# Fixed Measurements¶

The primary difference between fixed_measurement and measurement is the idea that in a fixed_measurement the unit part is a constant. It does not change. Therefore any addition or subtraction operation will produce another measurement with the same units. It also allows for interaction and comparisons with numerical types since the unit is known. This is unlike measurements where comparison and addition and subtraction operations with numbers are not allowed. Otherwise the behavior and operations are identical between measurement and fixed_measurement and likewise between fixed_precise_measurement and precise_measurement

## Relationship with numbers¶

Because the unit associated with a fixed measurement is fixed. It becomes legitimate to work with singular real valued numbers.

```
fixed_measurement dist(10, m);
if (dist>10.0) //this has meaning because the units of dist is known.
{
//all other operators are defined with doubles
}
dist=dist+3.0; // dist is now 13 meters
dist-=2.0; // dist is now 11 meters
dist=5.0; // dist is now 5 meters
```

## Interactions with measurement¶

Fixed measurements have an implicit conversion to Measurements, so all the methods that work with measurement work with fixed_measurements. The construction of a fixed_measurement from measurement is explicit. Likewise fixed_precise_measurement` have an implicit conversion to precise_measurement, so all the methods that work with precise_measurement work with fixed_precise_measurements. The construction of a fixed_measurement from measurement is explicit.