# Advanced Topic: Other Kinds of Iteration Spaces

# Advanced Topic: Other Kinds of Iteration Spaces#

The examples so far have used the class `blocked_range<T>`

to specify ranges.
This class is useful in many situations, but it does not fit every situation.
You can use oneAPI Threading Building Blocks (oneTBB) to define your own iteration space objects. The object
must specify how it can be split into subspaces by providing a basic splitting
constructor, an optional proportional splitting constructor, and two predicate
methods. If your class is called `R`

, the methods and constructors should be
as follows:

```
class R {
// True if range is empty
bool empty() const;
// True if range can be split into non-empty subranges
bool is_divisible() const;
// Splits r into subranges r and *this
R( R& r, split );
// (optional) Splits r into subranges r and *this in proportion p
R( R& r, proportional_split p );
...
};
```

The method `empty`

should return true if the range is empty. The
method `is_divisible`

should return true if the range can be split
into two non-empty subspaces, and such a split is worth the overhead.
The basic splitting constructor should take two arguments:

The first of type

`R`

The second of type oneapi::tbb::split

The second argument is not used; it serves only to distinguish the
constructor from an ordinary copy constructor. The basic splitting
constructor should attempt to split `r`

roughly into two halves, and
update `r`

to be the first half, and set the constructed object as the
second half.

Unlike the basic splitting constructor, the proportional splitting
constructor is optional and takes the second argument of type
`oneapi::tbb::proportional_split`

. The type has methods `left`

and `right`

that return the values of the proportion. These values should be used to
split `r`

accordingly, so that the updated `r`

corresponds to the
left part of the proportion, and the constructed object corresponds to
the right part.

Both splitting constructors should guarantee that the updated `r`

part
and the constructed object are not empty. The parallel algorithm
templates call the splitting constructors on `r`

only if
`r.is_divisible`

is true.

The iteration space does not have to be linear. Look at
`oneapi/tbb/blocked_range2d.h`

for an example of a range that is
two-dimensional. Its splitting constructor attempts to split the range
along its longest axis. When used with `parallel_for`

, it causes the
loop to be “recursively blocked” in a way that improves cache usage.
This nice cache behavior means that using `parallel_for`

over a
`blocked_range2d<T>`

can make a loop run faster than the sequential
equivalent, even on a single processor.