Boost C++ Libraries of the most highly regarded and expertly designed C++ library projects in the world. Herb Sutter and Andrei Alexandrescu, C++ Coding Standards



optional<bool> should be used with special caution and consideration.

First, it is functionally similar to a tristate boolean (false, maybe, true) —such as boost::tribool— except that in a tristate boolean, the maybe state represents a valid value, unlike the corresponding state of an uninitialized optional<bool>. It should be carefully considered if an optional<bool> instead of a tribool is really needed.

Second, although optional<> provides a contextual conversion to bool in C++11, this falls back to an implicit conversion on older compilers. This conversion refers to the initialization state and not to the contained value. Using optional<bool> can lead to subtle errors due to the implicit bool conversion:

void foo ( bool v ) ;
void bar()
    optional<bool> v = try();

    // The following intended to pass the value of 'v' to foo():
    // But instead, the initialization state is passed
    // due to a typo: it should have been foo(*v).

The only implicit conversion is to bool, and it is safe in the sense that typical integral promotions don't apply (i.e. if foo() takes an int instead, it won't compile).

Third, mixed comparisons with bool work differently than similar mixed comparisons between pointers and bool, so the results might surprise you:

optional<bool> oEmpty(none), oTrue(true), oFalse(false);

if (oEmpty == none);  // renders true
if (oEmpty == false); // renders false!
if (oEmpty == true);  // renders false!

if (oFalse == none);  // renders false
if (oFalse == false); // renders true!
if (oFalse == true);  // renders false

if (oTrue == none);   // renders false
if (oTrue == false);  // renders false
if (oTrue == true);   // renders true

In other words, for optional<>, the following assertion does not hold:

assert((opt == false) == (!opt));