class-equals

Flag comparisons of class(x) against a string literal—class(x) == "cls", class(x) != "cls", and the %in% membership spellings—which are inherits(x, "cls") (or its negation): class() returns the whole class vector, so the comparison is elementwise (an error in an if () condition on a multi-class object) and misses subclasses, while inherits() asks the intended question directly without materializing the vector.

The rule fires only on the clean single-argument shape and only when class resolves to base R; a local redefinition is left alone. The fix is unsafe: on a multi-class object the comparison yields an elementwise vector where inherits() yields a scalar, and for S4 objects inherits() follows the formal inheritance chain.

Testing an object’s class:

if (class(x) == "factor") levels(x)

warning: class-equals
 --> example.R:1:5
  |
1 | if (class(x) == "factor") levels(x)
  |     ^^^^^^^^^^^^^^^^^^^^ comparing `class(x)` to a string is fragile—`class()` returns a vector; `inherits()` asks directly
  = help: Use `inherits(x, "cls")` (or `!inherits(x, "cls")`).