# Storages#

There are several storages to choose from. To select a storage, pass the
`storage=bh.storage.`

argument when making a histogram.

## Simple storages#

These storages hold a single value that keeps track of a count, possibly a weighed count.

### Double#

By default, boost-histogram selects the `Double()`

storage. For most uses,
this should be ideal. It is just as fast as the `Int64()`

storage, it can fill
up to 53 bits of information (9 quadrillion) counts per cell, and supports
weighted fills. It can also be scaled by a floating point values without making
a copy.

```
h = bh.Histogram(bh.axis.Regular(10, 0, 1)) # Double() is the default
h.fill([0.2, 0.3], weight=[0.5, 2]) # Weights are optional
print(f"{h[bh.loc(.2)]=}\n{h[bh.loc(.3)]=}") # Python 3.8 print
```

```
h[bh.loc(.2)]=0.5
h[bh.loc(.3)]=2.0
```

### Unlimited#

The Unlimited storage starts as an 8-bit integer and grows, and converts to a double if weights are used (or, currently, if a view is requested). This allows you to keep the memory usage minimal, at the expense of occasionally making an internal copy.

### Int64#

A true integer storage is provided, as well; this storage has the `np.uint64`

datatype. This eventually should provide type safety by not accepting
non-integer fills for data that should represent raw, unweighed counts.

```
h = bh.Histogram(bh.axis.Regular(10, 0, 1), storage=bh.storage.Int64())
h.fill([0.2, 0.3], weight=[1, 2]) # Integer weights supported
print(f"{h[bh.loc(.2)]=}\n{h[bh.loc(.2)]=}")
```

```
h[bh.loc(.2)]=1
h[bh.loc(.3)]=2
```

### AtomicInt64#

This storage is like `Int64()`

, but also provides a thread safety guarantee.
You can fill a single histogram from multiple threads.

## Accumulator storages#

These storages hold more than one number internally. They return a smart view when queried
with `.view()`

; see Accumulators for information on each accumulator and view.

### Weight#

This storage keeps a sum of weights as well (in CERN ROOT, this is like calling
`.Sumw2()`

before filling a histogram). It uses the `WeightedSum`

accumulator.

### Mean#

This storage tracks a “Profile”, that is, the mean value of the accumulation instead of the sum.
It stores the count (as a double), the mean, and a term that is used to compute the variance. When
filling, you must add a `sample=`

term.

### WeightedMean#

This is similar to Mean, but also keeps track a sum of weights like term as well.