Beautiful code #5 - std::for_each, std::ranges::for_each, std::optional, std::execution::par_unseq, mutable 👋

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#include <algorithm>
#include <execution>
#include <vector>
#include <optional>
#include <iostream>

struct Custom {};
void process(Custom&) {}

int main() {
    std::vector<int> data{1, 2, 3, 4, 5};
    std::for_each(data.begin(), data.end(), [i = 5](int& v) mutable {
        // iterate over all elements in order
        v += i--;
    });
    // data == {6, 6, 6, 6, 6}
    for (auto v : data)
        std::cout << v << " ";
    std::cout << "\n";

    std::vector<Custom> rng(10,Custom{});
    std::for_each(std::execution::par_unseq, // parallel, in any order
        rng.begin(), rng.end(), // all elements
        process // invoke process on each element
    );

    std::vector<std::optional<int>> opt{{},2,{},4,{}};
    for (const auto& val : opt) {
        if (val.has_value()) {
            std::cout << val.value() << " ";
        } else {
            std::cout << "EMPTY ";
        }
    }
    std::cout << "\n" << std::endl;
    
    std::ranges::for_each(opt,
        [](int v) {
            // iterate over projected values
            // {0, 2, 0, 4, 0}
            std::cout << "value loop\n" << std::endl;
            std::cout << v << " " << "\n";
        },
        [](std::optional<int>& v){
            // projection that will return 
            // the contained value or zero
            std::cout << "projection loop\n" << std::endl;
            return v.value_or(555555);
        });
    std::cout << "\n";
}

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std::for_each(data.begin(), data.end(), [i = 5](int& v) mutable {
        // iterate over all elements in order
        v += i--;
    });

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std::for_each(std::execution::par_unseq, // parallel, in any order
        rng.begin(), rng.end(), // all elements
        process // invoke process on each element
    );