abap - 在 VALUE 的左侧和右侧使用相同的 itab 会导致空表。为什么？

Question

这是给你的……

为什么这段代码会导致一个空表 gt_test2...

gt_test2 = VALUE #(
  FOR gs_test1 IN gt_test1 WHERE ( v3 = gs_test3-v3 AND v4 = gs_test3-v4 )
  FOR gs_test2 IN gt_test2 WHERE ( v1 = gs_test1-v1 AND v2 = gs_test1-v2 )
  ( gs_test2 )
).
WRITE / lines( gt_test2 ).

这个产生了正确的结果？

DATA: gt_test2_2 LIKE gt_test2.
gt_test2_2 = VALUE #(
  FOR gs_test1 IN gt_test1 WHERE ( v3 = gs_test3-v3 AND v4 = gs_test3-v4 )
  FOR gs_test2 IN gt_test2 WHERE ( v1 = gs_test1-v1 AND v2 = gs_test1-v2 )
  ( gs_test2 )
).
WRITE / lines( gt_test2_2 ).

这是 ABAP 中的错误!？我一直认为首先评估表达式的右侧，然后将其分配给左侧。然而，在这个 ABAP 结构中，似乎在计算右侧时以某种方式在内部使用了左侧，这导致了不正确的结果。

最小的工作示例。

REPORT ZZZ.

TYPES: BEGIN OF ty_test1,
  v1 TYPE char40,
  v2 TYPE char6,
  v3 TYPE char6,
  v4 TYPE char32,
  END OF ty_test1,
  BEGIN OF ty_test2,
    v1 TYPE char40,
    v2 TYPE char6,
    v5 TYPE numc3,
  END OF ty_test2,
  BEGIN OF ty_test3,
    v3 TYPE char6,
    v4 TYPE char32,
  END OF ty_test3.
TYPES: tty_test1 TYPE STANDARD TABLE OF ty_test1 WITH EMPTY KEY,
  tty_test2 TYPE STANDARD TABLE OF ty_test2 WITH EMPTY KEY.

INITIALIZATION.
  DATA(gt_test1) = VALUE tty_test1(
    ( v1 = '1' v2 = '007001' v3 = '0012345678' v4 = '001061' )
    ( v1 = '2' v2 = '007001' v3 = '0999999953' v4 = '001061' )
  ).
  DATA(gt_test2) = VALUE tty_test2(
    ( v1 = '1' v2 = '007001' v5 = '10' )
    ( v1 = '2' v2 = '007001' v5 = '15' )
  ).
  DATA(gs_test3) = VALUE ty_test3( v3 = '0999999953' v4 = '001061' ).

START-OF-SELECTION.
*  DATA: gt_test2_2 LIKE gt_test2.
*  gt_test2_2 =
  gt_test2 =
  VALUE #(
    FOR gs_test1 IN gt_test1 WHERE ( v3 = gs_test3-v3 AND v4 = gs_test3-v4 )
    FOR gs_test2 IN gt_test2 WHERE ( v1 = gs_test1-v1 AND v2 = gs_test1-v2 )
    ( gs_test2 )
  ).
  WRITE / lines( gt_test2 ).
*  WRITE / lines( gt_test2_2 ).

Answer 1

使用构造函数表达式，目标数据对象首先被清除，因此您遇到的结果是:内部表 gt_test2 在 FOR 循环时为空。

在您的情况下，您可以使用在清除目标数据对象之前调用的 LET 将内部表传输到辅助内部表。

ABAP Documentation - "VALUE, Internal Tables" :

If the VALUE operator is used as the source of an assignment to an internal table, it is first initialized after the evaluation of a possible LET expression or is assigned the content of itab. Then, line_spec data is then evaluated and inserted directly into the target table.

In assignments of constructor expressions to an internal table, its existing lines cannot be used directly as an argument in line_spec. This is because this table is deleted before line_spec is evaluated or overwritten by the content of itab. If the entire internal table or lines from the left side are needed on the right side, however, they can be saved in local helper variables using a LET expression, since this expression is evaluated first.