fpc-defer

Defer mechanism for Free Pascal.

This is similar to defer from other languages:

• defer in V
• defer in go

This can be approximated (but is not identical) in Free Pascal.

But it is basically equivalent in that one can defer:

• A function call.
• Code that will be be executed.

This project demonstrates how that can be done.

Requirements

FPC 3.3.1 or later (What currently can be built from FPC gitlab main)

Reason:

• This mechanism uses Anonymous Functions
• This mechanism uses Function References

These are used so that code using defer can access data in their parent scope.

Options for installing FPC 3.3.1:

• FPCUPdeluxe
• Building FPC from sources using git or zip

To use

Study the code (fpcdefer.pas and main.pas) then execute make run.

Basically the main.pas example shows what to do:

• Create a variable of type tDefer in the function you need to defer something.
• For ease of use, you can name that variable defer.
  • var defer: tDefer;
• You then can use defer to add function references to a LIFO list.
• Both of these do the same thing:
  • defer.x := some function reference
  • defer.add(some function reference)
• You can do defer.x := @some_global_procedure and it will work as expected.
• You can also use a local procedure and it will work as expected, but you must cast it.
  • defer.x := tDeferProc(@some_local_procedure)
• You can do defer.x := @some_class_instance.some_procedure_method and it will work as expected.
• You can do defer.x := @some_class_instance.free and it will work as expected.

You can also use an anonymous function as the function reference.

defer.x := procedure
begin
	writeln('Some anonymous function...');
end;

Deferred execution

• When the defer goes out of scope, all the functions in the LIFO list will be called.
• LIFO, as in, Last in, First out, meaning:
  • The last function reference added will be called first.
  • The first function reference added will be called last.

Insure the deferred code is not executed too early

Place defer.anchor at the bottom of the function, before it's last end;

Notes

• The .anchor method must be used to make sure the deferred code is not called too early.
  • In and of itself, the .anchor method does nothing.
  • The .anchor method is an empty placeholder.
• It needs to be placed either:
  • At the end of scope it is intended to be used in.
  • At the earliest point the deferred code can be called.
• This is because FPC does in no way guarantee when exactly a managed type is finalized.
  • It can be at the end of the function or directly after its last use.
  • It is only guaranteed that it will be finalized at the latest at the end of the scope.
• Free Pascal scoping.
  • FPC is function scope.
  • FPC is not block scope (Unless one considers the whole function to be a block).
  • Nested functions are nested scopes.

References

• See Topic: Defer Mechanism for FreePascal in the Lazarus forums.