boost/optional.hpp must be included for boost::optional. make_optional creates an optional object In addition, a program is ill-formed if it instantiates an optional with the tag types std::nullopt_t or std::in_place_t. exchanges the contents Example21.1. (function) Boost.Optional speaks about initialized and uninitialized objects hence, the name of the member function is_initialized(). nullopt_t indicator of optional type with uninitialized state (class) (C++17)
P0798R0: Monadic operations for std::optional.
With is_initialized() you can check whether an object of type boost::optional is not empty. bad_optional_access exception indicating checked access to an optional that doesn't contain a value, bad_optional_access exception indicating checked access to an optional that doesn't contain a value.
(class) The type of the variable explicitly states that destructor (public member function) Various helper functions of Boost.Optional. operator!=
This constructor does not participate in overload resolution unless std::is_move_constructible_v is true. Helper classes operator<= (function template) (function) The optional object contains a value in the following conditions: * The object is initialized with/assigned from a value of type T or another optional that contains a value. When an object of type optional is contextually_converted_to_bool, the conversion returns true if the object contains a value and false if it does not contain a value. operator> Modifiers operator<= (function template) If get_even_random_number() generates an even random number, the value is returned directly, automatically wrapped in an object of type boost::optional, because boost::optional provides a non-exclusive constructor. (C++17) This class provides a special constructor that takes a condition as the first parameter. (class) destroys any contained value operator< compares optional objects
Defined in header operator!= Non-member functions Example21.1 illustrates how optional return values are usually implemented without Boost.Optional. accesses the contained value
(function) (C++17)
The following is an example of a function that returns a value, if it exists, or null optional: Since C++ 11 allows the movement of variables, the below demonstrates what happens when std::optional is moved: This example portrays how std::move does not change the state of optional if the contained value is primitive, as (C++17) For objects like std::string, it actually moves the value (and empties the operator< compares optional objects
If get_even_random_number() does not generate an even random number, an empty object of type boost::optional is returned.
(C++17) In order to eliminate all doubt, I would suggest to std::optional::reset after moving an optional object. Dmitry Danilov swap (public member function)
(class template specialization) reset (public member function) raw pointer, when used solely to express that a value is indeed optional. operator>= operator* If the condition is true, an object of type boost::optional is initialized with the second parameter. The reset would call the destructor of the contained object (if the destructor exists) and set the has_value flag to false. Many functions use special values like -1 to denote that no result can be returned. (function template) Helper classes
template< class T > (since C++17) std::swap(std::optional) specializes the std::swap algorithm bad_optional_access exception indicating checked access to an optional that doesn't contain a value, operator== std::hash specializes the std::hash algorithm There are no optional references; a program is ill-formed if it instantiates an optional with a reference type. Thus, get_even_random_number() cant guarantee that an even random number is returned. value_or (public member function)
(C++17) class optional; The class template std::optional manages an optional contained value, i.e. Example21.3 uses this constructor in the function get_even_random_number().
(C++17)
(C++17) ddanilov.me, Dockerized build environments for C/C++ projects, How I solved a bug by disabling C++ extensions, Implementations of std::async and how they might Affect Applications. Special values to denote optional return values, Example21.2.
If it isnt empty, the number stored in i is accessed with operator*. If you want a default value to be returned when boost::optional is empty, you can call boost::get_optional_value_or(). (C++17) A common use case for optional is the return value of a function that may fail. make_optional creates an optional object The behavior is perfectly legal in terms of the standard, but can seem illogical from the users perspective, who would naturally expect the optional to have no value if it has been moved. You signed in with another tab or window.
main() checks whether i is empty. operator> cppreference.comLicensed under the Creative Commons Attribution-ShareAlike Unported License v3.0. operator!= std::swap(std::optional) specializes the std::swap algorithm
operator<= (function template) In Example21.2 the return value of get_even_random_number() has a new type, boost::optional.
Helper classes Command to display std::optional manual in Linux: $ man 3 std::optional. empty: a moved-from optional still contains a value, but the value itself is moved from. (class template specialization) * The object is initialized with/assigned from a value of type std::nullopt_t or an optional object that does not contain a value. (C++17) For example, instead of writing this code: The interface is the same as std::optional, but the following member functions are also defined. This paper proposes adding map, and_then, and or_else to std::optional. Modifiers If other does not contain a
reset (public member function) boost::make_optional() can be called to create an object of type boost::optional. swap (public member function) C++11/14/17 std::optional with functional-style extensions and reference support. In this example, -1 means that no even random number could be generated. This work is published from: United Kingdom. operator== std::hash specializes the std::hash algorithm operator_bool (public member function) (class template specialization) bad_optional_access exception indicating checked access to an optional that doesn't contain a value, std::hash specializes the std::hash algorithm (C++17) destroys any contained value (C++17) (C++17) (class) swap (public member function)
assigns contents std::swap(std::optional) specializes the std::swap algorithm checks whether the object contains a value Thus, an optional object models an object, not a pointer, even though operator*() and operator->() are defined. operator-> (public member function) nullopt_t indicator of optional type with uninitialized state
reset (public member function) T - the type of the value to manage initialization state for. value (public member function) (C++17)
a contained variable is optional and it is stored by value. a value that may or may not be nullopt_t indicator of optional type with uninitialized state value_or (public member function) cppreference.com provides a clear explanation of the behavior: Move constructor: If other contains a value, initializes the contained value as if direct-initializing (but not
Non-member functions As opposed to other approaches, such as std::pair, optional handles expensive-to-construct objects well and is more readable, as the intent is expressed explicitly.
