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Quick Q: Regarding shared_ptr reference count block

By Adrien Hamelin | Nov 28, 2018 12:03 PM | Tags: intermediate community

Quick A: it is an implementation detail for the std.

Recently on SO:

Regarding shared_ptr reference count block

(1) Regarding size: How can I programatically find the exact size of the control block for a std::shared_ptr?

There is no way. It's not directly accessible.

(2) Regarding logic: Additionally, boost::shared_ptr mentions that they are completely lock-free with respect to changes in the control block.(Starting with Boost release 1.33.0, shared_ptr uses a lock-free implementation on most common platforms.) I don't think std::shared_ptr follows the same - is this planned for any future C++ version? Doesn't this also mean that boost::shared_ptr is a better idea for multithreaded cases?

Absolutely not. Lock-free implementations are not always better than implementations that use locks. Having an additional constraint, at best, doesn't make the implementation worse but it cannot possibly make the implementation better.

Consider two equally competent programmers each doing their best to implement shared_ptr. One must produce a lock-free implementation. The other is completely free to use their best judgment. There is simply no way the one that must produce a lock-free implementation can produce a better implementation all other things being equal. At best, a lock-free implementation is best and they'll both produce one. At worse, on this platform a lock-free implementation has huge disadvantages and one implementer must use one. Yuck.

Meeting C++ and Meeting Embedded trip report--Conan C/C++ Package Manager

By Adrien Hamelin | Nov 28, 2018 11:55 AM | Tags: community

Were you there? 

Meeting C++ and Meeting Embedded trip report

by Conan C/C++ Package Manager

From the article:

On Wednesday, before Meeting C++, we attended and presented a talk at Meeting Embedded, a new conference about many topics related to embedded systems. C++ and C had a relevant role in this conference, obviously (accordingly to Dan Saks statistics around 60% in embedded code is C, then around 20% is C++, followed by assembly), but where other topics presented, like Rust, protocols for embedded (MQTT), academic and professional education, real-time systems.

We did our own talk Continuous Integration of C/C++ for embedded and IoT with Jenkins, Docker and Conan, which went quite well, especially considering that we were running a real demo, live updating the embedded code in a Raspberry PI, that was built with Docker in Jenkins, using cross-compiled (from Windows) packages, and uploaded to Artifactory, all of that done in the live demo...

Quick Q: What does it mean to return a reference?

By Adrien Hamelin | Nov 27, 2018 02:10 PM | Tags: basics

Quick A: The returned variable can be modified.

Recently on SO:

What does it mean to return a reference?

It means you return by reference, which is, at least in this case, probably not desired. It basically means the returned value is an alias to whatever you returned from the function. Unless it's a persistent object it's illegal.

For example:

int& foo () {
    static int x = 0;
    return x;
}

//...
int main()
{
    foo() = 2;
    cout << foo();
}

would be legal and print out 2, because foo() = 2 modifies the actual value returned by foo.

However:

int& doit () {
    int x = 0;
    return x;
}

would be illegal (well, accessing the returned value would), because x is destroyed when the method exits, so you'd be left with a dangling reference.

Returning by reference isn't common for free functions, but it is for methods returning members. For example, in the std, the operator [] for common containers return by reference. For example, accessing a vector's elements with [i] returns an actual reference to that element, so v[i] = x actually changes that element.

Also, I hope that "is essentially equal to this code" means that they're semantically sort of (but not really) similar. Nothing more.

Inline Namespaces 101--Jonathan Müller

By Adrien Hamelin | Nov 27, 2018 01:58 PM | Tags: c++11 advanced

Obscure feature? Not for long.

Inline Namespaces 101

by Jonathan Müller

From the article:

Almost three years ago — wow, how time flies — I blogged about namespace aliases and called them one of C++ most underrated features (which probably was a bit of a click bait).

