Class template value_ref

boost::log::value_ref — Reference wrapper for a stored attribute value.

Synopsis

// In header: <boost/log/utility/value_ref_fwd.hpp>

template<typename T, typename TagT> 
class value_ref {
public:

  // public member functions
   BOOST_DEFAULTED_FUNCTION(value_ref(value_ref const &that), 
                            noexcept :base_type(static_cast< base_type const & >(that)) {}) const noexcept;
  explicit operator bool() const noexcept;
  bool operator!() const noexcept;
  bool empty() const noexcept;
  void swap(value_ref &) noexcept;
};

Description

The value_ref class template provides access to the stored attribute value. It is not a traditional reference wrapper since it may be empty (i.e. refer to no value at all) and it can also refer to values of different types. Therefore its interface and behavior combines features of Boost.Ref, Boost.Optional and Boost.Variant, depending on the use case.

The template parameter T can be a single type or an MPL sequence of possible types being referred. The reference wrapper will act as either an optional reference or an optional variant of references to the specified types. In any case, the referred values will not be modifiable (i.e. value_ref always models a const reference).

Template parameter TagT is optional. It can be used for customizing the operations on this reference wrapper, such as putting the referred value to log.

`value_ref` public member functions

 BOOST_DEFAULTED_FUNCTION(value_ref(value_ref const &that), 
                          noexcept :base_type(static_cast< base_type const & >(that)) {}) const noexcept;

Default constructor. Creates a reference wrapper that does not refer to a value.

Copy constructor.

Initializing constructor. Creates a reference wrapper that refers to the specified value.

```
explicit operator bool() const noexcept;
```
The operator verifies if the wrapper refers to a value.
```
bool operator!() const noexcept;
```
The operator verifies if the wrapper does not refer to a value.
```
bool empty() const noexcept;
```
Returns:

true if the wrapper does not refer to a value.
```
void swap(value_ref & that) noexcept;
```
Swaps two reference wrappers

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