How do I use a derived type inside a function?

[kyung]

In the following function, I got not 1.0 but “( )” with the line with std::cout.

If I use T(1.0), I got “(-4.8367e-26 )”.

Do I miss something here?

Thanks in advance.

template <typename T>
void somefunction(blaze::Vector<T, blaze::columnVector> &x, const blaze::Vector<T, blaze::columnVector> &y)
{
    .....(some code)

    blaze::DynamicVector<T> temp(2 * N + 1);
    temp[0] = 1.0;
    std::cout << temp[0];

    .....(some code)    
}

‌

[Klaus Iglberger]

Hi Kyung!

As the wiki explains the Vector class represents the base class for all kinds of vectors in the Blaze library. Vector is a CRTP base class, which means that the type T is the type of the concrete vector (e.g. DynamicVector, StaticVector, ...).

Considering your code example:

template< typename T >  // <- This template parameter 'T' represents the concrete type of vector the function accepts
void somefunction(blaze::Vector<T,blaze::columnVector> &x, const blaze::Vector<T,blaze::columnVector> &y)
{
   // ...
}

Given the following function call ...

blaze::DynamicVector<int,blaze::columnVector> a{ 1, 2, 3, 4, 5 };  // Create a 5D vector with the values (1, 2, 3, 4, 5)

somefunction( a );  // 'T' now represents 'DynamicVector<int,columnVector>'

... the template parameter T of the function template now represents the kind of vector, i.e. DynamicVector<int,blaze::columnVector>.

Assuming that T is DynamicVector, the expression T() creates a new empty vector, and T(1.0) creates an uninitialised dynamic vector of size 1 (hence the single element of value -4.8367e-26).

If you want to access elements of the given vectors, you have to perform an explicit cast back to the concrete vector type via the tilda operator (i.e. operator~()):

template< typename T >  // <- This template parameter 'T' represents the concrete type of vector the function accepts
void somefunction(blaze::Vector<T,blaze::columnVector> &x, const blaze::Vector<T,blaze::columnVector> &y)
{
   T temp(2*N+1);      // Create another vector of type 'T'
   temp[0] = 1.0;      // Set the element at index 0 to 1.0
   (~x)[0] = (~y)[0];  // Copy an element from y to x
}

For further examples, please consult the wiki.

I hope this resolves your problems. Please feel free to ask additional questions.

Best regards,

Klaus!