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Quick Q: What is a lambda expression in C++11?

By Adrien Hamelin | Mar 19, 2019 11:42 AM | Tags: c++11 basics

Quick A: A convenient way to create a functor.

Recently on SO:

What is a lambda expression in C++11?

C++ includes useful generic functions like std::for_each and std::transform, which can be very handy. Unfortunately they can also be quite cumbersome to use, particularly if the functor you would like to apply is unique to the particular function.

#include <algorithm>
#include <vector>

namespace {
  struct f {
    void operator()(int) {
      // do something
    }
  };
}

void func(std::vector<int>& v) {
  f f;
  std::for_each(v.begin(), v.end(), f);
}

If you only use f once and in that specific place it seems overkill to be writing a whole class just to do something trivial and one off...

A gentle introduction to jump threading optimizations--Aldy Hernandez

By Adrien Hamelin | Mar 18, 2019 10:50 AM | Tags: performance advanced

Your complier does magic.

A gentle introduction to jump threading optimizations

by Aldy Hernandez

From the article:

As part of the GCC developers‘ on-demand range work for GCC 10, I’ve been playing with improving the backward jump threader so it can thread paths that are range-dependent. This, in turn, had me looking at the jump threader, which is a part of the compiler I’ve been carefully avoiding for years. If, like me, you’re curious about compiler optimizations, but are jump-threading-agnostic, perhaps you’ll be interested in this short introduction...

Thanks for the memory (allocator)--Glennan Carnie

By Adrien Hamelin | Mar 18, 2019 10:49 AM | Tags: c++17

Unifying the types.

Thanks for the memory (allocator)

by Glennan Carnie

From the article:

One of the design goals of Modern C++ is to find new – better, more effective – ways of doing things we could already do in C++.  Some might argue this is one of the more frustrating aspects of Modern C++ – if it works, don’t fix it (alternatively: why use lightbulbs when we have perfectly good candles?!)

This time we’ll look at a new aspect of Modern C++:  the Allocator model for dynamic containers.  This is currently experimental, but has been accepted into C++20.

The Allocator model allows programmers to provide their own memory management strategy in place of their default library implementation.  Although it is not specified by the C++ standard, many implementations use malloc/free.

Understanding this feature is important if you work on a high-integrity, or safety-critical, project where your project standards say ‘no’ to malloc...

Quick Q: Pointer to class data member “::*”

By Adrien Hamelin | Mar 13, 2019 12:53 PM | Tags: advanced

Quick A: a pointer that lets you access the value of the member of an instance.

Recently on SO:

Pointer to class data member “::*”

It's a "pointer to member" - the following code illustrates its use:

#include <iostream>
using namespace std;

class Car
{
    public:
    int speed;
};

int main()
{
    int Car::*pSpeed = &Car::speed;

    Car c1;
    c1.speed = 1;       // direct access
    cout << "speed is " << c1.speed << endl;
    c1.*pSpeed = 2;     // access via pointer to member
    cout << "speed is " << c1.speed << endl;
    return 0;
}

As to why you would want to do that, well it gives you another level of indirection that can solve some tricky problems. But to be honest, I've never had to use them in my own code.

Edit: I can't think off-hand of a convincing use for pointers to member data. Pointer to member functions can be used in pluggable architectures, but once again producing an example in a small space defeats me. The following is my best (untested) try - an Apply function that would do some pre &post processing before applying a user-selected member function to an object:

void Apply( SomeClass * c, void (SomeClass::*func)() ) {
    // do hefty pre-call processing
    (c->*func)();  // call user specified function
    // do hefty post-call processing
}

The parentheses around c->*func are necessary because the ->* operator has lower precedence than the function call operator.

C++ Core Guidelines: Mixing C with C++--Rainer Grimm

By Adrien Hamelin | Mar 13, 2019 12:39 PM | Tags: community

Mixing C and C++ from the point of view of a C++ engineer.

C++ Core Guidelines: Mixing C with C++

by Rainer Grimm

From the article:

The chapter in the C++ core guidelines is called: C-style programming. Honestly, my first thought was to skip it, but after more thoughts I decided to write about it. My reason is twofold:

  1. This are the typical issues we have when dealing with legacy code.
  2. One reader wanted that I write more about the challenges of legacy code.

Understanding C++ Modules: Part 1: Hello Modules, and Module Units--Colby Pike

By Adrien Hamelin | Mar 12, 2019 12:04 PM | Tags: experimental advanced

Complex, but useful!

Understanding C++ Modules: Part 1: Hello Modules, and Module Units

by Colby Pike

From the article:

My previous posts on modules have received a lot of attention. I’m happy that I’ve been able to kick-start a lot of conversation, but I’ve also seen that a large part of the community is still unclear on what modules actually are.

There is a lot of ground to cover. I can’t do it all in one sitting, and I doubt you’d want to read the entire thing in one go. I’ll be breaking this up, starting at the most high-level aspects and drilling down over time. I intend these posts will clarify and discuss what modules are, what they can do, and what they are intended to do, what they cannot do, and how they are used...

Lambdas: From C++11 to C++20, Part 2--Bartlomiej Filipek

By Adrien Hamelin | Mar 12, 2019 11:54 AM | Tags: experimental c++17

The series continues!

Lambdas: From C++11 to C++20, Part 2

by Bartlomiej Filipek

From the article:

In the first part of the series we looked at lambdas from the perspective of C++03, C++11 and C++14. In that article, I described the motivation behind this powerful C++ feature, basic usage, syntax and improvements in each of the language standards. I also mentioned several corner cases.

Now it’s time to move into C++17 and look a bit into the future (very near future!): C++20...

STL Algorithms on Tuples--Jonathan Boccara

By Adrien Hamelin | Mar 12, 2019 11:46 AM | Tags: stl c++17

Do you need them?

STL Algorithms on Tuples

by Jonathan Boccara

From the article:

When you manipulate a collection of objects in C++–which is quite a common thing to do when programming in C++–STL algorithms are your loyal companions to perform operations with expressive code.

But the STL algorithms, shipped in the standard library with C++, only apply to collections that are filled at runtime, during the execution of a program (or in C++20, during the execution of constepxr code during compilation). This include the ubiquitous std::vector and std::map.

But STL algorithms don’t operate on std::tuples.

However, it could be useful to iterate over the elements of a tuple, at runtime, and perform transformations or extract information, like STL algorithms do. We will see in detail a situation where this is useful with the demux output iterator, in a future post.

Can we design algorithms that are do what STL algorithms do, but on the contents of std::tuples instead of std::vectors and std::maps?

It turns out we can.