SizeOf

│ Deutsch (de) │ English (en) │ suomi (fi) │ русский (ru) │

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
 3type
 4	integerArray = array of integer;
 5
 6function sum(const f: integerArray): int64;
 7{$ifdef CPUx86_64}
 8assembler;
 9{$asmMode intel}
10asm
11	// ensure f is in a particular register
12	mov rsi, f                       // rsi := f  (pointer to an array)
13	
14	// check for nil pointer (i.e. empty array)
15	test rsi, rsi                    // rsi = 0 ?
16	jz @sum_abort                    // if rsi = nil then goto abort
17	
18	// load last index of array [theoretically there is highF]
19	mov rcx, [rsi] - sizeOf(sizeInt) // rcx := (rsi - sizeOf(sizeInt))^
20	
21	// load first element, since loop condition won't reach it
22	{$if sizeOf(integer) = 4}
23	mov eax, [rsi]                   // eax := rsi^
24	{$elseif sizeOf(integer) = 2}
25	mov ax, [rsi]                    // ax := rsi^
26	{$else} {$error unexpected integer size} {$endif}
27	
28	// we're done, if f doesn't contain any more elements
29	test rcx, rcx                    // rcx = 0 ?
30	jz @sum_done                     // if high(f) = 0 then goto done
31	
32@sum_iterate:
33	{$if sizeOf(integer) = 4}
34	mov edx, [rsi + rcx * 4]         // edx := (rsi + 4 * rcx)^
35	{$elseif sizeOf(integer) = 2}
36	mov dx, [rsi + rcx * 2]          // dx := (rsi + 2 * rcx)^
37	{$else} {$error unexpected scale factor} {$endif}
38	
39	add rax, rdx                     // rax := rax + rdx
40	
41	jo @sum_abort                    // if OF then goto abort
42	
43	loop @sum_iterate                // dec(rcx)
44	                                 // if rcx <> 0 then goto iterate
45	
46	jmp @sum_done                    // goto done
47	
48@sum_abort:
49	// load neutral element for addition
50	xor rax, rax                     // rax := 0
51	
52@sum_done:
53end;
54{$else}
55unimplemented;
56begin
57	sum := 0;
58end;
59{$endif}
60
61begin
62	writeLn(sum(integerArray.create(2, 5, 11, 17, 23)));
63end.

With FPC the size of an integer depends on the used compiler mode. However, sizeOf(sizeInt) was inserted for demonstration purposes only. In the {$ifdef CPUx86_64} branch sizeOf(sizeInt) is always 8.

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). Do not forget that dynamic arrays have management data in front of the referenced payload block. So if you really want to know, how much memory has been reserved for one array, you would have to take the high (last index in array) and reference count fields into account, 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.

see also

• article “sizeOf” in the English Wikipedia (primarily about C’s unary operator)