How to Reliably Set All Bits of an Unsigned Integer to True?

Ensuring Bits are Set to True: Exploring Options

When initializing an unsigned integer to set all its bits to true, developers often encounter a dilemma. Is it preferable to use -1, 0xffffffff, or ~0?

-1: A Straightforward Solution

Setting the variable to -1 is considered the most direct method. It assigns the highest possible value for an unsigned integer, effectively setting all bits to true. This approach is independent of the sign representation and works consistently.

0xffffffff and ~0: Potential Surprises

Using 0xffffffff or ~0 may lead to unexpected behavior. ~0 inverts all bits of the variable, yielding the desired -1 only when the variable is of unsigned int type. For other types, it may not result in the expected value. For instance, ~0u is necessary for setting all bits of an unsigned long to true.

Furthermore, ~0 on a non-two's complement machine will not yield -1. This can result in initializing the variable to zero instead of the desired highest value.

Conclusion

When the goal is to set all bits of an unsigned integer to true, the most reliable method is to assign -1. This approach is portable and works for all variable types and machine representations.

Avoiding Misconceptions

It's important to clarify that the true/false interpretation is misleading. -1 doesn't necessarily have all bits set to one, and ~0 always sets all bits to one. Instead, the focus should be on the value being set: -1 effectively initializes the variable to the highest possible unsigned value.

The above is the detailed content of How to Reliably Set All Bits of an Unsigned Integer to True?. For more information, please follow other related articles on the PHP Chinese website!