How do I get the class name of a specific object?

[kasperl]

While running my Toit code, I have an object and I want to get the class name of it as a string. Something like this:

foo x/any:
  print x.class_name // <-- doesn't work

[kasperl]

Toit programs use class ids (small integers) instead of class names at runtime. This is to save space on memory constrained devices like the ESP32, where Toit programs are stripped of their unnecessary debug information. The information can easily be preserved in a snapshot file, where it can be queried through the tools/snapshot.toit utility code.

Getting the class id

Sometimes, it is useful to get the class name of a specific object or to translate class ids to class names.
If you write this code and run it:

class MyExampleClass:
  // Deliberately left empty.

main:
  x := MyExampleClass
  print x

you will see something along the lines of an instance with class-id 31 printed. Another way is to use the static Object.class_id method:

main:
  ...
  print "class = $(Object.class_id x)"

which prints class = 31.

Translating class ids to class names

We can use a snapshot that contains the debug information to translate the class id to the corresponding class name. First we need to put the above code in a file. Let's call it example.toit and give it this content:

class MyExampleClass:
  // Deliberately left empty.

main:
  x := MyExampleClass
  print "class = $(Object.class_id x)"

Then we can compile the program into a snapshot using the Toit compiler and run it from the snapshot:

build/host/bin/toitc -w example.snapshot example.toit
build/host/bin/toitvm example.snapshot

This generates the example.snapshot file and prints class = 31 when it is run.

The toitp.toit tool in the tools folder can print all classes with their ids:

build/host/bin/toitvm tools/toitp.toit -c example.snapshot

Alternatively, we can also write a small piece of Toit code that uses tools/snapshot.toit to decode the snapshot and ask for the name of the class with id 31:

import .tools.snapshot
import .tools.file as file

main:
  snapshot := file.read_content "example.snapshot"
  program := (SnapshotBundle snapshot).decode
  print (program.class_name_for 31)

It prints MyExampleClass 🎉