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Yeah, this is a hot take :-)

But Standard containers, Qt containers, etc. call the direct erasing operation erase, not remove:

// C++20
std::vector<int> v = ~~~;
erase(v, 42);

// Qt 6, C++17
QLits<int> list = ~~~;
erase(list, 42);

A container's type may include non-type template parameters, e.g.

QVarLengthArray<int, 32> container;
                  // ^^ NTTP

Accepting them with template <typename ...> typename Container will therefore fail:

    QVector<int> vec{1, 2, 3, 4};
    const auto toString = [](int i) { return QString::number(i); };
    auto result = KDAlgorithms::transformed<QVarLengthArray>(vec, toString); // ERROR

For some reason this was a deliberate choice. But why simply using template <typename> typename Container doesn't work here?

kdalgorithms::find_if if called with a temporary r-value reference, results in name returning a reference to the collection, which has already been deleted.

find_if only has an overload for l-value references, which returns an iterator result.
With l-values, this works fine, but if used with an r-value reference, as in the example below, the collection will be deleted as soon as the find_if has finished executing.
This means any dereference of the returned name may segfault (in my case it didn't until a few days after implementing this 🙈 ).

Ideally find_if should have an overload for r-value references, that returns the result by-value, instead of by-reference.
That way the code below would work without issue and avoid any copies, as the result could simply be moved out of the container and still wouldn't need to be copied.

Example:

auto name = kdalgorithms::find_if(match.getAll("message-name"), [&range](const auto &name) {
        return range.contains(name);
    });

Because they don't work universally. Yes, it's SUPER annoying.

• C arrays don't have .begin()
• initializer_list, span, etc. don't have .cbegin()

Replace them by std::begin / std::cbegin, or even better, by centralized KDAlgorithm::detail::begin etc. functions that call

• the std::ranges::begin() etc. functions, if ranges are available
• the std::begin() etc. functions otherwise

I'm using kdalgorithms with Qt 5.15.12.

The following code generates warnings in Visual Studio about the type of size in reverse_helper.h:

	QVector<int> a{ 1,2,3 };
	auto b = kdalgorithms::filtered(a,
		[](const auto& i) { return i % 2; });

1>...\kdalgorithms\bits\reserve_helper.h(27,5): warning C4267: 'argument': conversion from 'size_t' to 'int', possible loss of data
1>...kdalgorithms\bits\reserve_helper.h(36,5): message : see reference to function template instantiation 'bool kdalgorithms::detail::reserve_helper<Container>(Container &,size_t,std::true_type)' being compiled
1>        with
1>        [
1>            Container=QVector<int>
1>        ]

See #15 (comment)

Can I concatenate a vector to itself using copy?

std::vector<int> v{1, 2, 3};
KDAlgorithms::copy(v, v); // duplicate it

The current answer is "maybe", which is a bad answer. If you want this to be a precondition, it should be documented (and assert()ed, e.g. assert(&input != &output)).

This one should be fairly incontroversial: don't discard information that the algorithms may return to you.

In my own class called Node, I wrote

	        using value_type     = std::unique_ptr<AbstractPrintItem>;
		using const_iterator = std::vector<std::unique_ptr<AbstractPrintItem>>::const_iterator;
		const_iterator begin() const
		{
			return m_printItems.begin();
		}
		const_iterator end() const
		{
			return m_printItems.end();
		}

This works fine for range-for, and STL algorithms.
But kdalgorithms::min_element failed to build with

kdalgorithms.h:214:19: error: ‘const class Node’ has no member named ‘empty’
  214 |     if (container.empty())

Shouldn't this be a begin()==end() test instead?

The convenience operators should be constrained to only take predicates or similar. Otherwise, if they're in scope, then all sorts of nonsense starts compiling:

using namespace KDAlgorithms::Operators;
std::string to_be;

to_be || !to_be; // that is the question

(Granted, one can't really use the return value of these operators, as it would immediately then trigger compilation errors)

I'm using transformed to convert a QVector into a QJsonArray of QJsonObjects as per the code below. It compiles OK with g++ 10 and clang++ 11, but not in Visual Studio 2022.

