\section{The \texttt{VError} class}

The \verb+VError+ class is the class thrown by the VIPS C++ API when an
error is detected. It is derived from \verb+std::exception+ in the usual way.

\subsection{Constructors}

There are two constructors for \verb+VError+:

\begin{verbatim}
VError( std::string str );
VError();
\end{verbatim}

The first form creates an error object initialised with the specified
string, the last form creates an empty error object.

\subsection{Projection functions}

A function gives access to the string held by \verb+VError+:

\begin{verbatim}
const char *what();
\end{verbatim}

You can also send to an \verb+ostream+.

\begin{verbatim}
std::ostream& operator<<( 
    std::ostream&, const error& );
\end{verbatim}

\subsection{Computing with \texttt{VError}}

Two member functions let you append elements to an error:

\begin{verbatim}
VError &app( std::string txt );
VError &app( const int i );
\end{verbatim}

For example:

\begin{verbatim}
VError wombat;
int n = 12;

wombat.app( "possum: no more than " ).
	app( n ).app( " elements\n" );
throw( wombat );
\end{verbatim}

\noindent
will throw a \verb+VError+ with a diagnostic of:

\begin{verbatim}
possum: no more than 12 elements
\end{verbatim}

The member function \verb+perror()+ prints the error message to \verb+stdout+
and exits with a code of 1.

\begin{verbatim}
void perror( const char * );
void perror();
\end{verbatim}

\subsection{Convenience function}

The convenience function \verb+verror+ creates an \verb+VError+ with the
specified error string, and throws it. If you pass \verb+""+ for the string,
verror uses the contents of the VIPS error buffer instead.

\begin{verbatim}
extern void verror( std::string str = "" );
\end{verbatim}