Class attribute

boost::log::attribute — A base class for an attribute value factory.

Synopsis

// In header: <boost/log/attributes/attribute.hpp>


class attribute {
public:
  // member classes/structs/unions

  // A base class for an attribute value factory.

  struct impl : public boost::intrusive_ref_counter< impl > {

    // public member functions
    virtual ~impl();
    virtual attribute_value get_value() = 0;

    // public static functions
    static void * operator new(std::size_t);
    static void operator delete(void *, std::size_t) noexcept;
  };

  // public member functions
  attribute() = default;
  attribute(attribute const &) noexcept;
  attribute(attribute &&) noexcept;
  explicit attribute(intrusive_ptr< impl >) noexcept;
  attribute & operator=(attribute const &) noexcept;
  attribute & operator=(attribute &&) noexcept;
  explicit operator bool() const noexcept;
  bool operator!() const noexcept;
  attribute_value get_value() const;
  void swap(attribute &) noexcept;

  // protected member functions
  impl * get_impl() const noexcept;
  void set_impl(intrusive_ptr< impl >) noexcept;

  // friend functions
  template<typename T> T attribute_cast(attribute const &);
};

Description

Every attribute is represented with a factory that is basically an attribute value generator. The sole purpose of an attribute is to return an actual value when requested. A simplest attribute can always return the same value that it stores internally, but more complex ones can perform a considerable amount of work to return a value, and the returned values may differ each time requested.

A word about thread safety. An attribute should be prepared to be requested a value from multiple threads concurrently.

`attribute` public member functions

```
attribute() = default;
```
Default constructor. Creates an empty attribute value factory, which is not usable until set_impl is called.

attribute(attribute const & that) noexcept;

Copy constructor

```
attribute(attribute && that) noexcept;
```
Move constructor
```
explicit attribute(intrusive_ptr< impl > p) noexcept;
```
Initializing constructor

Parameters:

p

Pointer to the implementation. Must not be NULL.

attribute & operator=(attribute const & that) noexcept;

Copy assignment

attribute & operator=(attribute && that) noexcept;

Move assignment

```
explicit operator bool() const noexcept;
```
Verifies that the factory is not in empty state
```
bool operator!() const noexcept;
```
Verifies that the factory is in empty state
```
attribute_value get_value() const;
```
Returns:

The actual attribute value. It shall not return empty values (exceptions shall be used to indicate errors).
```
void swap(attribute & that) noexcept;
```
The method swaps two factories (i.e. their implementations).

`attribute` protected member functions

```
impl * get_impl() const noexcept;
```
Returns:

The pointer to the implementation
```
void set_impl(intrusive_ptr< impl > p) noexcept;
```
Sets the pointer to the factory implementation.

Parameters:

p

Pointer to the implementation. Must not be NULL.

`attribute` friend functions

template<typename T> T attribute_cast(attribute const &);

The function casts one attribute factory to another

Boost C++ Libraries

Class attribute

Synopsis

Description

attribute public member functions

attribute protected member functions

attribute friend functions

`attribute` public member functions

`attribute` protected member functions

`attribute` friend functions