Dispatch table

In computer science, a dispatch table is a table of pointers or memory addresses to functions or methods. Use of such a table is a common technique when implementing late binding in object-oriented programming.

It is primarily used to simplify program lines and reduce lines of code.

In different programming languages

Perl

The following shows one way to implement a dispatch table in Perl, using a hash to store references to code (also known as function pointers).

# Define the table using one anonymous code-ref and one named code-ref
my %dispatch = (
    "-h" => sub {  return "hello\n"; },
    "-g" => \&say_goodbye
);
 
sub say_goodbye {
    return "goodbye\n";
}
 
# Fetch the code ref from the table, and invoke it
my $sub = $dispatch{$ARGV[0]};
print $sub ? $sub->() : "unknown argument\n";

Running this Perl program as perl greet -h will produce "hello", and running it as perl greet -g will produce "goodbye".

JavaScript

Following is a demo of implementing a dispatch table in JavaScript:

const thingsWeCanDo = {
    doThisThing() { /* behavior */ },
    doThatThing() { /* behavior */ },
    doThisOtherThing() { /* behavior */ },
    default() { /* behavior */ }
};

function doSomething(doWhat) {
    const thingToDo = Object.hasOwn(thingsWeCanDo, doWhat) 
        ? doWhat 
        : "default";
    return thingsWeCanDo[thingToDo]();
}

Lua

In Lua, this behavior is supported and encouraged. Here is an example:

local colors = { -- This is a lookup table improving speed and simplifying program logic
   red = "#ff0000",
   green = "#00ff00",
   blue = "#0000ff"
}

print("Enter a color:")
local choice = io.read("*l")
local color = colors[choice]
if color then
    print("Your color in Hex format is: " .. color)
end

Virtual method tables

Main article: Virtual method table

In object-oriented programming languages that support virtual methods, the compiler will automatically create a dispatch table for each object of a class containing virtual methods. This table is called a virtual method table or vtable, and every call to a virtual method is dispatched through the vtable.

See also

• Branch table

References

• Diomidis Spinellis (2003). Code Reading: The Open Source Perspective. Boston, MA: Addison-Wesley. ISBN 0-201-79940-5