Produce the structure of an internal table

If you want to know the structure of internal table dynamically, you can use CL_ABAP_TYPEDESCR=>DESCRIBE_BY_DATA method.
 

TYPE-POOLS: ABAP.
TYPES: BEGIN OF TY_ISEG ,
        IBLNR LIKE ISEG-IBLNR,            "Doc no
        GJAHR LIKE ISEG-GJAHR,            "Year
        ZEILI LIKE ISEG-ZEILI,            "Position
        MATNR LIKE ISEG-MATNR,            "Material
        WERKS LIKE ISEG-WERKS,            "Plant
        USNAD LIKE ISEG-USNAD,            "User
        BUDAT LIKE ISEG-BUDAT,            "Posting Date
        DMBTR LIKE ISEG-DMBTR,            "Posted Amount (Line Item)
        WAERS LIKE ISEG-WAERS,            "Local Currency
        WRTBM LIKE ISEG-WRTBM,            "Doc Total Amt
      END   OF TY_ISEG.


DATA: T_ISEG TYPE STANDARD TABLE OF TY_ISEG,
      V_ISEG TYPE TY_ISEG,
      FNAME  TYPE FIELDNAME.

DATA: L_REF TYPE REF TO CL_ABAP_TABLEDESCR,
      L_TYP TYPE REF TO CL_ABAP_TYPEDESCR.


FIELD-SYMBOLS: &ltKEY_COMP_WA> TYPE ABAP_KEYDESCR,
               &ltFS> TYPE ANY.

V_ISEG-IBLNR = '0100000053'.
V_ISEG-GJAHR = '2005'.
V_ISEG-ZEILI = '001'.
V_ISEG-MATNR = '000000000000000041'.
V_ISEG-WERKS = 'Z1I1'.
V_ISEG-USNAD = 'RSUBBARAYAN'.
V_ISEG-BUDAT = '20050405'.
V_ISEG-DMBTR = 40000.
V_ISEG-WAERS = 'USD'.
V_ISEG-WRTBM = 50000.

APPEND V_ISEG TO T_ISEG.

V_ISEG-IBLNR = '0100000053'.
V_ISEG-GJAHR = '2005'.
V_ISEG-ZEILI = '001'.
V_ISEG-MATNR = '000000000000000041'.
V_ISEG-WERKS = 'Z1I1'.
V_ISEG-USNAD = 'RSUBBARAYAN'.
V_ISEG-BUDAT = '20050405'.
V_ISEG-DMBTR = 40000.
V_ISEG-WAERS = 'USD'.
V_ISEG-WRTBM = 50000.

APPEND V_ISEG TO T_ISEG.

L_REF ?= CL_ABAP_TYPEDESCR=&gtDESCRIBE_BY_DATA( T_ISEG ).

LOOP AT T_ISEG INTO V_ISEG.
  WRITE:/.
  LOOP AT L_REF->KEY ASSIGNING &ltKEY_COMP_WA&gt.    WRITE:/ &ltKEY_COMP_WA&gt-NAME.    CONCATENATE 'V_ISEG-' &ltKEY_COMP_WA&gt-NAME INTO FNAME.
    ASSIGN (FNAME) TO &ltFS&gt.    CHECK SY-SUBRC = 0.
    L_TYP ?= CL_ABAP_TYPEDESCR=>DESCRIBE_BY_DATA_REF( &ltFS> ).
    WRITE: &ltFS>, 50 L_TYP-&gtTYPE_KIND.  ENDLOOP.
ENDLOOP.