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:

 1 program sizeOfDemo(input, output, stderr);
 2 
 3 {$typedAddress on}
 4 
 5 uses
 6 	heaptrc;
 7 
 8 type
 9 	s = record
10 		c: char;
11 		i: longint;
12 	end;
13 
14 var
15 	x: ^s;
16 
17 begin
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));
32 end.

Direct handling of structured data types in assembly language requires awareness of data sizes, too:

 1 program sizeOfDemo(input, output, stderr);
 2 
 3 type
 4 	integerArray = array of integer;
 5 
 6 function sum(const f: integerArray): int64;
 7 {$ifdef CPUx86_64}
 8 assembler;
 9 {$asmMode intel}
10 asm
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:
53 end;
54 {$else}
55 unimplemented;
56 begin
57 	sum := 0;
58 end;
59 {$endif}
60 
61 begin
62 	writeLn(sum(integerArray.create(2, 5, 11, 17, 23)));
63 end.

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.

 1 program dynamicArraySizeDemo(input, output, stderr);
 2 
 3 uses
 4 	sysUtils;
 5 
 6 resourcestring
 7 	enteredN = 'You''ve entered %0:d integers';
 8 	totalData = 'occupying a total of %0:d Bytes.';
 9 
10 var
11 	f: array of longint;
12 
13 begin
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])]));
24 end.

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