Let’s talk about some other namespace feature, that is, well, not quite underrated, but relatively obscure: inline namespace. They are namespaces that don’t really introduce a scope, except when they do.

So what can you do with them?

How to teach C++--Marius Elvert

By Adrien Hamelin | Nov 27, 2018 01:53 PM | Tags: community

Do you have a way?

How to teach C++

by Marius Elvert

From the article:

In the closing Keynote of this year’s Meeting C++, Nicolai Josuttis remarked how hard it can be to teach C++ with its ever expanding complexity. His example was teaching rookies about initialization in C++, i.e. whether to use assignment =, parens () or curly braces {}. He also asked for more application-level programmers to participate.

Well, I am an application programmer, and I also have experience with teaching C++. Last year I held a C++ introductory course for experienced C programmers who mostly had never used C++ before. From my experience, I can completely agree with what Nico had to say about teaching C++. The language and its subtlety can be truly overwhelming.

Most of the complexity in C++ boils down to tuning your code for optimal performance. We cannot just leave that out, can we? After all, the sole reason to use C++ is performance, right?

How to Design Function Parameters That Make Interfaces Easier to Use (1/3)--Jonathan Boccara

By Adrien Hamelin | Nov 26, 2018 02:30 PM | Tags: intermediate community

Readability is important.

How to Design Function Parameters That Make Interfaces Easier to Use (1/3)

by Jonathan Boccara

From the article:

When you look at an function in an interface, 3 prominent things give you indications about how to use it: its name, its parameters and its return type. And when you look at a piece of code calling that function, it’s just its name and its function parameters.

We’ve already covered in details how to give good names to the components of your code. Now we’re going to examine how to design function parameters in a way that both your interfaces and the code that calls them are as expressive as can be.

Summed up in one sentence, you want to make the decision of what arguments to pass to your functions a no-brainer.

There are a lot of things to say about how to achieve this. So much so that you will find the contents broken down into 3 articles in order to make it easier to digest:

  • Part 1: interface-level parameters, one-parameter functions, const parameters,
  • Part 2: calling contexts, strong types, parameters order,
  • Part 3: packing parameters, processes, levels of abstraction.

To support this series I’ve taken many examples from interfaces I’ve worked on, except that I’ve stripped out all domain aspects to make them both simpler and disclosable...

A zero cost abstraction?--Josh Peterson

By Adrien Hamelin | Nov 26, 2018 02:12 PM | Tags: performance

Safe and performant?

A zero cost abstraction?

by Josh Peterson

From the article:

Recently Joachim (CTO at Unity) has been talking about “performance by default”, the mantra that software should be as fast as possible from the outset. This is driving the pretty cool stuff many at Unity are doing around things like ECS, the C# job system, and Burst (find lots more about that here).

One question Joachim has asked internally of Unity developers is (I’m paraphrasing here): “What is the absolute lower bound of time this code could use?” This strikes me as a really useful way to think about performance. The question changes from “How fast is this?” to “How fast could this be?”. If the answers to those two questions are not the same, the next question is “Do we really need the additional overhead?”

Another way to think about this is to consider the zero-cost abstraction, a concept much discussed in the C++ and Rust communities. Programmers are always building abstractions, and those abstractions often lead to the difference between “how fast it is” and “how fast it could be”. We want to provide useful abstractions that don’t hurt performance...

San Diego Committee Meeting: A Trip Report--Corentin Jabot

By Adrien Hamelin | Nov 19, 2018 01:08 PM | Tags: experimental community

Trip report.

San Diego Committee Meeting: A Trip Report

by Corentin Jabot

From the article:

As I left Rapperswil earlier this year, I said very firmly that I would not go to the San Diego Meeting.

Crossing an ocean to work on C++ 12 hours a day for a week is indeed madness.

And so naturally, I found myself in a San Diego hotel straight from the 60s, to do some C++ for a week. With the exception of the author of this blog, all people there are incredibly smart and energetic, and so a lot of great work was done...