Rust for C-Programmers

21.1 Comparison: C switch vs. Rust match

The switch statement in C provides basic conditional branching based on the value of an expression, but it has several limitations compared to Rust’s match:

• Limited Types: C’s switch works reliably only with integral types (like int, char) and enumeration constants. It cannot directly handle strings, floating-point numbers, or complex data structures.
• Fall-through Behavior: By default, execution “falls through” from one case label to the next unless explicitly stopped by a break statement. This is a notorious source of bugs if break is accidentally omitted.
• Non-Exhaustiveness: The C compiler typically does not enforce that all possible values of an enum or integer range are handled within a switch. While warnings might be available, missing cases can lead to unhandled states and runtime errors.
• Simple Comparisons: case labels only permit direct equality comparisons against constant values.

Rust’s match expression systematically addresses these points:

• Type Versatility: match works with any type, including complex data structures like structs, enums, tuples, and slices.
• Exhaustiveness Checking: The Rust compiler requires that a match expression covers all possible variants for the type being matched (especially enums). This compile-time check eliminates entire classes of bugs related to unhandled cases. The wildcard pattern (_) can be used to explicitly handle any remaining possibilities.
• No Fall-through: Each arm of a match expression (PATTERN => EXPRESSION) is self-contained. Execution does not automatically fall through to the next arm, preventing related bugs.
• Powerful Pattern Syntax: match arms use patterns that go far beyond simple equality checks. They can destructure data, bind values to variables, match ranges, combine multiple possibilities (|), and use conditional guards (if condition).
• Value Binding: Patterns can extract parts of the matched value and bind them to new variables available only within the scope of the matching arm.

Overall, match provides a safer, more expressive, and more versatile tool for control flow based on the structure and value of data compared to C’s switch.