#include <kdalgorithms.h>

#include <QVector>
#include <QJsonArray>
#include <QJsonObject>

void f()
{
	struct S { int a, b; };
	QVector<S> s;

	auto j = kdalgorithms::transformed<QJsonArray>(s, [](const auto& v)
		{
			return QJsonObject{
				{ QStringLiteral("a"), v.a },
				{ QStringLiteral("b"), v.b }
			};
		});
}

The VS compiler reports

1>test.cpp(973,30): error C2039: 'a': is not a member of 'QJsonValue'
1>C:\qt\Qt5.15.12\5.15.12\msvc2019_64\include\QtCore\qjsonvalue.h(59,21): message : see declaration of 'QJsonValue'
1>C:\Program Files\Microsoft Visual Studio\2022\Professional\VC\Tools\MSVC\14.34.31933\include\type_traits(1470,1): message : see reference to function template instantiation 'auto f::<lambda_1>::operator ()<QJsonValue>(const _T1 &) const' being compiled
1>        with
1>        [
1>            _T1=QJsonValue
1>        ]
1>C:\Program Files\Microsoft Visual Studio\2022\Professional\VC\Tools\MSVC\14.34.31933\include\type_traits(1675,38): message : see reference to alias template instantiation 'std::_Decltype_invoke_nonzero<f::<lambda_1>,QJsonValue,>' being compiled
1>...\include\kdalgorithms\bits\transform.h(82,19): message : see reference to variable template 'const bool is_invocable_v<`f'::`2'::<lambda_1>,QJsonValue>' being compiled
1>test.cpp(974,30): error C2039: 'b': is not a member of 'QJsonValue'
1>C:\qt\Qt5.15.12\5.15.12\msvc2019_64\include\QtCore\qjsonvalue.h(59,21): message : see declaration of 'QJsonValue'
1>test.cpp(972,22): error C2440: '<function-style-cast>': cannot convert from 'initializer list' to 'QJsonObject'
1>test.cpp(972,22): message : No constructor could take the source type, or constructor overload resolution was ambiguous

However if I replace the const auto& v parameter in the lambda with const S& v then it does compile.

It seems that with auto a QJsonValue is being passed, being the output container's value_type perhaps?

kdalgorithms-src/src/bits/find_if.h:135:43: warning: moving a temporary object prevents copy elision [-Wpessimizing-move]
  135 |     return detail::create_result<Iterator>(std::move(it), std::move(range.begin()),
      |            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  136 |                                            std::move(range.end()),
      |                                            ~~~~~~~~~~~~~~~~~~~~~~~
  137 |                                            std::is_lvalue_reference<Container>());
      |                                            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
kdalgorithms-src/src/bits/find_if.h:135:43: note: remove ‘std::move’ call
kdalgorithms-src/src/bits/find_if.h:135:43: warning: moving a temporary object prevents copy elision [-Wpessimizing-move]
kdalgorithms-src/src/bits/find_if.h:135:43: note: remove ‘std::move’ call

The code is quite standard I think:

			const auto &objects = aMethodThatReturnsAConstRefToAQList();
			auto object = kdalgorithms::find_if(objects, [product](object) { return object->product() == product; });

We cannot pretend that ranges don't exist. While rightfully one does NOT want to reimplement the algorithms themselves¹, one should still

• reason in terms of iterator/sentinel pairs, instead of iterator/iterator
• detect if <ranges> is available
• if so, use the algorithm overloads in std::ranges (as they're the only ones that support iterator/sentinel)

Example:

QRegularExpression re = "\\d+";
auto matches = re.globalMatch(string);
auto matchesLengths = KDAlgorithms::transform(matches, [](const auto &m) { return m.capturedLength(); });

¹ even if simple (e.g. find_if). In other words, ranges should be supported up to the algorithmic call, after that one can require the user to be using a C++20 standard library.

Take a vector of QPoint, and use kdalgorithms::remove_duplicates with kdalgorithms::do_not_sort: I would expect this to do:
points.erase(std::unique(points.begin(), points.end()), points.end());

But it complains that the type has no operator<

If you have a std::list, you're supposed to call list.remove_if(predicate), and not std::remove_if(list, predicate).

That's because shuffling around the nodes of the list can be much much cheaper than shuffling around the contents of those nodes.

To cope with these differences, C++20 introduced free erase and erase_if functions, which I have also added to Qt containers.

remove/remove_if (or erase, cf #7 ) should therefore have some dispatching logic inside:

• if you can call (through ADL!) erase(container, ...) or erase_if(container, ...) then do that
• otherwise do the erase/remove idiom

max_element is used to retrieve to the position (as in, an iterator) of the maximum element. But the implementation here returns the value (wrapped in an optional), not the iterator pointing to it. Should these _element functions be renamed to something like max_value?