When Does an Explicit `operator bool` Work Without Casting in C ?

Explicit Operator bool Without Casting

When designing a class that has an explicit conversion to bool, one might wonder where it can be used as a boolean without an explicit cast.

Contextual Conversion to bool

The C standard defines specific scenarios where a value can be "contextually converted to bool." These situations fall under four main categories:

1. Statements:

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

2. Expressions:

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

3. Compile-Time Tests:

• static_assert(t);
• noexcept(t)
• explicit(t)
• if constexpr (t)

4. Algorithms and Concepts:

• NullablePointer T
• std::remove_if(...)
• std::sort(...)

Caveats with Const and Non-Const Operators

It's worth noting that a combination of const and non-const conversion operators can lead to confusion. Refer to the related questions below for more information:

• [Why doesn't explicit bool() conversion happen in contextual conversion?](https://stackoverflow.com/questions/5580156/why-doesnt-explicit-bool-conversion-happen-in-contextual-conversion)
• [Why does the explicit operator bool not in effect as expected?](https://stackoverflow.com/questions/26578447/why-does-the-explicit-operator-bool-not-in-effect-as-expected)

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