Why swap with xor works fine in c++ but in java do

问题:

Possible Duplicate:
Why is this statement not working in java x ^= y ^= x ^= y;

Sample code

int a=3;
int b=4;
a^=(b^=(a^=b));

In c++ it swaps variables, but in java we get a=0, b=4 why?

回答1:

By writing your swap all in one statement, you are relying on side effects of the inner a^=b expression relative to the outer a^=(...) expression. Your Java and C++ compilers are doing things differently.

In order to do the xor swap properly, you have to use at least two statements:

a ^= b; 
a ^= (b ^= a);

However, the best way to swap variables is to do it the mundane way with a temporary variable, and let the compiler choose the best way to actually do it:

int t = a;
a = b;
b = t;

In the best case, the compiler will generate no code at all for the above swap, and will simply start treating the registers that hold a and b the other way around. You can't write any tricky xor code that beats no code at all.

回答2:

That's not guaranteed to work in C++ either. It's undefined behavior.

You should do it in three separate statements:

a ^= b; 
b ^= a;
a ^= b;