Is Using -1 to Set All Bits of an Unsigned Integer to True the Safest and Most Portable Approach?

Is Using -1 to Set All Bits to True a Safe Approach?

Various programming languages, including C and C , often employ this technique:

unsigned int flags = -1;  // all bits are true

This approach aims to initialize a variable with all bits set to 1. However, its safety and portability remain questionable.

The Pros and Cons of -1, ~0, and 0xffffffff

To answer this question, we must consider the behavior of these three options in various scenarios:

• -1: This value is a constant integer with all bits set to 1, regardless of the underlying integer representation (two's complement, one's complement, or signed magnitude). Hence, it is a straightforward and reliable method.
• ~0: This bitwise NOT operation inverts all bits of 0, resulting in a value with all bits set to 1. However, its behavior can vary depending on the operand type. Using an unsigned int operand, it will produce the correct result, but using a smaller type (e.g., unsigned short) may not yield the desired outcome.
• 0xffffffff: This hexadecimal constant also represents a value with all bits set to 1. Similar to ~0, its portability may be an issue since it assumes a 32-bit integer size.

Recommendation

Based on the analysis above, it is recommended to initialize the flags variable with -1. This approach is the most straightforward and guarantees a consistent result across different integer representations and machine architectures.

As the provided reference explains, the choice of -1 focuses on the value being set rather than the underlying bit patterns. By initializing with -1, we obtain the highest possible value for the unsigned int type, ensuring that all bits are effectively set to true.

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