先来看numpy round:
numpy round reference
For values exactly halfway between rounded decimal values, NumPy rounds to the nearest even value. Thus 1.5 and 2.5 round to 2.0, -0.5 and 0.5 round to 0.0, etc.
可以看到, numpy的round是四舍五入到最近的偶数
c++ round reference

value | round | floor ceil | trunc
------- | -------- | -------- | ------- | --------
2.3 | 2.0 | 2.0 | 3.0 | 2.0
3.8 | 4.0 | 3.0 | 4.0 | 3.0
5.5 | 6.0 | 5.0 | 6.0 | 5.0
-2.3 | -2.0 | -3.0 | -2.0 | -2.0
-3.8 | -4.0 | -4.0 | -3.0 | -3.0
-5.5 | -6.0 | -6.0 | -5.0 | -5.0

看起来c++的操作是不管符号是啥, 统统四舍五入