emplace (public member function) It is called get_value_or(). (C++17) If the generated random number is odd, -1 is returned. It is a trivial constructor if std::is_trivially_move_constructible_v is true. The following behavior-changing defect reports were applied retroactively to previously published C++ standards. (C++17) bad_optional_access exception indicating checked access to an optional that doesn't contain a value, accesses the contained value 2022 bad_optional_access exception indicating checked access to an optional that doesn't contain a value, exchanges the contents
value, constructs an object that does not contain a value. destroys the contained value, if there is one (function template) has_value This implementation provides a number of utilities to make coding with optional cleaner. constructs the contained value in-place
std::swap(std::optional) specializes the std::swap algorithm (C++17) operator!= returns the contained value if available, another value otherwise constructor (public member function)
std::hash specializes the std::hash algorithm (class template specialization) Other useful member functions of, Example21.4. returns the contained value if available, another value otherwise Optional return values with, Example21.3.
(class) operator>= returns the contained value http://en.cppreference.com/w/cpp/utility/optional/optional, http://en.cppreference.com/w/cpp/utility/optional/optional, value with which to initialize the contained value, arguments with which to initialize the contained value, initializer list with which to initialize the contained value, copy/move constructors may not be trivial even if underlying constructor is trivial. nullopt_t indicator of optional type with uninitialized state The library Boost.Optional provides the class boost::optional, which can be used for optional return values. constructs the contained value in-place The return value is created with a call to the default constructor. value (public member function) * The member function reset() is called. present[1]. Observers The return value is optional.
boost::optional appears to work like a pointer. boost::optional is a template that must be instantiated with the actual type of the return value. operator< compares optional objects (C++17) Helper classes (C++17) operator< compares optional objects
operator<= (function template) These are return values from functions that may not always return a result.
(C++17) If std::rand() generates an even random number, that number is returned by get_even_random_number(). returns the contained value If an optional contains a value, the value is guaranteed to be allocated as part of the optional object footprint, i.e. Example21.3 introduces other useful member functions of boost::optional. For example, the member function find() of the class std::string returns the special value std::string::npos if a substring cant be found. (C++17) make_optional creates an optional object operator-> (public member function) checks whether the object contains a value In addition to those member functions, optional references are also supported: Assignment has rebind semantics rather than assign-through semantics: This library also serves as an implementation of WG21 standards paper P0798R0: Monadic operations for std::optional. If the condition is false, an empty object of type boost::optional is created. Any instance of optional at any given point in time either contains a value or does not contain a value. The class template std::optional manages an optional contained value, i.e. destroys any contained value Functions whose return value is a pointer often return 0 to indicate that no result exists.
Boost.Optional provides boost::optional, which makes it possible to clearly mark optional return values. This commit does not belong to any branch on this repository, and may belong to a fork outside of the repository.
emplace (public member function) However, you should not think of boost::optional as a pointer because, for example, values in boost::optional are copied by the copy constructor while a pointer does not copy the value it points to. (function template)
operator>
Along with boost/optional/optional_io.hpp, Boost.Optional provides a header file with overloaded stream operators, which let you write objects of type boost::optional to, for example, standard output. nullopt_t indicator of optional type with uninitialized state The member function get() is equivalent to operator*. no dynamic memory allocation ever takes place. string contained in s1) but s1 still has_value. std::hash specializes the std::hash algorithm
operator== operator>= direct-list-initializing) an object of type T with the expression std::move(*other) and does not make other (C++17) The function boost::get_optional_value_or() is also provided as a member function of boost::optional. (class) The object does not contain a value in the following conditions: * The object is default-initialized. operator>=, make_optional creates an optional object (C++17) Special thanks to Petar Ivanov for the idea and Rina Volovich for editing. It is a great alternative for std::unique_ptr, or bad_optional_access exception indicating checked access to an optional that doesn't contain a value, nullopt_t indicator of optional type with uninitialized state operator_bool (public member function) a value that may or may not be present. The following visuals illustrate the various states of std::optional discussed thus far. It does this in a rather naive fashion by calling the function std::rand() from the standard library.
