Option base 0

[stevech]

I'm probably missing a concept here, but below, txPayload is supposed to be a pointer to a byte array with the first element being 0

Code: Select all

Option Base 0
...
public txPayload() as byte byref
...
txPayload(0) = x

Compiler says invalid index in the above if 0 is used, but 1 yields no error.
The value of txPayload varies at run-time.

[dkinzer]

txPayload is supposed to be a pointer to a byte array with the first element being 0

The Option Base directive only affects the number of elements that are allocated for an array whose lower bound is not specified when the array is defined. In all other cases, the lower bound of an array of elements is taken to be 1.

Consider this ZBasic code:

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Public txPayload() as Byte ByRef
Dim array1(0 to 19) as Byte
Dim array2(-5 to 14) as Byte
Dim x as Byte

Sub Main()
    txPayload.DataAddress = array1.DataAddress
    txPayload(1) = x
    txPayload.DataAddress = array2.DataAddress
    txPayload(1) = x
End Sub

The C code generated for this is:

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static uint8_t mzv_array1[20];
static uint8_t mzv_array2[20];
static uint8_t mzv_x;

uint8_t *zv_txPayload;

void
zf_Main(void)
{
    zv_txPayload = (uint8_t *)(uint16_t)mzv_array1;
    zv_txPayload[1 - 1] = mzv_x;
    zv_txPayload = (uint8_t *)(uint16_t)mzv_array2;
    zv_txPayload[1 - 1] = mzv_x;
}

You can see, then, that referring to element(1) accesses the first element of the array irrespective of the bounds used to define it.

[stevech]

I see. I was confused because the manual says Option Base 0 controls whether array elements index beginning with 0 vs. 1. That's how I read

The Option Base directive is provided to allow you
to specify that the default array base is either 0 or 1...

I recall in VB6, Option Base 0 meant 0 is the index of the first element. So the ByRef (pointer) would use the same convention (base 0), it seemed to me.

It's still a bit fuzzy.