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## Chapter 1. The Type Traits Introspection Library

### Edward Diener

Copyright © 2011-2013 Tropic Software East Inc

Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)

## Introduction

Welcome to the Boost Type Traits Introspection library, abbreviated TTI.

TTI is a library which provides the ability to introspect by name the elements of a type at compile time.

TTI works through macros generating metafunctions. Metafunctions are class templates of a particular syntax, having a nested 'type' member. So wherever in C++ class templates can occur, TTI macros can be used. The metafunctions generated by TTI are no different from any other metafunction as defined by the Boost MPL library.

The metafunctions generated by TTI are used to introspect elements of a type at compile time, always passing at minimum to each metafunction the enclosing type being introspected.

The name of the library has been chosen because the library offers compile time functionality on a type, similar to the Boost Type Traits library, and because the functionality the library offers is the ability to introspect a type about the existence of a specific element within that type.

I use the word "introspect" in a very broad sense here. Normally computer language introspection means initially asking for information to be returned by name, which can then further be used to introspect for more specific information. In the TTI library one must always know and supply the name, and use the functionality provided for the correct type of inner element to find out if that particular named entity exists.

You may prefer the term "query" instead of "introspection" to denote what this library does, but I use terminology based on the word "introspect" throughout this documentation.

The functionality of the library may be summed up as:

- Provide the means to introspect a type at compile time using a set of macros. Each macro takes the name of the type's element and generates a metafunction which can be subsequently invoked to determine whether or not the element exists within the type. These generated metafunctions will be called "macro metafunctions" in the documentation. The type to be introspected can be a class, struct, or union.
- Provide the means to create a typedef for a type which may not exist. If the type does not exist an empty marker type is returned as the typedef type. This typedef type can be used as a type in the metafunctions of the library without producing compile-time errors.

The library is dependent on Boost PP, Boost MPL, Boost Type Traits, and Boost Function Types.

The library is also dependent on the variadic macro support of the Boost PP library if the variadic macros in the library are used.

The library is a header only library.

Since the dependencies of the library are all header only libraries, there is no need to build a library in order to use the TTI library.

### Header Files

There is a single header file, `boost/tti/tti.hpp`, which includes all the header files in the library.

There are also separate specific header files for each of the elements to be introspected by the library. This allows for finer-grained inclusion of the nested elements to be introspected. These header files are:

Table 1.1. TTI Header Files

Introspected Element | Specific Header File
--- | ---
Type | `has_type.hpp`
Class/Struct | `has_class.hpp`
Enumeration | `has_enum.hpp`
Union | `has_union.hpp`
Class Template | `has_template.hpp`
Member data | `has_member_data.hpp`
Member function | `has_member_function.hpp`
Member function template | `has_member_function_template.hpp`
Static member data | `has_static_member_data.hpp`
Static member function | `has_static_member_function.hpp`
Static member function template | `has_static_member_function_template.hpp`
Data | `has_data.hpp`
Function | `has_function.hpp`
Function template | `has_function_template.hpp`
Member Type Creation | `member_type.hpp`