Tables and call_indirect in WebAssembly

Function pointers: a table of funcrefs picked by runtime index.

(module
  ;; A table of function references, picked at run time by index:
  ;; this is how function pointers and vtables compile to wasm.
  (type $unop (func (param i32) (result i32)))
  (table 2 funcref)
  (elem (i32.const 0) $double $negate)

  (func $double (param $n i32) (result i32)
    (i32.mul (local.get $n) (i32.const 2)))
  (func $negate (param $n i32) (result i32)
    (i32.sub (i32.const 0) (local.get $n)))

  (func (export "apply") (param $which i32) (param $n i32) (result i32)
    (call_indirect (type $unop) (local.get $n) (local.get $which))))

How it works

1. (table 2 funcref) and (elem ...) fill two slots with functions.
2. (type $unop ...) declares the signature the call will check.
3. call_indirect takes the argument, then the index — wrong types trap.

Keywords and builtins used here

• call_indirect
• elem
• export
• func
• funcref
• i32
• module
• param
• result
• table
• type

The run, in numbers

Lines

14

Characters to type

534

Tokens

120

Three-star pace

60 tpm

At the three-star pace of 60 tokens a minute, this run takes about 120 seconds.

Type this snippet

Step 3 of 3 in Calls, step 15 of 28 in Language basics.

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