std::unique_copy(3)

🦋 std::unique_copy — C++ Standard Library manual, page (3) 🦋

okay bestie, welcome to the std::unique_copy(3) page from the C++ Standard Library manual 💖 This one is giving every overload, every caveat, and every little “behavior is undefined” jumpscare. No cap.

✨ NAME ✨

std::unique_copy — literally std::unique_copy. That’s the name, bestie; it’s the function is the moment 💅

📜 Synopsis 📜

Defined in header <algorithm>:

template< class InputIt, class OutputIt >
OutputIt unique_copy( InputIt first, InputIt last, OutputIt d_first ); // (1) (constexpr since C++20)

template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2 >
ForwardIt2 unique_copy( ExecutionPolicy&& policy, ForwardIt1 first, ForwardIt1 last, ForwardIt2 d_first ); // (2) (since C++17)

template< class InputIt, class OutputIt, class BinaryPred >
OutputIt unique_copy( InputIt first, InputIt last, OutputIt d_first, BinaryPred p ); // (3) (constexpr since C++20)

template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2, class BinaryPred >
ForwardIt2 unique_copy( ExecutionPolicy&& policy, ForwardIt1 first, ForwardIt1 last, ForwardIt2 d_first, BinaryPred p ); // (4) (since C++17)

So what does it do? It copies elements from the range [first, last) to another range beginning at d_first, in such a way that there are no consecutive equal elements. Only the first element of each group of equal elements is copied. Periodt. ✨

• (1) Elements are compared using operator==. If operator== does not establish an equivalence relation, the behavior is undefined. 😭
• (3) Elements are compared using the given binary predicate p. If p does not establish an equivalence relation, the behavior is undefined. 💀
• (2,4) Same as (1,3), but executed according to policy. These overloads participate in overload resolution only if:
  • std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true. (until C++20)
  • std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true. (since C++20)

If *d_first = *first is invalid (until C++20), or *first is not writable to d_first (since C++20), the program is ill-formed. Not cute.

If source and destination ranges overlap, the behavior is undefined. Do not do the overlap thing, bestie. 🫶

Given T as the value type of InputIt, if overload (1) or (3) does not satisfy all of the following conditions, the behavior is undefined:

• InputIt meets the requirements of LegacyForwardIterator. (until C++20)
• InputIt models forward_iterator. (since C++20)
• T is both CopyConstructible and CopyAssignable.
• All following conditions are satisfied:
  • OutputIt meets the requirements of LegacyForwardIterator.
  • The value type of OutputIt is also T.
  • T is CopyAssignable.

🧸 Parameters 🧸

• first, last — the range of elements to process.
• d_first — the beginning of the destination range.
• policy — the execution policy to use. See execution policy for details.
• p — binary predicate which returns true if the elements should be treated as equal.

The signature of the predicate function should be equivalent to the following:

bool pred(const Type1 &a, const Type2 &b);

While the signature does not need to have const &, the function must not modify the objects passed to it and must be able to accept all values of type (possibly const) Type1 and Type2 regardless of value category (thus, Type1 & is not allowed, nor is Type1 unless for Type1 a move is equivalent to a copy (since C++11)). The types Type1 and Type2 must be such that an object of type InputIt can be dereferenced and then implicitly converted to both of them. 💅

🧚 Type requirements 🧚

• InputIt must meet the requirements of LegacyInputIterator.
• OutputIt must meet the requirements of LegacyOutputIterator.
• ForwardIt1, ForwardIt2 must meet the requirements of LegacyForwardIterator.

🎁 Return value 🎁

Output iterator to the element past the last written element. 🫶

⚡ Complexity ⚡

Given \(\scriptsize N\)N as std::distance(first, last):

• 1,2) Exactly \(\scriptsize max(0,N-1)\)max(0,N-1) comparisons using operator==.
• 3,4) Exactly \(\scriptsize max(0,N-1)\)max(0,N-1) applications of the predicate p.

For overloads (2,4), there may be a performance cost if the value type of ForwardIt1 is not both CopyConstructible and CopyAssignable. 🐌

😭 Exceptions 😭

The overloads with a template parameter named ExecutionPolicy report errors as follows:

• If execution of a function invoked as part of the algorithm throws an exception and ExecutionPolicy is one of the standard policies, std::terminate is called. For any other ExecutionPolicy, the behavior is implementation-defined.
• If the algorithm fails to allocate memory, std::bad_alloc is thrown.

🛠️ Possible implementation 🛠️

See also the implementations in libstdc++ and libc++. 👀

📝 Notes 📝

• If InputIt satisfies LegacyForwardIterator, this function rereads the input in order to detect duplicates.
• Otherwise, if OutputIt satisfies LegacyForwardIterator, and the value type of InputIt is the same as that of OutputIt, this function compares *d_first to *first.
• Otherwise, this function compares *first to a local element copy.

💻 Example 💻

// Run this code

#include <algorithm>
#include <iostream>
#include <iterator>
#include <string>

int main()
{
    std::string s1 {"A string with mmmany letters!"};
    std::cout << "Before: " << s1 << '\n';

    std::string s2;
    std::unique_copy(s1.begin(), s1.end(), std::back_inserter(s2),
                     [](char c1, char c2) { return c1 == 'm' && 'm' == c2; });

    std::cout << "After:  " << s2 << '\n';
}

🖨️ Output 🖨️

Before: A string with mmmany letters!
After:  A string with many letters!

🩹 Defect reports 🩹

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

• LWG 239, applied to C++98 — behavior as published: the predicate was applied std::distance(first, last) times; correct behavior: applied one time fewer (for non-empty ranges).
• LWG 241, applied to C++98 — behavior as published: the value type of InputIt was not required to be CopyConstructible; correct behavior: conditionally required.
• LWG 538, applied to C++98 — behavior as published: the value type of InputIt was not required to be CopyAssignable; correct behavior: conditionally required.
• LWG 2439, applied to C++98 — behavior as published: the value type of InputIt was not required to be CopyConstructible if OutputIt is a LegacyForwardIterator; correct behavior: conditionally required.

👀 See also 👀

• adjacent_find — finds the first two adjacent items that are equal (or satisfy a given predicate) (function template).
• unique — removes consecutive duplicate elements in a range (function template).
• copy — copies a range of elements to a new location (function template).
• copy_if (C++11) — (function template).
• ranges::unique_copy (C++20) — creates a copy of some range of elements that contains no consecutive duplicates (niebloid).

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That’s the whole page, bestie! 🌈 Source: http://cppreference.com, version/date: 2024.06.10, manual page std::unique_copy(3). Mwah 💋