When can I use explicit operator bool without a cast?

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The standard mentions places where a value may be “contextually converted to bool“. They fall into four main groups:

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)

The conversion operator needs to be constexpr for these.

Algorithms and concepts

  •    NullablePointer T

    Anywhere the Standard requires a type satisfying this concept (e.g. the pointer type of a std::unique_ptr), it may be contextually converted. Also, the return value of a NullablePointer‘s equality and inequality operators must be contextually convertible to bool.

  •    std::remove_if(first, last, [&](auto){ return t; });

    In any algorithm with a template parameter called Predicate or BinaryPredicate, the predicate argument can return a T.

  •    std::sort(first, last, [&](auto){ return t; });

    In any algorithm with a template parameter called Compare, the comparator argument can return a T.

(source1, source2)

Do be aware that a mix of const and non-const conversion operators can cause confusion:

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