…one 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
();
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
);
if
(
oEmpty
==
false
);
if
(
oEmpty
==
true
);
if
(
oFalse
==
none
);
if
(
oFalse
==
false
);
if
(
oFalse
==
true
);
if
(
oTrue
==
none
);
if
(
oTrue
==
false
);
if
(
oTrue
==
true
);
In other words, for optional<>, the following assertion does not
hold:
assert
((
opt
==
false
)
==
(!
opt
));
