Struct template has_env_var

Boost C++ Libraries

Struct template has_env_var

boost::proto::functional::has_env_var — A unary boolean PolymorphicFunctionObject used for determining whether a particular transform environment has a value associated with a particular key.

Synopsis

// In header: <boost/proto/transform/env.hpp>

template<typename Key> 
struct has_env_var :  proto::callable {
  // member classes/structs/unions
  template<typename Sig> 
  struct result {
    // types
    typedef see-below type;
  };

  // public member functions
  template<typename Env> see-below operator()(Env const &) const;
};

Description

`has_env_var` public member functions

```
template<typename Env> see-below operator()(Env const & e) const;
```
This function behaves as follows:
- If proto::is_env<Env>::value is true:
  - If e[Key()] returns an instance of proto::key_not_found, return mpl::false_. See proto::env::operator[] for more information.
  - Otherwise, return mpl::true_.
- Otherwise:
  - If Key is proto::data_type, return mpl::true_.
  - Otherwise, return mpl::false_.