When Can You Contextually Convert a Class to bool Without an Explicit Cast?

Contextual Conversion of Explicit Operator bool Without a Cast

When designing a class, you may encounter situations where you need to explicitly convert it to a bool type. However, there are instances where you can leverage contextual conversion to utilize your type as a bool without explicitly casting it.

The C Standard specifies various scenarios where values can be "contextually converted to bool." These fall into four primary categories:

Statements:

• if (t) /* statement */
• for (;t;) /* statement */
• while (t) /* statement */
• do { /* block */ } while (t);

Expressions:

• !t
• t && t2
• t || t2
• t ? "true" : "false"

Compile-Time Tests:

• static_assert(t);
• noexcept(t)
• explicit(t)
• if constexpr (t) (conversion operator must be constexpr)

Algorithms and Concepts:

• NullablePointer T: T can be contextually converted in contexts where that concept is required.
• Predicate or BinaryPredicate Arguments in algorithms: T can be returned as the predicate.
• Compare Argument in algorithms: T can be returned as the comparator.

Cautions:

Be mindful of mixed const and non-const conversion operators as they can introduce complexities and unexpected behavior. For further understanding, refer to resources like "Why doesn't explicit bool() conversion happen in contextual conversion?" and "Why does the explicit operator bool not in effect as expected?"

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