Metafunctions

Type Metafunctions

For example, the metafunction 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:

    String<char> str = "I am a string";
    Iterator<String<char> >::Type it = begin(str);
    while (! atEnd(it, str))
    {
        std::cout << *it;
        ++it;
    }
    std::cout << std::endl;

I am a string

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 LENGTH:

template <typename T>
void printLenOfFixedSizeString(T const &)
{
    std::cout << LENGTH<T>::VALUE << std::endl;
}

int main()
{
    String<char, Array<100> > my_str;
    printLenOfFixedSizeString(my_str);
    return 0;
}

100

Important

Different uses of “Value”:

Please note that 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

Objective

Write a generic function checkContainerForDna(T & container) that prints out a message if the value inside the container is of the type 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 Value to access the (alphabet-)type of the elements in the container.

  • Use the Value Metafunction IsSameType to check for type equality.

Solution

Your program should look something like this:

#include <iostream>
#include <seqan/basic.h>
#include <seqan/stream.h>

using namespace seqan2;

template <typename T>
void checkContainerForDna(T & /* container */)
{
    // Type Metafunction Value<>
    typedef typename Value<T>::Type TAlphType;

    // Value Metafunction IsSameType<> evaluated at compile time
    if (IsSameType<TAlphType, Dna>::VALUE)
        std::cout << "I have found a container with Dna!" <<  std::endl;
    else
        std::cout << "No Dna anywhere." <<  std::endl;
}

int main()
{
    typedef String<Dna> TDnaString;
    TDnaString dna = "AAAATTTT";

    typedef String<int> TIntString;

    TIntString numbers;
    appendValue(numbers, 1);
    appendValue(numbers, 3);

    checkContainerForDna(dna);
    checkContainerForDna(numbers);

    return 0;
}

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:

I have found a container with Dna!
No Dna anywhere.