odify-by-reference": `data.table`
* If one is not aware of modify-by-reference, it can have problematic consequences 
  + Unintentional overwriting of full objects, attributes or parts
  + Caching in rmarkdown does not work (but throws no errors)


---


##Warning: modify-by-reference


```r
v1 &lt;- validator(check1 = sex == "Male")
v1
```

```
## Object of class 'validator' with 1 elements:
##  check1: sex == "Male"
```

```r
v2 &lt;- v1  
names(v2) &lt;- "All males"
v1
```

```
## Object of class 'valida