如何将32位有符号整数放入64位无符号整数的高32位？

Question

Example:

qint32 si32 = -1; // that gives us 0xFFFFFFFF
quint64 ui64 = si32; // that gives us 0xFFFFFFFFFFFFFFFF, that is expected

Desired result:

0xFFFFFFFF00000000

When I'm trying to shift bits on si32 like this

quint64 ui64 = si32 << 32;

compiler warns me about bit-shifting overflow.

That should be an easy task, but I can't figure out how to do that. That is a unix C++ code (using QT, but that doesn't matter, plain unix C++ code will do).

I'd appreciate your help.

Answer 1

The warning has to do with the size of si32 itself. Basically the sequence of steps looks like this:

1. Take si32 (which is a 32bit value)
2. Shift that value by 32bits (ie shift a 32bit value by 32 bits)
3. Assign it to ui64 .

To fix that, you can first cast si32 to the proper 64bit type, before shifting it by 32 bits so the bit shifting will operate on 64bit values:

quint64 ui64 = ((quint64) si32) << 32;