Investigating Swift’sString(describing:)and Performance Issues

HowString(describing:)Works in Swift

Swift’s String(describing:) initializer is a convenience for converting any value to a string, primarily intended for debugging and logging purposes. Under the hood, it checks if the value conforms to certain protocols (like CustomStringConvertible or TextOutputStreamable) and uses those to produce a description . If no custom textual representation is available, Swift falls back on reflection — using the Mirror API to traverse the value’s structure and generate a description of all its fields . This reflection-based fallback means that complex or large values will be fully walked and described by default, which can be computationally expensive. As one expert notes, String(describing:) will “turn anything into a string representation” but is meant only for debugging rather than for any performance-critical or user-facing use . In other words, the implementation prioritizes completeness over speed, since it isn’t expected to run in tight loops or in production logic.

Why might this be slow? The generality of String(describing:) comes at a cost. It may perform multiple runtime checks and potentially invoke Swift’s reflection machinery (Mirror) to inspect properties and child objects . This involves branching on protocol conformances and iterating over object graphs, which is “more work” compared to a specialized conversion, hence taking more time . In normal use (printing the occasional value for debugging), this overhead…