class_equals

What it does

Checks for usage of class(...) == "some_class", class(...) %in% "some_class", and identical(class(...), "some_class").

For == and %in% operators, the only cases that are flagged (and potentially fixed) are cases that:

happen in the condition part of an if () statement or of a while () statement,
and are not nested in other calls.

For example, if (class(x) == "foo") would be reported, but not if (my_function(class(x) == "foo")).

For identical() calls, all cases are flagged regardless of context.

Why is this bad?

An R object can have several classes. Therefore, class(...) == "some_class" would return a logical vector with as many values as the object has classes, which is rarely desirable.

It is better to use inherits(..., "some_class") instead. inherits() checks whether any of the object’s classes match the desired class.

The same rationale applies to class(...) %in% "some_class". Similarly, identical(class(...), "some_class") would break if a class is added or removed to the object being tested.

Example

x <- lm(drat ~ mpg, mtcars)
class(x) <- c("my_class", class(x))

if (class(x) == "lm") {
  # <do something>
}

identical(class(x), "foo")

Use instead:

x <- lm(drat ~ mpg, mtcars)
class(x) <- c("my_class", class(x))

if (inherits(x, "lm")) {
  # <do something>
}

inherits(x, "foo")

References

See ?inherits