Boost C++ Libraries

...one of the most highly regarded and expertly designed C++ library projects in the world. — Herb Sutter and Andrei Alexandrescu, C++ Coding Standards

This is the documentation for an old version of boost. Click here for the latest Boost documentation.

Struct template value

boost::xpressive::value — value<> is a lazy wrapper for a value that can be used in xpressive semantic actions.

Synopsis

// In header: <boost/xpressive/xpressive_fwd.hpp>

template<typename T> 
struct value :
  public proto::extends< proto::terminal< T >::type, value< T > >
{
  // construct/copy/destruct
  value();
  explicit value(T const &);

  // public member functions
  T & get();
  T const  & get() const;
};

Description

Below is an example that shows where value<> is useful.

sregex good_voodoo(boost::shared_ptr<int> pi)
{
    using namespace boost::xpressive;
    // Use val() to hold the shared_ptr by value:
    sregex rex = +( _d [ ++*val(pi) ] >> '!' );
    // OK, rex holds a reference count to the integer.
    return rex;
}

In the above code, xpressive::val() is a function that returns a value<> object. Had val() not been used here, the operation ++*pi would have been evaluated eagerly once, instead of lazily when the regex match happens.

Template Parameters

```
typename T
```
The type of the value to store.

`value` public construct/copy/destruct

```
value();
```
Store a default-constructed T.
```
explicit value(T const & t);
```
Store a copy of t.

Parameters:

t

The initial value.

`value` public member functions

```
T & get();
```
This is an overloaded member function, provided for convenience. It differs from the above function only in what argument(s) it accepts.
```
T const  & get() const;
```
Fetch the stored value.