C++20 ranges let you pass a whole container to an algorithm (std::ranges::sort(v)) and build lazy pipelines with views: v | views::filter(...) | views::transform(...).

Range algorithms

C++

#include <algorithm>
#include <iostream>
#include <string>
#include <vector>

struct Person { std::string name; int age; };

int main() {
    std::vector<int> v{5, 3, 9, 1};
    std::ranges::sort(v);                                   // no begin()/end() noise
    std::cout << v.front() << ' ' << std::ranges::max(v) << ' ' << std::ranges::count(v, 3) << '\n';

    std::vector<Person> people{{"cy", 30}, {"al", 25}, {"bo", 35}};
    std::ranges::sort(people, {}, &Person::age);            // projection: sort by a member
    for (const auto& p : people) std::cout << p.name << ' ';
    std::cout << '\n';
    auto it = std::ranges::find(people, "bo", &Person::name);
    std::cout << it->age << '\n';
}

Output

1 9 1
al cy bo
35

Views: lazy pipelines

C++

#include <iostream>
#include <ranges>
#include <vector>

int main() {
    namespace views = std::views;
    std::vector<int> v{1, 2, 3, 4, 5, 6, 7, 8, 9, 10};

    auto evenSquares = v | views::filter([](int x) { return x % 2 == 0; })
                         | views::transform([](int x) { return x * x; });
    for (int x : evenSquares) std::cout << x << ' ';           // computed on the fly, no temp vector
    std::cout << '\n';

    for (int x : views::iota(1) | views::take(5)) std::cout << x;      // infinite range, first 5
    std::cout << '\n';
    for (int x : v | views::reverse | views::drop(7)) std::cout << x;
    std::cout << '\n';
    for (int x : v | views::take_while([](int x) { return x < 4; })) std::cout << x;
    std::cout << '\n';
}

Output

4 16 36 64 100
12345
321
123

View	Produces
`views::filter(pred)`	Elements satisfying `pred`
`views::transform(f)`	`f(x)` for each element
`views::take(n)` / `views::drop(n)`	First n / all but first n
`views::take_while(p)` / `views::drop_while(p)`	Prefix while `p` holds / the rest
`views::reverse`	Reverse order
`views::iota(a, b)`	`a, a+1, …, b-1` (unbounded without `b`)
`views::keys` / `views::values` / `views::elements<N>`	Parts of pairs/tuples
`views::split(delim)`, `views::join`	Split / flatten
`views::enumerate`, `views::zip`, `views::chunk`	C++23

Views over maps, strings & collecting results

C++

#include <iostream>
#include <map>
#include <ranges>
#include <string>
#include <string_view>
#include <vector>

int main() {
    std::map<std::string, int> stock{{"apple", 0}, {"kiwi", 4}, {"pear", 2}};
    for (const auto& name : stock | std::views::filter([](const auto& kv) { return kv.second > 0; })
                                  | std::views::keys)
        std::cout << name << ' ';
    std::cout << '\n';

    std::string_view csv = "red,green,blue";
    for (auto part : csv | std::views::split(','))
        std::cout << '[' << std::string_view(part.begin(), part.end()) << ']';
    std::cout << '\n';

    auto sq = std::views::iota(1, 6) | std::views::transform([](int x) { return x * x; });
    std::vector<int> out(sq.begin(), sq.end());          // materialise (C++23: std::ranges::to<std::vector>())
    std::cout << out.size() << ' ' << out.back() << '\n';
}

Output

kiwi pear
[red][green][blue]
5 25

What is the difference between a range algorithm and a view?