Today I ran smack into what is easily the nastiest C/C++ gotcha of my entire software engineering career. From the early 90s reading Sam’s Teach Yourself C in 21 Days with a shitty freeware C compiler from a local BBS, through to today, I have been bit by just about every imaginable C and C++ gotcha, but this one takes the cake.

If you see what’s wrong with this code, you’ve been bitten by this before:

UINT32 x = 0x80000000;
UINT32 y = 2;
UINT64 z = x * y;
cwout << L"x*y=" << std::hex << z << std::endl;

If you think this code will output 0x100000000, you’ve not been bitten by this before.

You see, 2 times 0x80000000 is 0x100000000, which is 2^32 in hex. Unfortunately, since both x and y are 32-bit unsigned integers, the result is a 32-bit unsigned integer as well, implicitly cast to a 64-bit unsigned integer only after the computation is performed.