Choosing Memory Management Strategies in Delphi: TObject vs. IInterface

The Memory Ownership Dilemma

In Delphi, the primary challenge in memory management is deciding who "owns" an object. When a developer allocates memory using a constructor, they must ensure a corresponding destructor is called. Failure to do so results in a memory leak; calling it too early or twice results in an Access Violation (AV). The decision typically falls between manual management via TObject and automated reference counting via IInterface.

Comparison of Management Strategies

Feature	TObject (Manual)	IInterface (ARC)	Smart Pointers (RAII)
Destruction	Explicit (.Free)	Automatic (Ref Count = 0)	Automatic (Scope-based)
Ownership	Single, defined owner	Shared ownership	Scoped ownership
Overhead	Minimal	Reference count increment/decrement	Small wrapper overhead
Risk	Memory leaks / Dangling pointers	Circular references	Implementation complexity

Trade-offs and Constraints

TObject is the standard for most Delphi applications. It provides the highest performance and most predictable behavior. However, it requires a strict ownership hierarchy. If Object A creates Object B, Object A must be responsible for freeing it, usually within a try...finally block.

IInterface (Automatic Reference Counting or ARC) is useful when an object's lifetime is unpredictable or shared across multiple modules. By inheriting from TInterfacedObject, the object tracks how many references point to it. When the last reference is released, the object destroys itself. The critical constraint here is the Circular Reference: if Object A holds a reference to Object B, and Object B holds one to Object A, neither reference count will ever reach zero, causing a permanent leak.

Mixing Strategies is a common source of crashes. If you cast a TInterfacedObject to a TObject and call .Free, the internal reference counter is bypassed. When the interface later attempts to decrement the counter and destroy the object, it will trigger an Access Violation because the memory has already been reclaimed.

Implementation: Interface Reference Counting

To implement automated management, define an interface and a class that inherits from TInterfacedObject. This ensures the _AddRef and _Release methods are handled correctly.

type
  IMyService = interface
    ['{GUID-HERE}'] // Required for interface identification
    procedure ExecuteTask;
  end;

  TMyService = class(TInterfacedObject, IMyService)
  public
    procedure ExecuteTask;
    destructor Destroy; override;
  end;

procedure RunService;
var
  Service: IMyService;
begin
  // Object is created and ref count is 1
  Service := TMyService.Create;
  Service.ExecuteTask;
  // When Service goes out of scope here, ref count becomes 0 and Destroy is called
end;

Validating Memory Behavior

To verify that your chosen strategy is working, use a memory manager like FastMM4 (integrated into modern Delphi versions).

• For TObject: Run the application, perform the task, and shut down. FastMM4 will report a "Memory Leak" if .Free was not called.
• For IInterface: Add a Writeln or log entry inside the destructor Destroy. If the log entry does not appear when the variable goes out of scope, you likely have a circular reference.

Risk Mitigation and Rollback

If you discover a circular reference in an interface-based system, you must manually break the link. This is typically done by creating a "Weak Reference" or providing a TObject-style Cleanup method that sets internal interface references to nil.

If you have accidentally introduced TInterfacedObject into a legacy codebase where manual .Free calls are prevalent, the rollback is to change the inheritance back to TObject. This removes the reference counting logic and restores manual ownership, preventing the double-free Access Violations.