Namespaces
Variants

    cpp/experimental/optional: Difference between revisions

    From cppreference.com
    ← Older editNewer edit →
    +nullopt
    engaged/disengaged is not in the TS. It's "contains a value" now.
    Line 1: Line 1:
    {{cpp/title|n=experimental::|optional}}
    {{cpp/title|n=experimental::|optional}}
    {{cpp/experimental/optional/navbar}}
    {{cpp/experimental/optional/navbar}}
    {{fmbox | style=font-size: 0.8em; background-color:#ffffbb | text=After reviewing national body comments to N3690, this library component was voted out from C++14 working paper into a separate Technical Specification. It is not a part of the draft C++14 as of n3797.}}
    {{fmbox | style=font-size: 0.8em; background-color:#ffffbb | text=This is work in progress, there may be inaccuracies}}
    {{ddcl | header=experimental/optional | since=libfund_ts |
    {{ddcl | header=experimental/optional | since=libfund_ts |
    template< class T >
    template< class T >
    Line 8: Line 6:
    }}
    }}


    The class template {{tt|std::experimental::optional}} manages an ''optional'' contained value, i.e. a value that semantically may not be present.
    The class template {{tt|std::experimental::optional}} manages an ''optional'' contained value, i.e. a value that may not be present.


    A common use case for {{tt|optional}} is the return value of a function that may fail. As opposed to other approaches, such as {{c|std::pair<T,bool>}}, {{tt|optional}} handles expensive to construct objects well and is more readable, as the intent is expressed explicitly.
    A common use case for {{tt|optional}} is the return value of a function that may fail. As opposed to other approaches, such as {{c|std::pair<T,bool>}}, {{tt|optional}} handles expensive to construct objects well and is more readable, as the intent is expressed explicitly.


    The value is guaranteed to be allocated within the {{tt|optional}} object itself, i.e. no dynamic memory allocation ever takes place. Thus, an {{tt|optional}} object models an object, not a pointer, even though the {{lc|operator*()}} and {{lc|operator->()}} are defined.
    {{tt|optional}} not a .


    The value inside an {{tt|optional}} object may be in either an initialized or uninitialized state.  An {{tt|optional}} object with a value in initialized state is called ''engaged'', whereas if the value is in uninitialized state, the object is called ''disengaged''.
    value {{tt|optional}} object an {{tt|optional}} object a , the .


    The {{tt|optional}} object is ''engaged'' on the following conditions:
     
    The {{tt|optional}} object '''' the following conditions:


    * The object is initialized with a value of type {{tt|T}}
    * The object is initialized with a value of type {{tt|T}}


    * The object is assigned an ''engaged'' {{tt|optional}}.
    The object '''' the following conditions:
     
    The object is ''disengaged'' on the following conditions:


    * The object is default-initialized.
    * The object is default-initialized.
     
    * The object is initialized with a value of {{lc|std::experimental::nullopt_t}} or {{tt|optional}} object .
    * The object is initialized with a value of {{lc|std::experimental::nullopt_t}} or a ''disengaged'' {{tt|optional}} object.
    * The object is assigned a value of {{lc|std::experimental::nullopt_t}} or {{tt|optional}}
     
    * The object is assigned a value of {{lc|std::experimental::nullopt_t}} or a ''disengaged'' {{tt|optional}}.


    ===Template parameters===
    ===Template parameters===

    Revision as of 03:14, 16 May 2015

     
    C++
     
    Experimental
    Technical Specification
    Filesystem library (filesystem TS)
    Library fundamentals (library fundamentals TS)
    Library fundamentals 2 (library fundamentals TS v2)
    Library fundamentals 3 (library fundamentals TS v3)
    Extensions for parallelism (parallelism TS)
    Extensions for parallelism 2 (parallelism TS v2)
    Extensions for concurrency (concurrency TS)
    Extensions for concurrency 2 (concurrency TS v2)
    Concepts (concepts TS)
    Ranges (ranges TS)
    Reflection (reflection TS)
    Mathematical special functions (special functions TR)
    Experimental Non-TS
    Pattern Matching
    Linear Algebra
    std::execution
    Contracts
    2D Graphics
     
    Library fundamentals