Question

which of the two is faster: ?

1.

char* _pos ..;
short value = ..;

*((short*)_pos = va;

2.

char* _pos ..;
short value = ..;

memcpy(_pos, &value, sizeof(short));

Answer 1

As with all “which is faster?” questions, you should benchmark it to see for yourself. And if it matters, then ask why and pick which you want.

In any case, your first example is technically undefined behavior since you are violating strict-aliasing. So if you had to choose without benchmarking, go with the second one.

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To answer the actual question, which is faster will probably depend on the alignment of pos. If it’s aligned properly, then 1 will probably be faster. If not, then 2 might be faster depending on how it’s optimized by the compiler. (1 might even crash if the hardware doesn’t support misaligned access.)

But this is all guess-work. You really need to benchmark it to know for sure.
At the very least, you should look at the compiled assembly:

:     *(short *)_pos = value;

mov WORD PTR [rcx], dx

vs.

:     memcpy(_pos, &value, sizeof(short));

mov WORD PTR [rcx], dx

Which in this case (in MSVC) shows the exact same assembly with default optimizations. So you can expect the performance to be the same.