abap - ABAP运算符中？=的含义是什么？

Question

我的问题就像标题一样，ABAP运算符中的=和?=有什么区别？

Answer 1

?=是(down)casting operator。它用于在引用变量之间进行分配，这些引用变量的可分配性最早在运行时开始时进行检查。

C.f. wikipedia的一般说明。

例子:

DATA fruit TYPE REF TO zcl_fruit.
DATA apple TYPE REF TO zcl_apple. " inherits from zcl_fruit
DATA apricot TYPE REF TO zcl_apricot. " inherits from zcl_fruit

...

case fruit->type.
  when 'apple'.
    apple ?= fruit.
    seeds = apple->seeds.
  when 'apricot'.
    apricot ?= fruit.
    seeds = VALUE #( ( apricot->kernel ) ).
endcase.

从7.40开始，可以使用构造函数运算符 CAST :

DATA fruit TYPE REF TO zcl_fruit.

...

case fruit->type.
  when 'apple'.
    seeds = CAST zcl_apple( fruit )->seeds.
  when 'apricot'.
    seeds = VALUE #( ( CAST zcl_apricot( fruit )->kernel ) ).
endcase.