fn Example (Lookup Table)Use the fn keyword to create a function, and use fn as a type to create a lookup table of functions.
This page was last reviewed on May 11, 2022.
Fn, function. In Rust the fn keyword can be used to declare a function. But we can also use fn in a type expression to store function pointers in a vector.
With a vec of function pointers, we can call functions based on an index. This eliminates the need for a match statement or if-else chain.
const fn
Example program. Consider this Rust example program—it has 4 functions declared at the top. In main, we create a Vec of function pointers.
And The "no_action" function is used for all 128 indexes. This function could be used to indicate no action is taken.
push u8 Vec
Next The core, art and top functions are indexed by the first letters of their names.
Finally We loop over the bytes of the string "cat" and call the core, art and top functions by the byte values.
Loop chars
fn no_action() -> &'static str { "no action" } fn core() -> &'static str { "core" } fn art() -> &'static str { "art" } fn top() -> &'static str { "top" } fn main() { // Create vector of function pointers. let mut lookup: Vec<fn() -> &'static str> = vec![]; for _ in 0..128 { lookup.push(no_action); } // Store functions as function pointers at specific indexes. lookup[b'c' as usize] = core; lookup[b'a' as usize] = art; lookup[b't' as usize] = top; // Loop over this value and call function for each byte. let value = "cat"; println!("VALUE: {}", value); for c in value.bytes() { println!("RESULT: {}, {}", c as char, lookup[c as usize]()); } }
VALUE: cat RESULT: c, core RESULT: a, art RESULT: t, top
Lookup table notes. It is possible to use a lookup table of function pointers to replace match or if-statements in Rust. This will affect performance and should be tested.
A summary. With a function pointer lookup table, we can develop a simple bytecode interpreter. Each byte can be handled with a function call based on the byte value.
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Sam Allen is passionate about computer languages. In the past, his work has been recommended by Apple and Microsoft and he has studied computers at a selective university in the United States.
This page was last updated on May 11, 2022 (edit link).
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