Posts (Page 2)

Implementing a Turing Tape for Use at Compile Time

May 15, 2016

Turing Machines consist of a tape with memory cells, a tape reader like cassette drives and a program table. Implementing the tape drive part with an array and a pointer is a trivial thing to do with imperative programming languages. It becomes more interesting when learning purely functional programming, especially in the context of template meta programming in C++. As a preparation for the next article, i will show how to implement a turing tape based on type lists, usable at compile time.

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Transformations between User Input/Output and Type Lists

May 14, 2016

Type lists are an important way to represent ordered and unordered sets of types at compile time. These types can be real structure- or class types bundling runtime algorithms etc., but they can also convey actual data at compile time. In order to apply certain compile time processing to data, this data needs to be transformed from and to other representations, which can be provided by the programmer and consumed by run time programs. This article shows how to transform back and forth between strings and character type lists.

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Type Lists

May 8, 2016

Homogenuous data in purely functional programs is typically managed in lists. Items can be appended or prepended to lists, different lists can be concatenated. Lists can be filtered, transformed, mapped, reduced, etc. Having all this nice stuff as a template meta library is quite an enabler for complex compile time meta programs.

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Template Meta Programming Basics

May 5, 2016

C++ template meta programs are at first really hard to read. This is because the template mechanism accidentally became turing complete, although it was invented to enable for type-agnostic programming of generic algorithms. However, now the world knows that the C++ template part of the language is turing complete, people started writing full programs in it, and it enhances the power and flexibility of libraries quite a lot. This article aims to explain some very basic things one needs to know about the C++ template meta programming language, in order to be able to do things with it, or even understand foreign programs.

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Const References to Temporary Objects

March 23, 2016

In both C and C++, it is not a sane idea to hold a reference (or a pointer in C) to a temporarily created object, as the reference is quickly dangling as soon as the assignment is done. But actually, C++ provides an interesting feature, where the life time of a temporary object can be extended to the life time of the reference which points to it.

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Uncaught Exceptions in C++

March 18, 2016

What does actually happen, if an exception is thrown somewhere in the middle of a C++ program, but there is no try-catch clause which handles it? The program gets terminated. That is fine in general, but what happens to all objects which need to be properly destructed?

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Static Polymorphy in C++

February 27, 2016

In order to use polymorphy, virtual functions are the way to go in C++. They are nice and easy to use. However, polymorphy is not always needed at actual runtime. If it is only used to separate generic from specific functionality in order to have a common interface and avoid code duplication, the cost of having indirection introduced by vtables might not be desired. This article shows how to use the CRTP in order to get the compile time advantages of polymorphy, without using virtual methods.

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How to Use the CRTP to Reduce Duplication

February 26, 2016

Some objects have different interfaces for doing the same thing in a different way. One could either check if two objects are equal, or if both are not different. Or one could ask if some container is empty, or if it has zero size. Classes should sometimes provide multiple kinds to express the same thing to let the user decide which way to express something is more readable in a specific context. But that does not mean, that the class developer has to express everything multiple times. This article explains how CRTP can help out and remove potential duplicate code lines.

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ON_EXIT - Combining Automatic Destruction and Lambdas

February 21, 2016

When using C-style libraries, dealing with resources which need to be constructed and destructed again, the code doing the construction/allocation and destruction/release often ends up being ugly and repetitive, because one can’t easily stick with the RAII principle. Especially when a whole set of resources is allocated, and the case that some allocation inbetween fails, all already allocated resources need to be correctly released again. When wrapping such code in C++, it is possible to tidy such code paths up by using automatic destructor calls.

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Useful type traits with if_compiles semantics

February 21, 2016

SFINAE type traits are very mighty, because they can check a lot of properties of types in a non-intrusive way. Unfortunately, they are extremely bloaty to implement. The single interesting expression within an SFINAE type trait is surrounded by lots of boiler plate code, which is ugly to read and repetitive. This article shows a nice one-liner approach to define new SFINAE type traits.

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