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

By Adrien Hamelin | Nov 14, 2017 01:24 PM | Tags: intermediate c++11

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

By Vittorio Romeo | Nov 13, 2017 11:37 PM | Tags: intermediate c++14

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

By Adrien Hamelin | Nov 13, 2017 01:49 PM | Tags: community

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

By Adrien Hamelin | Nov 13, 2017 01:39 PM | Tags: community

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?

By Adrien Hamelin | Nov 9, 2017 02:46 PM | Tags: basics

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]]

By bfilipek | Nov 7, 2017 10:43 AM | Tags: None

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.

C++ User Group Meetings in November

By Meeting C++ | Nov 2, 2017 07:25 AM | Tags: usergroups meetingcpp community

The monthly overview on upcoming C++ User Group Meetings...

C++ User Group Meetings in November

by Jens Weller

From the article:

The monthly overview on upcoming C++ User Group Meetings! With next weeks Meeting C++ 2017 conference, many members of the new and established C++ User Groups will meet in Berlin! I hope to motivate again many visiting folks to start attending a near by C++ User Group, or to start their own User Group, if it does not yet exist.

There are 5 new C++ User Groups: Core C++, Israel, Brisbane, Moscow, Lissabon, Canterbury...

Common C++ Modules TS Misconceptions -- Boris Kolpackov

By Blog Staff | Nov 1, 2017 11:06 AM | Tags: None

boris-kolpackov.PNGAn illuminating "mini-FAQ" on a very current major feature progressing in ISO C++:

Common C++ Modules TS Misconceptions

by Boris Kolpackov

From the article:

It has become fashionable to criticize C++ Modules TS. My problem with this bandwagon criticism is that at best it's based on cursory reading of the specification but more commonly on just hearing others' knee-jerk reactions. Oftentimes the criticizing post cannot even get the terminology right. So in this article I would like to refute the most common Modules TS misconceptions...

  • I cannot have everything in a single file
  • I cannot export macros from modules
  • I cannot modularize existing code without touching it
  • No build system will be able to support modules

Better code understanding with Sourcetrail -- Bartlomiej Filipek

By bfilipek | Oct 31, 2017 11:54 PM | Tags: None

Let's have a look at Sourcetrail, a great tool for code (C++/Java) visualtiozation:

Better code understanding with Sourcetrail

by Bartlomiej Filipek

From the article:

I’m exploring the tool, and overall I am impressed! It works very well; the setup is easy to do, there’s a lot of help, beautiful and dynamic diagrams (even with smooth animations), under active development… what else would I want? smile