::isspace(c, std::locale()); }).base(), 
    return temp;

How It Works?

Another beautiful usage of Standard Library:

  • In order to trim the string, we trim from right and then from the left (what a discovery!)
  • trim left: std::find_if returns iterator to the first non space character in the string. Then we erase those characters.
  • trim right: also uses std::find_if but this time we use reverse iterators

Note: You can also use boost string algorithm to make life even easier.

Bonus :)

What does this code do?

while (std::next_permutation(start, end));

Simple, one line of code... should be nice! But...

Answer: It's another and 'wonderful' method of sorting containers - permutation sort! But please do not use it at home. :)

Complexity: O((n+1)!)

This algorithm is a variation of Bogosort and other similar 'sorting' algorithms. Read more on wiki. As victor_zverovich noticed, in Bogosort the next permutation is chosen at random, but std::next_permutation gives the next lexicographically greater permutation.


I've showed several, I think nice, code examples where algorithms from C++ Standard Library are heavily used. Maybe next time, when I'll be writing some ugly piece of code I'll stop, think for a minute, and maybe some existing algorithm/function could be called instead.

Do you know some more interesting examples? My list, definitely, does not show all of them!


Image 6Image 7


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

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Comments and Discussions

BugrandIter undefined Pin
D4rkTrick27-Feb-15 14:21
professionalD4rkTrick27-Feb-15 14:21 
GeneralRe: randIter undefined Pin
Bartlomiej Filipek1-Mar-15 7:18
Bartlomiej Filipek1-Mar-15 7:18 
GeneralRe: thx! Pin
D4rkTrick2-Mar-15 13:23
professionalD4rkTrick2-Mar-15 13:23 
Suggestionformatting and naming Pin
D4rkTrick27-Feb-15 14:14
professionalD4rkTrick27-Feb-15 14:14 
Questionrequest Pin
taranea24-Dec-14 9:33
taranea24-Dec-14 9:33 
QuestionSource Code Pin
geoyar22-Dec-14 14:40
professionalgeoyar22-Dec-14 14:40 
AnswerRe: Source Code Pin
Bartlomiej Filipek23-Dec-14 2:18
Bartlomiej Filipek23-Dec-14 2:18 
QuestionAlternative for string trimming Pin
LWessels18-Dec-14 1:46
LWessels18-Dec-14 1:46 
AnswerRe: Alternative for string trimming Pin
Bartlomiej Filipek18-Dec-14 1:52
Bartlomiej Filipek18-Dec-14 1:52 
QuestionString trimming Pin
Stian Andre Olsen17-Dec-14 22:01
Stian Andre Olsen17-Dec-14 22:01 
AnswerRe: String trimming Pin
Bartlomiej Filipek18-Dec-14 1:54
Bartlomiej Filipek18-Dec-14 1:54 
GeneralRe: String trimming Pin
Stian Andre Olsen18-Dec-14 2:40
Stian Andre Olsen18-Dec-14 2:40 
GeneralRe: String trimming Pin
Michael Gazonda30-Dec-14 16:49
professionalMichael Gazonda30-Dec-14 16:49 
Suggestionstd Pin
pates17-Dec-14 8:47
pates17-Dec-14 8:47 

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