Re: sizeof error?

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Michael R. Copeland wrote:
> The following code compiles and executes, but it produces incorrect
> results (on VC++6.0), asfaics. The sizeof function that reference both
> the structure (T1) and the declared variable (t) yield 8, whereas the
> components of the structure add up to 6 (2 + 4). Can any explain
> why...or what can be done to make it work? TIA
>
> int main(int argc, char* argv[])
> {
> struct T1
> {
> int int_val;
> short short_val;
> } t = {0};
>
> size_t t1 = sizeof(int); // yields 4
> size_t t2 = sizeof(short); // " 2
> size_t t3 = sizeof(t); // " 8!
> size_t t4 = sizeof(T1); // " 8!!
>
> return 0;
> }
>

The compiler "pads" the structure to align it to memory boundaries. In
this case, it adds an extra two bytes to it, to bring it to a total of 8
instead of 6.

You can use #pragma pack to change the alignment on a
per-structure-definition basis, or /Zp<num> to the compiler to do it on
a project wide basis.

However, you should not mess with that, unless you have a very good
reason to (such as trying to "mirror" a hardware-based data structure in
software), because it can negatively affect performance.

-n
.