[SOLVED] VectorData merge function question

[ErwanDouaille]

Hey,

I was looking for copying VectorData and i found the merge function.
Here is the source code of merge function :

/// merging several Data from a VectorData into a large Data (of the same type)
    static void merge(Data<T>& outputData, const vectorData<T>& vectorData)
    {
        size_t nbInput = vectorData.size();
        size_t nbElems = 0;

        for( size_t i=0 ; i<nbInput ; ++i )
            nbElems += vectorData[i]->getValue().size();

        helper::WriteOnlyAccessor< Data<T> > out = outputData;
        out.clear();
        out.reserve(nbElems);
        for( size_t i=0 ; i<nbInput ; ++i )
        {
            helper::ReadAccessor< Data<T> > in = vectorData[i];
            for( size_t j=0, jend=in.size() ; j<jend ; ++j )
                out.push_back(in[j]);
        }
    }

But I encounter than Data was the “output” and it feels weird from me …
The code of merge function, use push_back data in data, which looks like Data is a VectorData.

Finally here is my question, why Data has the same behavior of VectorData (push_back) ? What is the real difference between Data and VectorData, if Data can be considered as a vector ?

[jnbrunet]

Hi Erwan,

It seems the merge function is only used by MergeVectors<VecT>::update() (SofaGeneralEngine/MergeVectors.inl). In this case, the Data<T>& out variable you see will be of type Data<VecT>. So, when expending the templates, the out.push_back is in fact calling VecT.push_back() since the WriteOnlyAccessor is creating a bridge between Data<T>.push_back() and T.push_back().

I’m pretty sure that if you call this merge function with a T template that does not define a push_back() function somewhere, the compilation will fail.

For your second question, Data<T> can be of any type T, be it a vector or not. vectoData<T> is of type vector<Data<T>> which is always a vector. BUT, in the case of this merge function, the Data<T> must be of type vector since the template must have the push_back function. So here, Data<T> is a vector and vectorData<T> is a vector of vector.

This merge function is what is often call a flattening function :

[[0,1],[2,3]] will become [0,1,2,3].

Hope that helps.

[Author]

Posts