weird overflow error...

[wildcat_2000]

hi all, could someone tell me why:

Dim x As Integer, a As Byte, b As Byte
a = 0
b = 1
x = a - b

generates an overflow error on last line, while:

Dim x As Integer, a As Byte, b As Byte
a = 0
b = 1
x = a + b

does not. i have to modify first operation into:

Dim x As Integer, a As Byte, b As Byte
a = 0
b = 1
x = a * 1 - b * 1

to make it work. this is really weird.

thanks,

wc.

[EJ12N]

change data type... maybe

[wildcat_2000]

that's the whole point... x IS an integer value.

[si_the_geek]

The problem is that by default all calculatations are performed in the smallest data type that is appropriate, and as both variables are bytes then the calculation is done in bytes.

However, if you look in MSDN for the limits of the byte data type:

Data type   Storage size         Range
Byte           1 byte            0 to 255

and -1 is outside of the valid range for the data type.


The "* 1" you added to make it work fixes this by doing an Implicit data type conversion, an explicit one would be better (as the compiler can change what method it uses rather than doing extra work), eg:

x = CInt(a) - CInt(b)

[wildcat_2000]

agreed on the fact that the overflow is generated by the fact that 0 - 1 = -1 is outside the range of bytes. agreed also on the implicit conversion:

-> that's why i've added the *1 multiplication in the first place (and you are right, better do an explicit conversion).

however, the fact is that x IS AN INTEGER!

i've declared it as such, therefore i still can scarcely understand why this is generating an error. i believe this is a small memory management leak in vb6, assuming to compute on the smallest type even if the user (me) has correctly allowed memory space according on presumed results.

[si_the_geek]

Where the results are going has no effect whatsoever on how the calculation is performed

Even though it is only one line of code, there are several steps that take place "behind the scenes". Essentially the calculation is done first (which is where the error occurs), and then the result is returned to the required location (eg: your x variable).