Quick Q: Array Initialisation Compile Time - Constexpr Sequence

By Adrien Hamelin | Aug 30, 2017 12:45 PM | Tags: intermediate c++11

Quick A: Use integer_sequence with a helper function.

Recently on SO:

Array Initialisation Compile Time - Constexpr Sequence

1) How to implement that kind of integer_sequence?

template <std::size_t... Is>
constexpr auto make_sequence_impl(std::index_sequence<Is...>)
{
    return std::index_sequence<generate_ith_number(Is)...>{};
}

template <std::size_t N>
constexpr auto make_sequence()
{
    return make_sequence_impl(std::make_index_sequence<N>{});
}

2) Is it possible to build an std::array from that integer_sequence at compile time?

template <std::size_t... Is>
constexpr auto make_array_from_sequence_impl(std::index_sequence<Is...>)
{
    return std::array<std::size_t, sizeof...(Is)>{Is...};
}

template <typename Seq>
constexpr auto make_array_from_sequence(Seq)
{
    return make_array_from_sequence_impl(Seq{});
}

Usage:

int main()
{
    constexpr auto arr = make_array_from_sequence(make_sequence<6>());
    static_assert(arr[0] == 0);
    static_assert(arr[1] == 1);
    static_assert(arr[2] == 2);
    static_assert(arr[3] == 4);
    static_assert(arr[4] == 5);
    static_assert(arr[5] == 7);
}

Quick Q: vector of vector does not convert brace encloser list

By Adrien Hamelin | Aug 30, 2017 12:34 PM | Tags: None

Quick A: The constructor needs an extra {} pair.

Recently on SO:

vector does not convert brace encloser list

You have two options:

1. add a constructor taking std::initializer_list<std::initializer_list<T>>
2. eclose the init expression with another set of {} i.e.

Matrix<double> a{{

    { 17,    24,    1},

    { 23,    5,     7 },

    {  4,     6,    13 }

}};

Ok, I'll try a little explanation of what is going on here:

If there is no constructor taking a std::initializer_list then the outermost {} are always opening and closing the constructor call if you will, and not part of what you actually pass to the constructor.

Matrix<double> a{ {1, 2}, {3, 4} };
                ^ ^~~~~~~~~~~~~~ ^
                |  2 parameters  |
                |                |
                |                |
            opening            closing

As you can see this is taken as a constructor with 2 parameters, in this case 2 initializer_lists.

This is why you need another set of {}:

Matrix<double> a{ {{1, 2}, {3, 4}} };
                ^ ^~~~~~~~~~~~~~~~ ^
                |  1 parameter     |
                |                  |
                |                  |
            opening            closing

In order for the outermost {} to be considered an initializer_list then the constructor needs to have an overload taking a initializer_list. That is what is happening in the std::vector case.

C++ Annotated: Apr – Aug 2017--Anastasia Kazakova

By Adrien Hamelin | Aug 29, 2017 12:57 PM | Tags: experimental community

Did you miss something?

C++ Annotated: Apr – Aug 2017

by Anastasia Kazakova

From the article:

Today we are happy to share our next compilation of C++ news with you.

C++17: Inline Variables--Marc Gregoire

By Adrien Hamelin | Aug 29, 2017 12:43 PM | Tags: intermediate c++17

A handy new feature:

C++17: Inline Variables

by Marc Gregoire

From the article:

Before C++17, if your class had any non-const static data members, you had to allocate memory for them.

Top 20 C++ multithreading mistakes and how to avoid them--Deb Haldar

By Adrien Hamelin | Aug 28, 2017 01:09 PM | Tags: efficiency c++11

You use threads? You should know this.

Top 20 C++ multithreading mistakes and how to avoid them

by Deb Haldar

From the article:

Threading is one of the most complicated things to get right in programming, especially in C++. I've made a number of mistakes myself over the years. Most of these mistakes were  luckily caught in code review and testing ; however, some arcane ones did slip through and make it into production code and we had to patch live systems, which is always expensive.

