Cannot return non_const reference to contained value for uninitialized optional variable type.

[robertramey]

I'm having an issue with the following code:

From boost/optional.hpp - lines 1267-1281 (problem lines marked with ~)

// Returns a reference to the value if this is initialized, otherwise,
// the behaviour is UNDEFINED
// No-throw
reference_const_type get() const { BOOST_ASSERT(this->is_initialized()) ; return this->get_impl(); }
~ reference_type get() { BOOST_ASSERT(this->is_initialized()) ; return this->get_impl(); }

/// Returns a copy of the value if this is initialized, 'v' otherwise
reference_const_type get_value_or ( reference_const_type v ) const { return this->is_initialized() ? get() : v ; }
reference_type get_value_or ( reference_type v ) { return this->is_initialized() ? get() : v ; }

/// Returns a pointer to the value if this is initialized, otherwise,
// the behaviour is UNDEFINED
// No-throw
pointer_const_type operator->() const { BOOST_ASSERT(this->is_initialized()) ; return this->get_ptr_impl() ; }
~ pointer_type operator->() { BOOST_ASSERT(this->is_initialized()) ; return this->get_ptr_impl() ; }

The code returns non-const reference and pointer. These used for getting a reference to the contained variable holding the type presumably so it can be modified. (this is my case). BUT, the BOOST_ASSERT traps if the variable isn't initialized. This makes it impossible to modify the contained value. The is necessary in the context of the serialization library.

[akrzemi1]

Can you show a small program that uses optional and suffers from this problem?

[akrzemi1]

The use case you describe is intentionally not supported. It is an incorrect usage of this library to try to get a reference to a non-existing contained value.

Instead, I would recommend first creating the contained value using T's default constructor, and then getting a reference to it:

optional<int> oi;  // no contained value
oi.emplace();      // now containing a default-initialized value
fill_as_you_please(*oi);