🚀 SAP ABAP Mock Interview – Question 7/50

 

Downcasting in ABAP: ?= vs CAST

One of the most common interview questions in Modern ABAP is about downcasting. Many developers know the syntax, but interviewers often look for your understanding of how and why each approach works.

Let's break it down.

Scenario

Consider the following class hierarchy:

lcl_vehicle    (Super Class)
      ↑
lcl_car        (Sub Class)

Assume we have a reference of the parent type:

DATA lo_vehicle TYPE REF TO lcl_vehicle.

At runtime, this reference may point to an object of lcl_car (or any subclass).

There are two ways to downcast it.

Approach 1 – Using ?=

DATA lo_vehicle TYPE REF TO lcl_vehicle.
DATA lo_car     TYPE REF TO lcl_car.

lo_car ?= lo_vehicle.

Characteristics

• Statement-based syntax
• Available since Classic ABAP
• Requires an existing reference variable
• Commonly seen in legacy code

Approach 2 – Using CAST

DATA(lo_car) = CAST lcl_car( lo_vehicle ).

Characteristics

• Expression-based syntax
• Supports inline declarations
• Can be nested inside larger expressions
• Preferred in Modern ABAP

Example:

DATA(lv_name) =
    CAST lcl_employee( lo_object )->name.

This kind of expression chaining is not possible with ?=.

1️⃣ What is the Difference Between ?= and CAST?

A common interview answer is:

"Use CAST when the hierarchy is known."

❌ This is not the real difference.

Both ?= and CAST require a valid inheritance hierarchy.

The actual difference is how the cast is expressed in ABAP.

?=	CAST
Statement	Expression
Classic ABAP syntax	Modern ABAP syntax
Needs an existing variable	Supports inline declaration
Cannot be embedded in expressions	Can be used inside larger expressions
Mostly used in older code	Recommended for Modern ABAP

2️⃣ Do Both Perform Runtime Type Checking?

Yes.

Both

lo_car ?= lo_vehicle.

and

CAST lcl_car( lo_vehicle )

perform exactly the same runtime type validation.

If the runtime object is not actually an instance of lcl_car (or one of its subclasses), the cast fails.

There is:

• ✅ No performance difference
• ✅ No difference in runtime validation

The difference is purely syntactical and stylistic.

3️⃣ Which Exception Can Occur?

If the runtime object cannot be converted to the requested type, ABAP raises:

CX_SY_MOVE_CAST_ERROR

This happens when the runtime object is not compatible with the target type.

4️⃣ How Would You Handle It?

Wrap the cast inside a TRY...CATCH block.

TRY.

    DATA(lo_car) =
        CAST lcl_car( lo_vehicle ).

  CATCH cx_sy_move_cast_error
        INTO DATA(lx_cast).

    "Handle invalid cast
    "Log the error, skip processing,
    "or notify the user

ENDTRY.

This prevents a runtime dump and allows your program to handle invalid casts gracefully.

🎯 Interview Tip

A basic answer is:

"CAST is the new syntax."

A stronger answer is:

"CAST is an expression operator, whereas ?= is an assignment statement. Both perform identical runtime type checking, but CAST integrates naturally with Modern ABAP features such as inline declarations and expression chaining."

That answer demonstrates a deeper understanding of Modern ABAP.

💡 Bonus Question

What will happen here?

DATA lo_vehicle TYPE REF TO lcl_vehicle.

CREATE OBJECT lo_vehicle TYPE lcl_vehicle.

DATA(lo_car) =
    CAST lcl_car( lo_vehicle ).

Will the cast succeed?

❌ No.

lo_vehicle actually refers to an object of type lcl_vehicle, not lcl_car.

Although lcl_car inherits from lcl_vehicle, the reverse is not true.

At runtime, ABAP checks the actual object type, not the reference type.

Since the object is not an instance of lcl_car (or its subclass), the cast fails and raises:

CX_SY_MOVE_CAST_ERROR

📌 Key Takeaways

• ?= and CAST perform the same runtime type checking.
• The difference is statement-based (?=) vs expression-based (CAST) syntax.
• CAST is preferred in Modern ABAP because it supports:
  • Inline declarations
  • Expression chaining
  • Cleaner and more readable code
• Invalid downcasting raises CX_SY_MOVE_CAST_ERROR.
• Always use TRY...CATCH when the runtime type is uncertain.

💬 Interview Question for You

Suppose the following code executes:

DATA lo_vehicle TYPE REF TO lcl_vehicle.

CREATE OBJECT lo_vehicle TYPE lcl_car.

DATA(lo_car) = CAST lcl_car( lo_vehicle ).

Will the cast succeed? Why?

Share your answer in the comments! 👇