What does binary plus operator do to int Array in C

Question

I am porting an obscure library form C to dart.

a property is defined on a struct like this unsigned char payload[256];

I interpret that as an array of chars. So I converted it to List<int> payload;

later in the original library there is this code

parser->handleDataValue( extendedCodeLevel, code, numBytes, parser->payload+i, parser->customData );

where 'i' is an index for a loop

I translated that to

parser.handleDataValue(extendedCodeLevel, code, numBytes, parser.payload + i, parser.customData);`

Now I am dealing with the error The argument type 'int' can't be assigned to the parameter type 'List<int>'.

I understand the dart side of the problem but I don't understand what the original C means to write its dart equivalent.

It performs pointer arithmetic. The other way to write it in C would be &(parser->payload[i]) — UnholySheep
– UnholySheep, Commented Dec 2, 2021 at 10:38
For any array or pointer p and index i, the expression p[i] is exactly equal to *(p + i). From this can be deduced that p + i is a pointer to the element at index i (i.e. &p[i]). — Some programmer dude
– Some programmer dude, Commented Dec 2, 2021 at 10:38
@Someprogrammerdude ah, so does that mean that it is pointing to the next element in the payload array? — xerotolerant
– xerotolerant, Commented Dec 2, 2021 at 10:40
@xerotolerant: No. Adding 1 points to the next element. Adding i points to the element displaced i elements from the starting point. — Eric Postpischil
– Eric Postpischil, Commented Dec 2, 2021 at 10:45

lrn · Accepted Answer · 2021-12-03 10:14:51Z

1

I'd probably translate unsigned char payload[256]; in C to Uint8List payload; in Dart. That's an actual list of unsigned "char"s.

The pointer arithmetic can then be emulated by Uint8List.sublistView(parser.payload, i), which creates a view of the data of payload starting from the ith value, without copying, just like the C code.

Q.v. Uint8List.sublistView.

answered Dec 3, 2021 at 10:14

lrn

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xerotolerant Over a year ago

Than you. that's exactly what the code was doing. In ended up with a much hacker solution but I'll change to this as It makes the most. sense

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