Is this static_cast valid/legit?

Question

double diff = static_cast<int64_t>(a- b);

a and b are of type int64_t.

I saw this code in our project. I think it is suspicious, but I am really not sure. I am familiar with static_cast, and I would not write code like this.

Is this static_cast valid/legit? Is it useful?

It does seem odd to cast something to an `int64` only to assign it to a `double`. Maybe not "wrong", but "odd". Does your compiler throw any warnings when you use `-Wall -pedantic`? And now that we know `a` and `b` are already of byte `int64_t`, another cast ought to do nothing... Looks like some "left over" code that should have been cleaned out a long time ago. — Floris, Oct 07 '13 at 19:42
The `static_cast(value)` notation is C++ only. It is invalid in C. Retagging. — Jonathan Leffler, Oct 07 '13 at 19:43
If `a` and `b` are already `int64_t`s then the `static_cast` is indeed pointless. — Simple, Oct 07 '13 at 19:49
If your compiler has an `int` larger than 64 bit, integral promotion is applied to `a` and `b`, converting them to `int`s. The `static_cast` then would narrow the result down to the range of `int64_t`, invoking UB (signed integer over-/underflow). — dyp, Oct 07 '13 at 19:52
@DyP: I suppose that's possible, but I've never heard of a system having a native `int` type wider than 64 bits. It's much more likely that the author of this code intended to write `static_cast` rather than guard against an out-of-range error on such an arcane system. — Adam Rosenfield, Oct 07 '13 at 19:55

score 1 · Answer 1 · answered Oct 07 '13 at 19:51

1

If a and b are both int64_t, there is no point to the cast. It is casting a result of type int64_t to type int64_t. It would be like doing this:

int a = 10, b = 5;
double c = (int)(a - b); // the cast is not needed, but also not "harmful"

answered Oct 07 '13 at 19:51

Zac Howland

15,777
1
26
42

score 0 · Accepted Answer · answered Oct 07 '13 at 20:35

This cast is valid but does not do anything. I agree that it is suspicious. You should carefully review what the code is supposed to be doing, and if it's correct then you may want to re-write it so that the intent is clearer and so that it doesn't look as suspicious.

Is this static_cast valid/legit?

2 Answers2