SizeOf
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The compile-time function sizeOf
evaluates to the size in Bytes of a given data type name or variable identifier.
sizeOf
can appear in compile-time expressions, inside compiler directives, too.
usage
sizeOf
is especially encountered in assembly language or when doing manual allocation of memory:
1program sizeOfDemo(input, output, stderr);
2
3{$typedAddress on}
4
5uses
6 heaptrc;
7
8type
9 s = record
10 c: char;
11 i: longint;
12 end;
13
14var
15 x: ^s;
16
17begin
18 returnNilIfGrowHeapFails := true;
19
20 getMem(x, sizeOf(x));
21
22 if not assigned(x) then
23 begin
24 writeLn(stderr, 'malloc for x failed');
25 halt(1);
26 end;
27
28 x^.c := 'r';
29 x^.i := -42;
30
31 freeMem(x, sizeOf(x));
32end.
Direct handling of structured data types in assembly language requires awareness of data sizes, too:
1program sizeOfDemo(input, output, stderr);
2
3function sum(const f: array of integer): int64;
4{$ifdef CPUx86_64}
5assembler;
6{$asmMode intel}
7asm
8 // ensure f is in a particular register
9 mov rsi, f // rsi := f (pointer to an array)
10
11 // check for nil pointer (i.e. empty array)
12 test rsi, rsi // rsi = 0 ?
13 jz @sum_abort // if rsi = nil then goto abort
14
15 // load last index of array [theoretically there is highF]
16 mov rcx, [rsi] - sizeOf(sizeInt) // rcx := (rsi - sizeOf(sizeInt))^
17
18 // load first element, since loop condition won't reach it
19 {$if sizeOf(integer) = 4}
20 mov eax, [rsi] // edx := rsi^
21 {$elseif sizeOf(integer) = 2}
22 mov ax, [rsi] // ax := rsi^
23 {$else} {$error unexpected integer size} {$endif}
24
25@sum_iterate:
26 {$if sizeOf(integer) = 4}
27 mov edx, [rsi + rcx * 4] // rdx := (rsi + 4 * (rcx-1))^
28 {$elseif sizeOf(integer) = 2}
29 mov dx, [rsi + rcx * 2] // dx := (rsi + 2 * (rcx-1))^
30 {$else} {$error unexpected scale factor} {$endif}
31
32 add rax, rdx // rax := rax + rdx
33
34 jo @sum_abort // if OF then goto abort
35
36 loop @sum_iterate // dec(rcx)
37 // if rcx <> 0 then goto iterate
38
39 // TODO: j @done somehow compiles as jo @done
40 jno @sum_done // goto done
41
42@sum_abort:
43 // load neutral element for addition
44 xor rax, rax // rax := 0
45
46@sum_done:
47end;
48{$else}
49unimplemented;
50begin
51 sum := 0;
52end;
53{$endif}
54
55var
56 f: array of integer;
57
58begin
59 setLength(f, 5);
60 f[0] := 2;
61 f[1] := 5;
62 f[2] := 11;
63 f[3] := 17;
64 f[4] := 23;
65 writeLn(sum(f));
66end.
comparative remarks
dynamic arrays and alike
Since dynamic arrays are realized as pointers to a block on the heap, sizeOf
evaluates to the pointer's size.
In order to determine the array's size – of its data – sizeOf
has to be used in conjunction with the function length
.
1program dynamicArraySizeDemo(input, output, stderr);
2
3uses
4 sysUtils;
5
6resourcestring
7 enteredN = 'You''ve entered %0:d integers';
8 totalData = 'occupying a total of %0:d Bytes.';
9
10var
11 f: array of longint;
12
13begin
14 setLength(f, 0);
15
16 while not eof() do
17 begin
18 setLength(f, length(f) + 1);
19 readLn(f[length(f)]);
20 end;
21
22 writeLn(format(enteredN, [length(f)]));
23 writeLn(format(totalData, [length(f) * sizeOf(f[0])]));
24end.
The approach is the same for ANSI strings (depending on the {$longstrings}
compiler switch state possibly denoted by string
, too).
classes
Classes as well are pointers.
The class TObject
provides the function instanceSize
.
It returns an object's size as it is determined by the class's type definition.
Additional memory that's allocated by constructors or any method, is not taken into account.
Note, that classes might contain dynamic arrays or ANSI strings, too.