std::common...mon_iterator(3)

🌸 NAME 🌸

This is the page for std::common_iterator::common_iterator — the manual header shows it as std::common...mon_iterator(3) — and it comes from the C++ Standard Libary manual. Yes, “Libary” with one r: iconic typo, no notes 💅.

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✨ Synopsis ✨

constexpr common_iterator() requires std::default_initializable<I>
= default;

(1) (since C++20)

constexpr common_iterator( I i );

(2) (since C++20)

constexpr common_iterator( S s );

(3) (since C++20)

template< class I2, class S2 >

requires std::convertible_to<const I2&, I> &&
         std::convertible_to<const S2&, S>

constexpr common_iterator( const common_iterator<I2, S2>& x );

(4) (since C++20)

Okay so bestie, this constructor is literally constructing a new iterator adaptor, and it effectively initializes the underlying std::variant<I, S> member object var so it holds an I (iterator) or an S (sentinel) object. Slay.

1. Default constructor. Default-initializes var. After construction, var holds a value-initialized I object. Operations on the resulting iterator adaptor have defined behavior if and only if the corresponding operations on a value-initialized I also have defined behavior.
2. After construction, var holds an I object move-constructed from i.
3. After construction, var holds an S object move-constructed from s.
4. After construction, var holds an I or S object initialized from the I2 or S2 held by x.var, if x.var holds that alternative, respectively. The behavior is undefined if x is in an invalid state, that is, x.var.valueless_by_exception() is equal to true.

🚨 No cap: that last one is undefined behavior when x.var.valueless_by_exception() == true, so don’t play with invalid states, bestie.

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🎀 Parameters 🎀

• i — iterator to adapt
• s — sentinel to adapt
• x — iterator adaptor to copy

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💖 Example 💖

       // Run this code

        #include <algorithm>
        #include <iostream>
        #include <iterator>
        #include <numeric>
        #include <vector>

        int main()
        {
            std::vector v{3, 1, 4, 1, 5, 9, 2};

            using CI = std::common_iterator<
                           std::counted_iterator<std::vector<int>::iterator>,
                           std::default_sentinel_t>;
            CI unused; // (1)
            CI start{std::counted_iterator{std::next(begin(v)), ssize(v) - 2}};
       // (2)
            CI finish{std::default_sentinel}; // (3)
            CI first{start}; // (4)
            CI last{finish}; // (4)

            std::copy(first, last, std::ostream_iterator<int>{std::cout, " "});
            std::cout << '\n';

            std::common_iterator<
                std::counted_iterator<
                    std::ostream_iterator<double>>,
                    std::default_sentinel_t>
                    beg{std::counted_iterator{std::ostream_iterator<double>{std::cout,";
       "}, 5}},
                    end{std::default_sentinel};
            std::iota(beg, end, 3.1);
            std::cout << '\n';
        }

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🌈 Output: 🌈

1 4 1 5 9
3.1;  4.1;  5.1;  6.1;  7.1;

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👑 See also 👑

• operator= — assigns another iterator adaptor (C++20) (public member function)

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Source/version/date sign-off: this page is from http://cppreference.com, version 2024.06.10, and it’s the std::common...mon_iterator(3) page. Bye bestie, go construct something iconic 💅✨🫶