Articles & Books

First Meeting C++ Trip Report—Simon Brand

Simon Brand attended the recent Meeting C++ conference and wrote down his impressions:

Meeting C++ Trip Report

by Simon Brand

From the article:

This year was my first time at Meeting C++. It was also the first time I gave a full-length talk at a conference. But most of all it was a wonderful experience filled with smart, friendly people and excellent talks. This is my report on the experience. I hope it gives you an idea of some talks to watch when they’re up on YouTube, and maybe convince you to go along and submit talks next year! I’ll be filling out links to the talks as they go online.

 

Implementing the spaceship operator for optional—Barry Revzin

The future implementation?

Implementing the spaceship operator for optional

by Barry Revzin

From the article:

Last week, the C++ Standards Committee added operator<=>, known as the spaceship operator, to the working draft for what will eventually become C++20. This is an exciting new language feature for two reasons: it allows you to write one function to do all your comparisons where you used to have to write six, and it also allows you to write zero functions — just declare the operator as defaulted and the compiler will do all the work for you! Exciting times...

CppCon2017 Trip Report—Gordon Brown

Trip report from CppCon 2017. Highlights, trends and some tips for getting the most out of the conference.

CppCon2017 Trip Report

by Gordon Brown

From the article:

CppCon this year was bigger than ever with approaching 1200 attendees and 7 tracks; making it even harder to pick which talks to go to. Thankfully all the talks are made available online, so if you couldn’t make it to the conference or even if you did but couldn’t get to see all the talks you wanted to, you can go online and watch them. I only got to a handful of the talks as I often had conflicting appointments, but I’ve highlighted a few from what I saw that I would recommend checking out and some which I didn’t make it to but heard good things about.

Quick Q: What is the usefulness of `enable_shared_from_this`?

Quick A: It allows you to get a valid shared pointer from your instance directly.

Recently on SO:

What is the usefulness of `enable_shared_from_this`?

It enables you to get a valid shared_ptr instance to this, when all you have is this. Without it, you would have no way of getting a shared_ptr to this, unless you already had one as a member. This example from the boost documentation for enable_shared_from_this:

class Y: public enable_shared_from_this<Y>
{
public:

    shared_ptr<Y> f()
    {
        return shared_from_this();
    }
}

int main()
{
    shared_ptr<Y> p(new Y);
    shared_ptr<Y> q = p->f();
    assert(p == q);
    assert(!(p < q || q < p)); // p and q must share ownership
}

The method f() returns a valid shared_ptr, even though it had no member instance. Note that you cannot simply do this:

class Y: public enable_shared_from_this<Y>
{
public:

    shared_ptr<Y> f()
    {
        return shared_ptr<Y>(this);
    }
}

The shared pointer that this returned will have a different reference count from the "proper" one, and one of them will end up losing and holding a dangling reference when the object is deleted.

enable_shared_from_this is going to be a part of the new C++0x standard as well, so you can also get it from there as well as from boost.

Abstraction design and implementation: `repeat`—Vittorio Romeo

This series of two articles covers the train of thought behind the design and implementation of a simple `repeat` abstraction that, given a number `n` and a function object `f`, invokes `f` `n` times. The abstraction can be used to repeat actions both at run-time and compile-time. The articles also cover importance of propagating `noexcept`-correctness (and the pain caused by it!).

abstraction design and implementation: `repeat`

compile-time `repeat` & `noexcept`-correctness

by Vittorio Romeo

From the articles:

In my previous "passing functions to functions" and "zero-overhead C++17 currying & partial application" articles I've praised C++11 (and newer standards) for allowing us to write "more functional" code. [...] If I want to repeat an action n times, I exactly want to write that in my code:

    repeat(10, []
    {
        foo();
    });

Cannot get simpler than that - let's implement it!

2017 Albuquerque ISO C++ Committee Reddit Trip Report

Another report:

2017 Albuquerque ISO C++ Committee Reddit Trip Report

From the article:

The ISO C++ Committee met in Albuquerque, New Mexico, USA last week to continue work on C++ Technical Specifications and the next International Standard, C++20. C++17 is done; the final version was sent to ISO for publication in September. We started to firm up the schedule and feature set for C++20 at this meeting; we're hoping to land most of the major features by the first meeting of 2019...

Trip report: Fall ISO C++ standards meeting (Albuquerque)—Herb Sutter

C++ continues to evolve.

Trip report: Fall ISO C++ standards meeting (Albuquerque)

by Herb Sutter

From the article:

A few minutes ago, the ISO C++ committee completed its fall meeting in Albuquerque, New Mexico, USA, hosted with our thanks by Sandia National Laboratories. We had some 140 people at the meeting, representing 10 national bodies. As usual, we met for six days Monday through Saturday, including several evenings.

Quick Q: What does the explicit keyword mean?

Quick A: It tell the compiler not to do any implicit conversions of types.

Recnetly on SO:

What does the explicit keyword mean?

The compiler is allowed to make one implicit conversion to resolve the parameters to a function. What this means is that the compiler can use constructors callable with a single parameter to convert from one type to another in order to get the right type for a parameter.

Here's an example class with a constructor that can be used for implicit conversions:

class Foo
{
public:
  // single parameter constructor, can be used as an implicit conversion
  Foo (int foo) : m_foo (foo)
  {
  }

  int GetFoo () { return m_foo; }

private:
  int m_foo;
};

Here's a simple function that takes a Foo object:

void DoBar (Foo foo)
{
  int i = foo.GetFoo ();
}

and here's where the DoBar function is called.

int main ()
{
  DoBar (42);
}

The argument is not a Foo object, but an int. However, there exists a constructor for Foo that takes an int so this constructor can be used to convert the parameter to the correct type.

The compiler is allowed to do this once for each parameter.

Prefixing the explicit keyword to the constructor prevents the compiler from using that constructor for implicit conversions. Adding it to the above class will create a compiler error at the function call DoBar (42). It is now necessary to call for conversion explicitly with  DoBar (Foo (42))

The reason you might want to do this is to avoid accidental construction that can hide bugs. Contrived example:

  • You have a MyString(int size) class with a constructor that constructs a string of the given size. You have a function print(const MyString&), and you call print(3) (when you actually intended to call print("3")). You expect it to print "3", but it prints an empty string of length 3 instead.

Enforcing code contracts with [[nodiscard]]

More details about a new C++17 attribute

Enforcing code contracts with [[nodiscard]]

by Bartlomiej Filipek

From the article:

[[nodiscard]] is an excellent addition to all the important code: public APIs, safety-critical systems, etc. Adding this attribute will at least enforce the code contract. The compiler will help you detect bugs - at compile time, rather than finding in in the runtime.