diff -r 47fbc814aa31 -r cbe3ec2d59d2 lemon/lgf_writer.h
--- a/lemon/lgf_writer.h	Wed Oct 01 12:44:16 2008 +0200
+++ b/lemon/lgf_writer.h	Wed Oct 01 13:56:40 2008 +0200
@@ -55,7 +55,7 @@
 
     template <typename T>
     bool operator<(const T&, const T&) {
-      throw DataFormatError("Label map is not comparable");
+      throw FormatError("Label map is not comparable");
     }
 
     template <typename _Map>
@@ -203,7 +203,7 @@
         typename std::map<Value, std::string>::const_iterator it =
           _map.find(str);
         if (it == _map.end()) {
-          throw DataFormatError("Item not found");
+          throw FormatError("Item not found");
         }
         return it->second;
       }
@@ -223,7 +223,7 @@
         typename std::map<typename Graph::Edge, std::string>
           ::const_iterator it = _map.find(val);
         if (it == _map.end()) {
-          throw DataFormatError("Item not found");
+          throw FormatError("Item not found");
         }
         return (_graph.direction(val) ? '+' : '-') + it->second;
       }
@@ -462,7 +462,9 @@
     /// output file.
     DigraphWriter(const Digraph& digraph, const std::string& fn)
       : _os(new std::ofstream(fn.c_str())), local_os(true), _digraph(digraph),
-        _skip_nodes(false), _skip_arcs(false) {}
+        _skip_nodes(false), _skip_arcs(false) {
+      if (!(*_os)) throw IoError("Cannot write file", fn);
+    }
 
     /// \brief Constructor
     ///
@@ -470,7 +472,9 @@
     /// output file.
     DigraphWriter(const Digraph& digraph, const char* fn)
       : _os(new std::ofstream(fn)), local_os(true), _digraph(digraph),
-        _skip_nodes(false), _skip_arcs(false) {}
+        _skip_nodes(false), _skip_arcs(false) {
+      if (!(*_os)) throw IoError("Cannot write file", fn);
+    }
 
     /// \brief Destructor
     ~DigraphWriter() {
@@ -1019,7 +1023,9 @@
     /// output file.
     GraphWriter(const Graph& graph, const std::string& fn)
       : _os(new std::ofstream(fn.c_str())), local_os(true), _graph(graph),
-        _skip_nodes(false), _skip_edges(false) {}
+        _skip_nodes(false), _skip_edges(false) {
+      if (!(*_os)) throw IoError("Cannot write file", fn);
+    }
 
     /// \brief Constructor
     ///
@@ -1027,7 +1033,9 @@
     /// output file.
     GraphWriter(const Graph& graph, const char* fn)
       : _os(new std::ofstream(fn)), local_os(true), _graph(graph),
-        _skip_nodes(false), _skip_edges(false) {}
+        _skip_nodes(false), _skip_edges(false) {
+      if (!(*_os)) throw IoError("Cannot write file", fn);
+    }
 
     /// \brief Destructor
     ~GraphWriter() {
@@ -1578,13 +1586,17 @@
     ///
     /// Construct a section writer, which writes into the given file.
     SectionWriter(const std::string& fn)
-      : _os(new std::ofstream(fn.c_str())), local_os(true) {}
+      : _os(new std::ofstream(fn.c_str())), local_os(true) {
+      if (!(*_os)) throw IoError("Cannot write file", fn);
+    }
 
     /// \brief Constructor
     ///
     /// Construct a section writer, which writes into the given file.
     SectionWriter(const char* fn)
-      : _os(new std::ofstream(fn)), local_os(true) {}
+      : _os(new std::ofstream(fn)), local_os(true) {
+      if (!(*_os)) throw IoError("Cannot write file", fn);
+    }
 
     /// \brief Destructor
     ~SectionWriter() {