optional/doc/18_gotchas.qbk

[section Gotchas]

[section A note about optional<bool>]

`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():
        foo(v);
        // 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));
[endsect]

[section Moved-from `optional`]

When an optional object that contains a value is moved from (is a source of move constructor or assignment) it still contains a value and its contained value is left in a moved-from state. This can be illustrated with the following example.

    optional<std::unique_ptr<int>> opi {std::make_unique<int>(1)};
    optional<std::unique_ptr<int>> opj = std::move(opi);
    assert (opi);
    assert (*opi == nullptr);

Quite a lot of people expect that when an object that contains a value is moved from, its contained value should be destroyed. This is not so, for performance reasons. Current semantics allow the implementation of `boost::optional<T>` to be trivially copyable when `T` is trivial.
[endsect]

[section Mixed relational comparisons]

Because `T` is convertible to `optional<T>` and because `optional<T>` is __STD_LESS_THAN_COMPARABLE__ when `T` is __STD_LESS_THAN_COMPARABLE__,
you can sometimes get an unexpected runtime result where you would rather expect a compiler error:

    optional<double> Flight_plan::weight(); // sometimes no weight can be returned

    bool is_aircraft_too_heavy(Flight_plan const& p)
    {
       return p.weight() > p.aircraft().max_weight(); // compiles!
    }                                                 // returns false when the optional contains no value 

[endsect]

[section False positive with -Wmaybe-uninitialized]

Sometimes on GCC compilers below version 5.1 you may get an -Wmaybe-uninitialized warning when compiling with option -02 on a perfectly valid `boost::optional` usage. For instance in this program:

    #include <boost/optional.hpp>

    boost::optional<int> getitem();

    int main(int argc, const char *[])
    {
      boost::optional<int> a = getitem();
      boost::optional<int> b;

      if (argc > 0)
        b = argc;

      if (a != b)
        return 1;

      return 0;
    }
    
This is a bug in the compiler. As a workaround (provided in [@http://stackoverflow.com/questions/21755206/how-to-get-around-gcc-void-b-4-may-be-used-uninitialized-in-this-funct this Stack Overflow question]) use the following way of initializing an optional containing no value:

    boost::optional<int> b = boost::make_optional(false, int());

This is obviously redundant, but makes the warning disappear.

[endsect]

[endsect]