Introducing emplacer -- Tim

By Blog Staff | May 9, 2014 06:44 AM | Tags: intermediate

Another nice way to have a polymorphic collection without sacrificing that performance-precious contiguous memory layout:

Introducing emplacer: allocate subtypes of an abstract base class directly in a container

by Tim

From the article:

So I complained to my friend Jeremy that C++ should be able to do this. This isn't Java, I shouldn't have to do a separate allocation for each object...

As alluded to earlier, you can also use it in containers. Here's an example:

std::vector<ShapeAny> shapes;

shapes.emplace(shapes.end())->emplace<Rect>(3, 4);
shapes.emplace(shapes.end())->emplace<Square>(3);

//emplace returns reference to *this, so you can use it like this too
shapes.push_back(ShapeAny().emplace<Rect>(5, 6));
shapes.push_back(ShapeAny().emplace<Circle>(1));

for (auto shape: shapes) {
  std::cout << shape->area() << std::endl;
}

Preface to Programming: Principles and Practice Using C++, 2nd Edition -- Bjarne Stroustrup

By Blog Staff | May 8, 2014 10:16 AM | Tags: basics

Stroustrup's introduction to programming book has now been updated for modern C++. The Preface was posted on InformIT:

Preface to Bjarne Stroustrup's Programming: Principles and Practice Using C++, 2nd Edition

by Bjarne Stroustrup

From the preface:

This book is for someone who has never programmed before but is willing to work hard to learn. It helps you understand the principles and acquire the practical skills of programming using the C++ programming language. My aim is for you to gain sufficient knowledge and experience to perform simple useful programming tasks using the best up-to-date techniques. How long will that take? As part of a first-year university course, you can work through this book in a semester...

Dangerous int-to-string conversions -- Andrzej Krzemieński

By Blog Staff | May 8, 2014 08:05 AM | Tags: None

Note that the article doesn't mention to_string, which is the preferred C++11 way to convert an int to a string. But it does point out an issue -- you can assign an int to a string, including by accident.

Dangerous int-to-string conversions

by Andrzej Krzemieński

From the article:

And now, because int is implicitly convertible to char, our unexpected conversion from int to std::string ‘works’ (under some definition of ‘work’)...

A tale of noexcept swap for user-defined classes in C++11 -- Sumant Tambe

By Blog Staff | May 8, 2014 06:17 AM | Tags: None

From the C++ Truths blog, with a creative use of std::tuple:

A tale of noexcept swap for user-defined classes in C++11

by Sumant Tambe

From the article:

I like my strong exception safety in my little copy-assignment operator so I don’t want my swap to throw but I don’t want my program to std::terminate() if an exception is really thrown. With all that in mind, I would rewrite the swap as follows...

This is not unlike what’s proposed by others but there’s more. The static assert looks awful and looks redundant. There is already a standard library utility that does the same thing: std::tuple. As mentioned before, std::tuple’s swap throws if any member swap throws. We use it here to make our job a lot easier...

C++ User Group Meetings in May

By Meeting C++ | May 7, 2014 03:11 AM | Tags: usergroup community basics

Again, a list of user group meetings for this month. In total its going to be 16 user groups meeting this month:

C++ User Group Meetings in May 2014

by Jens Weller

From the Article:

The Meetings

    8th May C++ UG Aachen - SIMD Programmierung
    8th May C++ UG Belgium
        Parallelism in the C++ Standard, what to expect from C++17
        Asynchronous Programming with futures and await
    8th May C++ UG Dresden - C++ Memory Model
    8th May C++ UG New York - Whats next for C++
    13th May C++ UG Philadelphia - Profiling and Perfomance
    14th May C++ UG San Francisco/Bay area
    15th May C++ UG Malmö/Sweden - Battle of the build systems
    19th May C++ UG Denver - CppNow, Eclipse Configuration, Qt Creator and more...
    20th May C++ UG Berlin - C++ in the demo scene
    21st May C++ UG Düsseldorf - Expression Templates
    21st May C++ UG Hamburg
    21st May C++ UG Seattle/Northwest - Agile Architecture
    23rd May C++ UG Paris
        Introspection et reflection en C++
        Le SIMD en pratique avec boost SIMD
        Clang & C++
    27th May C++ UG Chicago
    28th May C++ UG Heidelberg
    28th May C++ UG San Francisco/Bayarea - C++Now tripreport

Fast Polymorphic Collections -- Joaquín M López Muñoz

By Blog Staff | May 5, 2014 04:44 AM | Tags: performance intermediate efficiency

On the theme of "contiguous enables fast":

Fast Polymorphic Collections

by Joaquín M López Muñoz

From the article:

poly_collection behaves excellently and is virtually not affected by the size of the container. For n < 105, the differences in performance between poly_collection and a std::vector of std::unique_ptrs are due to worse virtual call branch prediction in the latter case; when n > 105, massive cache misses are added to the first degrading factor.

A C error handling style that plays nice with C++ exceptions

By Stefanus Du Toit | May 4, 2014 07:52 AM | Tags: intermediate

If you've ever wondered why someone might throw an exception from a destructor, this article provides a legitimate example:

A C Error Handling Style that Plays Nice with C++ Exceptions

by Stefanus Du Toit

From the article:

By choosing a particular convention for a C API's error handling, we were able to very conveniently translate errors from the C API to C++ exceptions. ... Every experienced C++ programmer I know has opinions on how errors should be handled. I've found this style to be quite useful when layering C++ on top of C, and I think there's some beauty in the interplay between C and C++ here.

Quick Q: If I use shared_ptrs to own objects, must I use weak_ptrs to observe them? -- StackOverflow

By Blog Staff | May 2, 2014 07:57 AM | Tags: None

Quick A: No. Remember that weak_ptr and * are both valid non-owning pointers. Use * when you know the pointed-at object will outlive this pointer. Use weak_ptr when you don't know whether the pointed-at object will outlive this pointer, and the pointed-at object is owned by shared_ptrs.

For more good information, see the top answer to this question yesterday on SO:

Smart pointers + cycles + “->”

Sometimes I'm really sure that I want to have circular dependence of pointers, and every object on cycle should be able to use his pointer (so it can't be weak_ptr).

My question is: Does this mean that I have bad design?

What if I want to implement graph? Can I use smart pointers? In graphs there are cycles, but with weak_ptr I can't use "->". What can I do?

I read some articles, reference and topics on StackOverflow, but it looks like I still don't get smart pointers. Really, why doesn't exists some variant of weak_ptr with "->"?

Quick Q: What is an undefined reference/unresolved external symbol error and how do I fix it? -- SO

By Blog Staff | May 1, 2014 07:54 PM | Tags: basics

If you're new to C++, the question isn't whether you will ask this question, but when.

A classic from SO:

What is an undefined reference/unresolved external symbol error and how do I fix it?

What are undefined reference/unresolved external symbol errors? What are common causes and how to fix them?

Qt & JSON

By Meeting C++ | Apr 30, 2014 10:16 AM | Tags: qt basics

Recently I could play around with Qt5 JSON API:

Qt & JSON

by Jens Weller

From the Article:

With Qt5 there is a new API for reading and writing JSON files in Qt. In the last days I had the chance to play around with this API, as I implemented importing and exporting different data sets from and to JSON.