Extract the Storage of a Dense ITensor

Dense ITensors store their data in an object of type Dense<Real> or Dense<Cplx>. These types are basically just a lightweight wrapper around a std::vector<Real> or a std::vector<Cplx>.

Here is a definition of Dense<V> which is equivalent to the one used in ITensor

template<typename V>
class Dense
    {
    std::vector<V> store;

    //There are also class methods such as
    //various constructors etc.
    //...
    };

Say we have the following ITensor

auto i = Index("i",2);
auto j = Index("j",3);
auto k = Index("k",4);

auto T = ITensor(i,j,k);
randomize(T);

and we want to extract its data, which is stored internally in a Dense<Real> object. There are two ways to do this:

1. Using applyFunc to apply a function to T's storage
2. Using the doTask system to create functions "dynamically overloaded" on T's storage type

Note: when working with the internal storage of an ITensor or IQTensor, it is important to remember that the true elements of an ITensor are defined by the product of its scale factor (accessible via the .scale() method) with one of the elements in the storage.

Using applyFunc

It is simple to extract the data as a std::vector<Real> using the function applyFunc, which first extracts the storage type of the ITensor then applies the provided function to it:

auto extractReal = [](Dense<Real> const& d)
    {
    return d.store;
    };

auto v = applyFunc(extractReal,T.store());

In the code above, extractReal is a lambda function that takes a Dense<Real> (by const reference so it does not make a copy) and returns its storage, which is a std::vector<Real>. The call to applyFunc takes extractReal as its first argument, and takes T's "storage pointer" T.store() as its second. It performs some magic to unwrap the storage pointer and discover that it is of type Dense<Real> in order to successfully call extractReal. If the storage had been of a different type, then applyFunc would throw an exception.

Using doTask

A more low-level approach to manipulating ITensor storage is to define an overload of doTask. First we need to define "task objects" which label our tasks.

struct ExtractReal {};
struct ExtractCplx {};

Next we define our overloads of doTask

std::vector<Real>
doTask(ExtractReal, Dense<Real> const& d)
    {
    return d.store;
    }

std::vector<Cplx>
doTask(ExtractCplx, Dense<Cplx> const& d)
    {
    return d.store;
    }

Finally we call doTask on T's storage pointer, which will automatically unwrap its type and call the appropriate overload

//If T has Dense<Real> storage:
auto v = doTask(ExtractReal{},T.store());

//If T has Dense<Cplx> storage:
auto v = doTask(ExtractCplx{},T.store());