std::forwar...forward_list(3)

💅 std::forward_list::forward_list 💅

Okay bestie, this is the std::forward_list::forward_list page from the C++ Standard Libary (yes, spelled exactly like that on the page 👀). It’s the constructor page for std::forward_list, and it has so many overloads. Iconic behavior, no cap. ✨

---

🌸 NAME 🌸

std::forward_list::forward_list — std::forward_list::forward_list.

That’s the name. That’s the whole vibe. 💖

---

🦋 Synopsis 🦋

forward_list();                                      (1) (since C++11)
explicit forward_list( const Allocator& alloc );     (2) (since C++11)
forward_list( size_type count,
              const T& value,                         (3) (since C++11)
              const Allocator& alloc = Allocator() );
explicit forward_list( size_type count );             (4) (since C++11)
                                                          (until C++14)
explicit forward_list( size_type count,
                       const Allocator& alloc = Allocator() ); (since C++14)
template< class InputIt >
forward_list( InputIt first, InputIt last,            (5) (since C++11)
              const Allocator& alloc = Allocator() );
forward_list( const forward_list& other );            (6) (since C++11)
forward_list( const forward_list& other, const        (7) (since C++11)
              Allocator& alloc );
forward_list( forward_list&& other );                 (8) (since C++11)
forward_list( forward_list&& other, const Allocator&  (9) (since C++11)
              alloc );
forward_list( std::initializer_list<T> init,
              const Allocator& alloc = Allocator()     (10) (since C++11)
            );
template< container-compatible-range<T> R >
forward_list( std::from_range_t, R&& rg,              (11) (since C++23)
              const Allocator& alloc = Allocator() );

So basically: it constructs a new container from a variety of data sources, and you can optionally use a user-supplied allocator alloc. 💫

1. Default constructor. Constructs an empty container with a default-constructed allocator. (1)
2. Constructs an empty container with the given allocator alloc. (2)
3. Constructs the container with count copies of elements with value value. (3)
4. Constructs the container with count default-inserted instances of T. No copies are made. (4)
5. Constructs the container with the contents of the range [first, last). This overload participates in overload resolution only if InputIt satisfies LegacyInputIterator, to avoid ambiguity with the overload (3). (5)
6. Copy constructor. Constructs the container with the copy of the contents of other. The allocator is obtained as if by calling std::allocator_traits<allocator_type>::select_on_container_copy_construction(other.get_allocator()). (6)
7. Constructs the container with the copy of the contents of other, using alloc as the allocator. During class template argument deduction, only the first argument contributes to the deduction of the container’s Allocator template parameter. (since C++23) (7)
8. Move constructor. Constructs the container with the contents of other using move semantics. Allocator is obtained by move-construction from the allocator belonging to other. (8)
9. Allocator-extended move constructor. Using alloc as the allocator for the new container, moving the contents from other; if alloc != other.get_allocator(), this results in an element-wise move. During class template argument deduction, only the first argument contributes to the deduction of the container’s Allocator template parameter. (since C++23) (9)
10. Constructs the container with the contents of the initializer list init. (10)
11. Constructs the container with the contents of the range rg. (11)

---

🎀 Parameters 🎀

• alloc — allocator to use for all memory allocations of this container
• count — the size of the container
• value — the value to initialize elements of the container with
• first, last — the range [first, last) to copy the elements from
• other — another container to be used as source to initialize the elements of the container with
• init — initializer list to initialize the elements of the container with
• rg — a container compatible range, that is, an input_range whose elements are convertible to T

---

⭐ Complexity ⭐

• 1, 2 — Constant.
• 3, 4 — Linear in count.
• 5 — Linear in distance between first and last.
• 6, 7 — Linear in size of other.
• 8 — Constant.
• 9 — Linear if alloc != other.get_allocator(), otherwise constant.
• 10 — Linear in size of init.
• 11 — Linear in ranges::distance(rg).

---

🦋 Exceptions 🦋

Calls to Allocator::allocate may throw. Yep, allocation can throw, bestie 😭.

---

🌈 Notes 🌈

After container move construction (overload (8)), references, pointers, and iterators (other than the end iterator) to other remain valid, but refer to elements that are now in *this. The current standard makes this guarantee via the blanket statement in [container.reqmts]/67, and a more direct guarantee is under consideration via LWG issue 2321. 👑

Feature-test macro tea:

• __cpp_lib_containers_ranges — value 202202L; std (C++23); feature: Ranges-aware construction and insertion; overload (11)

---

🧚 Example 🧚

// Run this code

#include <iostream>
#include <string>
#include <forward_list>

template<typename T>
std::ostream& operator<<(std::ostream& s, const std::forward_list<T>& v)
{
    s.put('{');
    for (char comma[]{'\0', ' ', '\0'}; const auto& e : v)
        s << comma << e, comma[0] = ',';
    return s << "}\n";
}

int main()
{
    // C++11 initializer list syntax:
    std::forward_list<std::string> words1{"the", "frogurt", "is", "also", "cursed"};
    std::cout << "1: " << words1;

    // words2 == words1
    std::forward_list<std::string> words2(words1.begin(), words1.end());
    std::cout << "2: " << words2;

    // words3 == words1
    std::forward_list<std::string> words3(words1);
    std::cout << "3: " << words3;

    // words4 is {"Mo", "Mo", "Mo", "Mo", "Mo"}
    std::forward_list<std::string> words4(5, "Mo");
    std::cout << "4: " << words4;

    auto const rg = {"cat", "cow", "crow"};
#ifdef __cpp_lib_containers_ranges
    std::forward_list<std::string> words5(std::from_range, rg); // overload (11)
#else
    std::forward_list<std::string> words5(rg.begin(), rg.end()); // overload (5)
#endif
    std::cout << "5: " << words5;
}

Output:

1: {the, frogurt, is, also, cursed}
2: {the, frogurt, is, also, cursed}
3: {the, frogurt, is, also, cursed}
4: {Mo, Mo, Mo, Mo, Mo}
5: {cat, cow, crow}

---

💀 Defect reports 💀

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

• LWG 2193 — applied to C++11; behavior as published: the default constructor is explicit; correct behavior: made non-explicit.

---

👑 See also 👑

• assign — assigns values to the container (public member function)
• operator= — assigns values to the container (public member function)

---

Source/version check, bestie: http://cppreference.com, 2024.06.10, std::forwar...forward_list(3) — that’s the footer, and that’s the post. Bye! 💖✨🎀