std::hash specializes the std::hash algorithm operator* (C++17) has_value
The type must meet the requirements of Destructible, constructs the optional object exchanges the contents operator== operator> (C++17) (function) (class template specialization) To the extent possible under law, Sy Brand has waived all copyright and related or neighboring rights to the optional library.
move for primitives just copies the values. Boost.Optional provides free-standing helper functions such as boost::make_optional() and boost::get_optional_value_or() (see Example21.4). Explicit types are for clarity. (function template) std::optional is the preferred way to represent an object which may or may not have a value. Non-member functions (function)
Example21.1 uses the function get_even_random_number(), which should return an even random number.
nullopt_t indicator of optional type with uninitialized state Unfortunately, chaining together many computations which may or may not produce a value can be verbose, as empty-checking code will be mixed in with the actual programming logic. Single header implementation of std::optional with functional-style extensions and support for references. (class) Helper classes Alternatively, an optional of a std::reference_wrapper of type T may be used to hold a reference. emplace (public member function) bad_optional_access exception indicating checked access to an optional that doesn't contain a value, std::swap(std::optional) specializes the std::swap algorithm constructs the contained value in-place (class template specialization) operator= (public member function)
P0798R0: Monadic operations for std::optional.
With is_initialized() you can check whether an object of type boost::optional is not empty. bad_optional_access exception indicating checked access to an optional that doesn't contain a value, bad_optional_access exception indicating checked access to an optional that doesn't contain a value.
(class) The type of the variable explicitly states that destructor (public member function) Various helper functions of Boost.Optional. operator!=
This constructor does not participate in overload resolution unless std::is_move_constructible_v is true. Helper classes operator<= (function template) (function) The optional object contains a value in the following conditions: * The object is initialized with/assigned from a value of type T or another optional that contains a value. When an object of type optional
Defined in header
(function) (C++17)
The following is an example of a function that returns a value, if it exists, or null optional: Since C++ 11 allows the movement of variables, the below demonstrates what happens when std::optional is moved: This example portrays how std::move does not change the state of optional if the contained value is primitive, as (C++17) For objects like std::string, it actually moves the value (and empties the operator< compares optional objects
If get_even_random_number() does not generate an even random number, an empty object of type boost::optional(C++17) In order to eliminate all doubt, I would suggest to std::optional::reset after moving an optional object. Dmitry Danilov swap (public member function)
(class template specialization) reset (public member function) raw pointer, when used solely to express that a value is indeed optional. operator>= operator* If the condition is true, an object of type boost::optional is initialized with the second parameter. The reset would call the destructor of the contained object (if the destructor exists) and set the has_value flag to false. Many functions use special values like -1 to denote that no result can be returned. (function template) Helper classes
template< class T > (since C++17) std::swap(std::optional) specializes the std::swap algorithm bad_optional_access exception indicating checked access to an optional that doesn't contain a value, operator== std::hash
(C++17) class optional; The class template std::optional manages an optional contained value, i.e. Example21.3 uses this constructor in the function get_even_random_number().
(C++17)
(C++17) ddanilov.me, Dockerized build environments for C/C++ projects, How I solved a bug by disabling C++ extensions, Implementations of std::async and how they might Affect Applications. Special values to denote optional return values, Example21.2.
If it isnt empty, the number stored in i is accessed with operator*. If you want a default value to be returned when boost::optional is empty, you can call boost::get_optional_value_or(). (C++17) A common use case for optional is the return value of a function that may fail. make_optional creates an optional object The behavior is perfectly legal in terms of the standard, but can seem illogical from the users perspective, who would naturally expect the optional to have no value if it has been moved. You signed in with another tab or window.
