$STREAMABLE ENABLE
$MESSAGE NotFound
@ path std::string const &
"not found: {$path}"

class NotFoundStreamable
{
    std::decay_t< std::string const & > member_path;

  public:
    explicit NotFoundStreamable( std::string const & path );
    virtual ~NotFoundStreamable( );

    std::decay_t< std::string const & > const &path( ) const;

  protected:
    virtual void PrintOn( std::ostream &stream ) const;
    friend std::ostream &operator<< ( std::ostream &stream, NotFoundStreamable const &data );
};

std::ostream &operator<< ( std::ostream &stream, NotFoundStreamable const &data );

std::cout << NotFoundStreamable( "a.cfg" ) << '\n';

The language comes from the stream's locale (std::cout.imbue(…)).

Base class

$STREAMABLE INHERITED [PUBLIC|PROTECTED|PRIVATE] Base — public by default. The base is default-constructed. If it declares a virtual PrintOn( std::ostream & ) const, the generated one overrides it, so the object prints through a reference to the base too.

Members generated into it

With MEMBER AS name a string, error, throw or syslog function becomes a member of this class, using the stored values — see Member functions.