## C++: < type_traits > header

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For this entry, I would like to introduce the type_traits header file. This file contains utility templates that greatly simplify work when writing template-based libraries. This is especially true for libraries that employ template meta-programming. The header file is available with C++ TR1 and above. This library provides tools to identify types, their qualifying properties and even peel-off properties one-by-one programmatically at compile-time. There are also transformation meta-functions that provide basic meta-programming operations such as a compile-time conditional.

The definitions in type_traits will save us a lot of time implementing Alchemy. As I introduce some of the tools that are available in the header, I will also demonstrate how these operations can be implemented. This will help you understand how to construct variations of the same type of solution when applying it in a different context. As an example of this, I will create a construct that behaves similarly to the tertiary operator, but is evaluated at compile-time.

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## Type Lists

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Previously I had discussed the tuple data type. The tuple is a general purpose container that can be comprised of any sequence of types. Both the types and the values can be accessed by index or traversing similar to a linked list.

The TypeList is a category of types that are very similar to the tuple, except no data is stored within the type list. Whether the final implementation is constructed in the form of a linked list, an array, or even a tree, they are all typically referred to as Typelists. I believe that the Typelist construct is credited to Andrei Alexandrescu. He first published an article in the C/C++ Users Journal, but a more thorough description can be found in his book, Modern C++ Design.

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## C++: Tuple

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During my design analysis for my Network Alchemy implementation I thought that the tuple may be the answer to allow me to iterate over the types defined in a network message definition. Tuples are data structures that are a generalization of the std::pair. Rather than having a limitation of 2 items in a tuple, potentially any number of items can be constructed within a custom tuple definition. The upper-limit will most likely be associated with the limit of your compilers ability to recurse down within a nested template structure. Conceptually, a tuple is similar to a compile-time linked list. If a tuple were implemented in terms of the std::pair, it would be constructed like this:

C++

 #include < utility > using std::pair;    pair< int ,        pair< float,              pair< char, long >           >     > tuple;

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## Desired Alchemy Syntax

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When I create a new library, I like to approach the design from two different directions. The first is the traditional route, analyzing the functionality that I need and designing a suitable interface to access those features. I also may write a set of pseudo-code that demonstrates what it would look like to use the library. Generally with writing unit tests I get to cover this second option. It is all the better if I write the tests while developing the interface. Then I discover if the interface is a clunky collection of garbage or an joy to work with.

Alchemy is a unique API, in that I don't actually want there to appear to be a library at work. One goal that I set for alchemy is to facilitate the proper handling of network data without requiring much work on the users part. In this sense, I need to work backwards. I would like to take the proposed syntax, and attempt to design my library API to meet the target syntax. The expressive ability of C++ is one of the things that I really like at the language.

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## An Eye on Refactoring

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Recently I ran across the article, "Is Design Dead?", from Martin Fowler's blog. Martin addresses the common misconception that design is discouraged in Extreme Programming. He describes the overall purpose of design in software and how the design emerges as part of the development phase. As more is learned about the problem domain, better decisions can be made and the design evolves with the implementation. Refactoring code is an activity that helps facilitate success with this approach.

At a different time I was learning about the structure of the human eye and some of the potential paths in evolution that could have led to the structure of our eyes today. I learned some surprising things with regards to how our eyes are structured. The evolved structure is counter-intuitive to what I would have imagined, especially when compared to the structure of a digital camera. I took a step back and starting thinking about the evolutionary path of the eye and compared it to typical events I have experienced during a software development cycle. I wanted to share the conclusions that I reached when I thought about these two topics.

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## Byte Sex

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Good! Now that I have your attention let's solve a relatively simple problem, byte sex. A less sensational name for this concept is byte endianess. This is one of those concepts that you should at least be aware of, even if you don't have to pay much attention to it in your day-to-day work. Each CPU architecture has it's own definition for memory. One of these properties is the endianess format of data registers. This is the first issue that I address for Network Alchemy.

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## My Unit Test Environment

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I discussed the merits of selecting a suitable unit test framework for your development project in my previous post. I described the qualities that I found most valuable in the test framework that I use, CxxTest. The qualities are xUnit framework, portability, simplicity, and flexibility. There are other frameworks for C++ that contain many more features, however, rarely have I had to expand on what is provided in order to test something. Moreover, CxxTest is lightweight enough, that other test tools can be integrated with it, such as Google's GMock library.

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## Unit Test Frameworks

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If you ask a group of 10 software engineers to develop unit tests for the same object, you will end up with 10 unique approaches to testing that object. Now imagine each engineer was given a different object. This was my experience with unit testing before I discovered how useful and how much more valuable your unit tests become when they are developed with a test framework. A unit test framework provides a consistent foundation to build all of your tests upon. The method to setup and run each and every test will be the same. Most importantly, the output provided by each test will be the same, no matter who developed the test.

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## Introduction to Network Alchemy

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While many of the principles of developing robust software are easy to explain, it is much more difficult to know how and when to apply these principles. Practice and learning from mistakes is generally the most productive way to understand these principles. However, it is much more desirable to understand the principles before a flawed system is built; an example from the physical world is the Tacoma Narrows bridge. Therefore I am starting a journey to demonstrate how to create robust software. This is not a simple task that can be summarized in a magazine article, a chapter in a book or a reference application with full source code.

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## C++: using and namespace

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using and namespace are two of the most useful C++ keywords when it comes to simplifying syntax, and clarifying your intentions with the code. You should understand the value and flexibility these constructs will add to your software and it maintenance. The benefits are realized in the form of organization, readability, and adaptability of your code. Integration with 3rd party libraries, code from different teams, and even the ability to simplify names of constructs in your programs are all situations where these two keywords will help. Beware, these keywords can also cause unnecessary pain when used incorrectly. There are some very simple rules to keep in mind, and you can avoid these headaches.

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