instanceOf: Returns a function that checks whether its 'argument' is...
In bedatadriven/hamcrest: Hamcrest framework for writing unit tests in R

Checks if the value is an instance of a class. The expected value checks where the class name of the actual object is inherited. This is about checking S3 and S4 classes and the call uses the base function inherits.

1	instanceOf(expected)

expected

object passed to the matcher function.

## Not run: 
res <- lm(Petal.Length ~ Petal.Width, iris)
assertThat(res, instanceOf("lm"))

df <- data.frame(x = c(1, 1, 1, 2, 2, 3, 3), y = c(1, 2, 3, 4, 5, 6, 7))
res <- by(df$y, df$x, sum)
assertThat(res, instanceOf("by"))

class(df) <- "result"
assertThat(df, instanceOf("result"))

## End(Not run)