main() checks whether i is empty. operator> cppreference.comLicensed under the Creative Commons Attribution-ShareAlike Unported License v3.0. operator!= std::swap(std::optional) specializes the std::swap algorithm
operator<= (function template) In Example21.2 the return value of get_even_random_number() has a new type, boost::optional
Helper classes Command to display std::optional manual in Linux: $ man 3 std::optional. empty: a moved-from optional still contains a value, but the value itself is moved from. (class template specialization) * The object is initialized with/assigned from a value of type std::nullopt_t or an optional object that does not contain a value. (C++17) For example, instead of writing this code: The interface is the same as std::optional, but the following member functions are also defined. This paper proposes adding map, and_then, and or_else to std::optional. Modifiers If other does not contain a
reset (public member function) boost::make_optional() can be called to create an object of type boost::optional. swap (public member function) C++11/14/17 std::optional with functional-style extensions and reference support. In this example, -1 means that no even random number could be generated. This work is published from: United Kingdom. operator== std::hash
assigns contents std::swap(std::optional) specializes the std::swap algorithm checks whether the object contains a value Thus, an optional object models an object, not a pointer, even though operator*() and operator->() are defined. operator-> (public member function) nullopt_t indicator of optional type with uninitialized state
reset (public member function) T - the type of the value to manage initialization state for. value (public member function) (C++17)
a contained variable is optional and it is stored by value. a value that may or may not be nullopt_t indicator of optional type with uninitialized state value_or (public member function) cppreference.com provides a clear explanation of the behavior: Move constructor: If other contains a value, initializes the contained value as if direct-initializing (but not
Non-member functions As opposed to other approaches, such as std::pair
emplace (public member function) It is called get_value_or(). (C++17) If the generated random number is odd, -1 is returned. It is a trivial constructor if std::is_trivially_move_constructible_v is true. The following behavior-changing defect reports were applied retroactively to previously published C++ standards. (C++17) bad_optional_access exception indicating checked access to an optional that doesn't contain a value, accesses the contained value 2022 bad_optional_access exception indicating checked access to an optional that doesn't contain a value, exchanges the contents
value, constructs an object that does not contain a value. destroys the contained value, if there is one (function template) has_value This implementation provides a number of utilities to make coding with optional cleaner. constructs the contained value in-place
std::swap(std::optional) specializes the std::swap algorithm (C++17) operator!= returns the contained value if available, another value otherwise constructor (public member function)
std::hash
(class) operator>= returns the contained value http://en.cppreference.com/w/cpp/utility/optional/optional, http://en.cppreference.com/w/cpp/utility/optional/optional, value with which to initialize the contained value, arguments with which to initialize the contained value, initializer list with which to initialize the contained value, copy/move constructors may not be trivial even if underlying constructor is trivial. nullopt_t indicator of optional type with uninitialized state The library Boost.Optional provides the class boost::optional, which can be used for optional return values. constructs the contained value in-place The return value is created with a call to the default constructor. value (public member function) * The member function reset() is called. present[1]. Observers The return value is optional.
boost::optional appears to work like a pointer. boost::optional is a template that must be instantiated with the actual type of the return value. operator< compares optional objects (C++17) Helper classes (C++17) operator< compares optional objects
operator<= (function template) These are return values from functions that may not always return a result.
(C++17) If std::rand() generates an even random number, that number is returned by get_even_random_number(). returns the contained value If an optional
Boost.Optional provides boost::optional, which makes it possible to clearly mark optional return values. This commit does not belong to any branch on this repository, and may belong to a fork outside of the repository.
emplace (public member function) However, you should not think of boost::optional as a pointer because, for example, values in boost::optional are copied by the copy constructor while a pointer does not copy the value it points to. (function template)
operator>
Along with boost/optional/optional_io.hpp, Boost.Optional provides a header file with overloaded stream operators, which let you write objects of type boost::optional to, for example, standard output. nullopt_t indicator of optional type with uninitialized state The member function get() is equivalent to operator*. no dynamic memory allocation ever takes place. string contained in s1) but s1 still has_value. std::hash
operator== operator>= direct-list-initializing) an object of type T with the expression std::move(*other) and does not make other (C++17) The function boost::get_optional_value_or() is also provided as a member function of boost::optional. (class) The object does not contain a value in the following conditions: * The object is default-initialized. operator>=, make_optional creates an optional object (C++17) Special thanks to Petar Ivanov for the idea and Rina Volovich for editing. It is a great alternative for std::unique_ptr, or bad_optional_access exception indicating checked access to an optional that doesn't contain a value, nullopt_t indicator of optional type with uninitialized state operator_bool (public member function) a value that may or may not be present. The following visuals illustrate the various states of std::optional
std::hash
The type must meet the requirements of Destructible, constructs the optional object exchanges the contents operator== operator> (C++17) (function) (class template specialization) To the extent possible under law, Sy Brand has waived all copyright and related or neighboring rights to the optional library.
move for primitives just copies the values. Boost.Optional provides free-standing helper functions such as boost::make_optional() and boost::get_optional_value_or() (see Example21.4). Explicit types are for clarity. (function template) std::optional is the preferred way to represent an object which may or may not have a value. Non-member functions (function)
Example21.1 uses the function get_even_random_number(), which should return an even random number.
nullopt_t indicator of optional type with uninitialized state Unfortunately, chaining together many computations which may or may not produce a value can be verbose, as empty-checking code will be mixed in with the actual programming logic. Single header implementation of std::optional with functional-style extensions and support for references. (class) Helper classes Alternatively, an optional of a std::reference_wrapper of type T may be used to hold a reference. emplace (public member function) bad_optional_access exception indicating checked access to an optional that doesn't contain a value, std::swap(std::optional) specializes the std::swap algorithm constructs the contained value in-place (class template specialization) operator= (public member function)