Is Using -1 to Set All Bits to True in C/C a Reliable and Portable Approach?

Using -1 to Set All Bits to True: A Portable Approach

In C and C , the expression unsigned int flags = -1 has often been employed to initialize an unsigned integer with all bits set to true. However, the question remains whether this method is a reliable and portable approach.

Advantages of -1

The recommended approach is to use -1 for initialization, as it offers several advantages:

• Independence from Sign Representation: -1 represents the most negative value for a signed integer, regardless of its sign representation (e.g., two's complement or one's complement). This ensures consistent behavior across different machines.
• Straightforward and Explicit: The use of -1 is clear and unambiguous in its intent, setting all bits to true.

Potential Pitfalls of ~0 and Other Expressions

While ~0 may seem like a suitable alternative, it can lead to unexpected results:

• Type Dependency: ~ operator performs bitwise complement on the operand's type. For example, ~0u results in -1, but ~0 (without the u suffix) may yield different values based on the operand's type.
• Platform-dependent Behavior: The bitwise complement behavior can vary in non-two's complement representations.

Conclusion

Setting all bits to true using -1 is the most reliable and portable approach. It is independent of the sign representation, easy to understand, and ensures consistent behavior across different platforms. Therefore, it is highly recommended to use -1 for this purpose, as it provides the most predictable and robust solution.

The above is the detailed content of Is Using -1 to Set All Bits to True in C/C a Reliable and Portable Approach?. For more information, please follow other related articles on the PHP Chinese website!