In this article, I've tried to catalog all the mistakes I know of, with potential solutions. If you know any more pitfalls, or have alternative suggestions for some of the mistakes – please leave a comment below and I'll factor them into the article.

C++17 in detail: Parallel Algorithms -- Bartlomiej Filipek

By bfilipek | Aug 23, 2017 10:23 AM | Tags: performance c++17

Let’s see how C++17 can make writing parallel code a bit easier.

C++17 in details: Parallel Algorithms

by Bartlomiej Filipek

From the article:

With C++17 we get a lot of algorithms that can be executed in a parallel/vectorized way. That’s amazing, as it’s a solid abstraction layer. With this making, apps is much easier. A similar thing could be achieved possibly with C++11/14 or third-party APIs, but now it’s all in the standard.

C++17 in details: Parallel Algorithms--Bartlomiej Filipek

By Adrien Hamelin | Aug 23, 2017 09:19 AM | Tags: intermediate c++17

THe series continue.

C++17 in details: Parallel Algorithms

by Bartlomiej Filipek

From the article:

Writing multithreaded code is hard. You’d like to utilize all of the machine’s processing power, keeping code simple and avoid data races at the same time.

Let’s see how C++17 can make writing parallel code a bit easier.

Overload 140 is now available

By Adrien Hamelin | Aug 21, 2017 12:10 PM | Tags: community

ACCU’s Overload journal of June 2017 is out. It contains the following C++ related articles.

Overload 140 is now available

From the journal:

Editorial: Gnomes and Misnomers.
What's in a name? Frances Buontempo decides some names are better than others.

The Path of the Programmer.
Charles Tolman provides a framework for personal development.

A Usable C++ Dialect that is Safe Against Memory Corruption.
Sergey Ignatchenko continues his investigation of allocators for (Re)Actors.

Metaclasses: Thoughts on Generative C++.
Herb Sutter shows how metaclasses could simplify C++ with minimal library extension.

A C++ Developer Sees Rustlang for the First Time.
Katarzyna Macias provides an introduction to Rust for a C++ developer.

Portable Console I/O via iostreams.
Alf Steinbach describes how his library fixes problems streaming non-ASCII characters in Windows.

A Functional Alternative to Dependency Injection in C++.
Satprem Pamudurthy showcases a functional alternative to dependency injection in C++.

About the C++ Core Guidelines.
Andreas Fertig shows us the C++ core guidelines.

Afterwood.
Chris Oldwood reminds us to fix the problem, not to blame.

Quick Q: Make C++ fail compilation on specific instantiation of template function

By Adrien Hamelin | Aug 18, 2017 12:11 PM | Tags: intermediate c++11

Quick Q: Delete its constructor

Recently on SO:

Make C++ fail compilation on specific instantiation of template function

Since foo is a complete specialization, it will always get compiled, and the static assert will always get called.

However, there’s an easier way:

template <>
Bar foo<Bar>(Bar val) = delete;

This will say that this specific version is deleted, and cannot be called.

Ranges, Coroutines, and React: Early Musings on the Future of Async in C++ -- Eric Niebler

By Eric Niebler | Aug 17, 2017 03:56 PM | Tags: intermediate experimental c++20

Eric Niebler shares his thoughts about the interaction of ranges and co-routines in his recent blog post.

Ranges, Coroutines, and React: Early Musings on the Future of Async in C++

by Eric Niebler

From the article:

Another way to look at this is that synchronous ranges are an example of a pull-based interface: the user extracts elements from the range and processes them one at a time. Asynchronous ranges, on the other hand, represent more of a push-based model: things happen when data shows up, whenever that may be. This is akin to the reactive style of programming.

By using ranges and coroutines together, we unify push and pull based idioms into a consistent, functional style of programming. And that’s going to be important, I think.