What Does the C Three-Way Comparison Operator `` Do?

Three-Way Comparison Operator in C

While studying C operators, you may have encountered the enigmatic "<=>" operator. What does this enigmatic symbol represent?

Understanding the Three-Way Comparison Operator

The "<=>" operator, known as the three-way comparison operator, allows for more nuanced comparisons compared to traditional relational operators. According to the P0515 proposal:

• If "a < b," "<=>" returns a value less than 0.
• If "a > b," "<=>" returns a value greater than 0.
• If "a == b," "<=>" returns 0.

Essentially, this operator enables comparisons to determine whether one value is less than, greater than, or equal to another.

Implementation

To implement this operator for your custom type, you need to define the "operator<=>" function and return the appropriate category as follows:

Ordering category: If your type supports comparisons using the "<" operator, you can return an ordering category to efficiently generate the following operators: "<", ">", "<=", ">=", "==", and "!=".

Equality category: If your type only supports equality comparisons, you can return an equality category to efficiently generate the "==" and "!=" operators.

Additionally, you can specify the strength of these operations. If "a == b" implies "f(a) == f(b)" for all "f" that access only publicly accessible comparison-salient state, the operation is strong. Otherwise, it's weak.

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