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In another question, David Heffernan posted a comment about his "all time least favourite Delphi construct":
SetLength(FItems, Length(FItems)+1);
The construct he dislikes was heavily used in the Pebongo project, such as in this excerpt:
procedure TBSONDocument.ReadStream( F: TStream );
var
len : integer;
elmtype : byte;
elmname : string;
begin
Clear;
f.Read( len, sizeof( len ) );
f.Read( elmtype, sizeof( byte ) );
while elmtype <> BSON_EOF do // Loop
begin
elmname := _ReadString( f );
SetLength( FItems, length( FItems ) + 1 ); // This, spotted by TOndrej
case elmtype of
BSON_ARRAY: FItems[high( FItems )] := TBSONArrayItem.Create;
BSON_BINARY: FItems[high( FItems )] := TBSONBinaryItem.Create;
...
end;
f.Read( elmtype, sizeof( byte ) );
end;
end;
What are the alternatives?
419
This is known as dynamic memory allocation in C programming. To allocate memory dynamically, library functions are malloc() , calloc() , realloc() and free() are used. These functions are defined in the <stdlib. h> header file.
To allocate memory for an array, just multiply the size of each array element by the array dimension. For example: pw = malloc(10 * sizeof(widget)); assigns pw the address of the first widget in storage allocated for an array of 10 widget s.
Because the data is added and removed in a last-in-first-out manner, stack-based memory allocation is very simple and typically much faster than heap-based memory allocation (also known as dynamic memory allocation) e.g. C's malloc .
Space is allocated by calling malloc with the number of bytes needed (for strings this is always one more than the maximum length of the string to be stored): char *pc = malloc(MAXSTR + 1) ; // can hold a string of up to MAXSTR characters.
It's not the SetLength() itself that is bad but incrementing the length in the loop. Example of bad code:
SetLength( Result.FItems, 0 );
for i := 0 to high( FItems ) do
begin
SetLength(Result.FItems, Length(Result.Fitems)+1);
Result.FItems[i] := FItems[i].Clone;
end;
In this case array is rearranged and reallocating memory on each iteration. Your posted example doesn't show bad usage of SetLength()
76
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