Is the Safe-Bool Idiom Obsolete in C 11 and Beyond?

Is the Safe-Bool Idiom a Relic of C 11?

The safe-bool idiom, once widely employed in C , has faced scrutiny in light of C 11's introduction of explicit boolean conversion operators. According to the C 11 standard (§4 [conv] p3), expressions can be implicitly converted to a type if a corresponding variable declaration is well-formed. This concept, referred to as "contextual conversion," allows for direct assignment of a value to a boolean variable.

The Safe-Bool Replacement

Prior to C 11, the safe-bool idiom was used to provide explicit control over boolean conversions. It involved defining a member function with an explicit name like is_valid() or is_active() that returned a boolean value. However, with C 11's introduction of explicit operator bool() const;, developers can now define an explicit boolean conversion operator for their classes.

Contextual Conversion in Action

Expressions involving the following language constructs require contextual conversion to boolean:

• if, while, for (§6.4 [stmt.select] p4)
• binary logical operators && and || (§5.14 [expr.log.and/or] p1 for both)
• logical negation operator ! (§5.3.1 [expr.unary.op] p9)
• conditional operator ?: (§5.14 [expr.cond] p1)
• static_assert (§7 [dcl.dcl] p4)
• noexcept (§15.4 [except.spec] p2)

The Answer

The safe-bool idiom is indeed considered obsolete in C 11. The introduction of explicit boolean conversion operators provides a cleaner and more logical approach for controlling boolean conversions. By allowing direct assignment to boolean variables, contextual conversion eliminates the need for the safe-bool idiom and simplifies code readability.

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