C++ 两个无符号长整数相乘的错误结果

Question

Consider the multiplication of the following unsigned long long integers:

unsigned long long a=4294967297;
unsigned long long b=4294967297;
cout<<a*b<<endl;

8589934593

I did not expect this answer. Instead I was expecting to get:

18446744065119617025

Note that, according to this source , the largest integer that can be represented by an unsigned long long is 18446744073709551615 (or in hexadecimal 0xffffffffffffffff ). This is by 8589934590 larger than my expected 18446744065119617025 , so that I was assuming this number could still be represented.

What is going on and how to resolve this issue?

EDIT:

You guys are right, I was playing around with different numbers and confused the results of 4294967297 and 4294967295 squared. With 7 at the end it does overflow. Thanks for pointing this out.

Answer 1

I think you're doing your math wrong:

slowpy3.8> 4294967297 * 4294967297
18446744082299486209
slowpy3.8> hex(_)
'0x10000000200000001'
slowpy3.8> 0xffffffffffffffff > 0x10000000200000001
False
slowpy3.8> math.log2(18446744082299486209)
64.0000000006718
slowpy3.8>