delphi-memory-exceptions

🧠 Memory and Exceptions Management in Delphi

Contexto

Delphi has manual memory management for class instances (not derived from interfaces) and uses ARC (Automatic Reference Counting) only for interfaces (IInterface), Strings, Dynamic Arrays and anonymous types. Poor exception handling and forgetting to release memory result in chronic Memory Leaks, catastrophic production failures and systemic instability.

Like AI, you must proactively ensure that every object you create is freed regardless of error streams.

Objectives of this Skill

• Teach how to generate safe blocks of try..finally.
• Prevent Memory Leaks by advising on Free and FreeAndNil.
• Promote the use of Interfaces for memory automation.
• Institute defensive and typed handling of Exceptions (try..except).
• Introduce custom Domain Exceptions.

---

🛑 Memory Management: Critical Rules

1. The Gold Standard: try..finally

Whenever an object instance is created and does not have an Owner who manages it, instantiate it in a try..finally block. try must occur IMMEDIATELY on the line following creation.

var
  LList: TStringList;
begin
  LList := TStringList.Create;
  try
    LList.Add('Item 1');
    // ...
  finally
    LList.Free;
  end;
end;

Anti-Pattern (DO NOT USE): Code between Create and try may generate an exception, leaking the newly created object.

  //WRONG - potential leak!
  LList := TStringList.Create;
  LList.Add('Item 1');
  try
    // ...

2. Multiple Objects in the Same Block

When allocating multiple temporary resources in the same method, apply this rule of thumb:

• Dependent objects (one wraps/consumes another, e.g. TStreamReader reading from a TMemoryStream) → nest one try..finally per object, freeing in reverse order of creation:

var
  LStream: TMemoryStream;
  LReader: TStreamReader;
begin
  LStream := TMemoryStream.Create;
  try
    LReader := TStreamReader.Create(LStream);
    try
      //logics with both
    finally
      LReader.Free;
    end;
  finally
    LStream.Free;
  end;
end;