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.. sidebar:: ToC
.. contents::
.. _tutorial-how-to-recipes-metafunctions:
Metafunctions
=============
Type Metafunctions
------------------
For example, the metafunction :dox:`ContainerConcept#Iterator` is a type metafunction, i.e. it is used to determine a type.
Type metafunctions have the form:
``typename TypeMetaFunc<T1, T2, ..., TN>::Type``
``TypeMetaFunc``
The name of the metafunction
``T1, T2, ..., TN``
Arguments (types or constants)
``Type``
The resulting type
The keyword ``typename`` must be stated if one of the arguments ``T1, T2, ..., TN`` is or uses a template parameter.
For example the following piece of code uses the metafunction ``Iterator`` to determine an iterator type for a string class:
.. includefrags:: demos/tutorial/metafunctions/base.cpp
:fragment: iterator
.. includefrags:: demos/tutorial/metafunctions/base.cpp.stdout
:fragment: iterator
Value Metafunctions
-------------------
Metafunctions can also be used to determine constant values at compile time.
The general form of value metafunctions is:
``VALUE_META_FUNC<T1, T2, ..., TN>::VALUE``
``VALUE_META_FUNC``
The name of the metafunction
``T1, T2, ..., TN``
Arguments (types or constants)
``VALUE``
The resulting constant value
For example the following function prints the length of a fixed sized string using the value metafunction :dox:`LENGTH`:
.. includefrags:: demos/tutorial/metafunctions/base.cpp
:fragment: length
.. includefrags:: demos/tutorial/metafunctions/base.cpp.stdout
:fragment: length
.. important::
Different uses of "*Value*":
Please note that :dox:`ContainerConcept#Value` (``Value<TSomeType>::Type``) is a **Type Metafunction**, because it returns a Type (e.g. of values in a container) and not a value.
Assignment 1
""""""""""""
.. container:: assignment
Objective
Write a generic function ``checkContainerForDna(T & container)`` that prints out a message if the value inside the container is of the type :dox:`Dna`. The type ``T`` of the container should be specified as a template argument. Test you function with some examples.
Hint
* Use the **Type Metafunction** :dox:`ContainerConcept#Value` to access the (alphabet-)type of the elements in the container.
* Use the **Value Metafunction** :dox:`IsSameType` to check for type equality.
Solution
.. container:: foldable
Your program should look something like this:
.. includefrags:: demos/tutorial/metafunctions/assignment1_solution.cpp
Note: Because the Value Metafunction ``IsSameType<>`` is evaluated at compile time, the part of the if-statement code that does not apply won't even appear in the compiled code. This can be an improvement to the runtime of your code.
The output is the following:
.. includefrags:: demos/tutorial/metafunctions/assignment1_solution.cpp.stdout
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