## Preprocessor Code Generation

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I really do not like MACROs in C and C++, at least the way they have been traditionally used starting with C. Many of these uses are antiquated because of better feature support with C++. The primary uses are inline function calls and constant declarations. With the possibility of MACRO instantiation side-effects, and all of the different ways a programmer can use a function, it is very difficult to write it correctly for all scenarios. And when a problem does occur, you cannot be certain what is being generated and compiled unless you look at the output from the preprocessor to determine if it is what you had intended.

However, there is still one use of MACROs, that I think makes them too valuable for the preprocessor to be removed altogether. This use is code generation in definitions. What I mean by that, is hiding cumbersome boiler-plate definitions that cannot easily be replicated without resorting to manual cut-and-paste editing. Cut-and-Paste is notorious for its likelihood of introducing bugs. I want to discuss some of the features that cannot be matched without the preprocessor, and set some guidelines that will help keep problems caused by preprocessor misuse to a minimum.

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## 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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