diff --git a/lemon/arg_parser.cc b/lemon/arg_parser.cc
--- a/lemon/arg_parser.cc
+++ b/lemon/arg_parser.cc
@@ -239,11 +239,11 @@
     return *this;
   }
 
-  void ArgParser::show(std::ostream &os,Opts::iterator i)
+  void ArgParser::show(std::ostream &os,Opts::const_iterator i) const
   {
     os << "-" << i->first;
     if(i->second.has_syn)
-      for(Opts::iterator j=_opts.begin();j!=_opts.end();++j)
+      for(Opts::const_iterator j=_opts.begin();j!=_opts.end();++j)
         if(j->second.syn&&j->second.help==i->first)
           os << "|-" << j->first;
     switch(i->second.type) {
@@ -261,9 +261,9 @@
     }
   }
 
-  void ArgParser::show(std::ostream &os,Groups::iterator i)
+  void ArgParser::show(std::ostream &os,Groups::const_iterator i) const
   {
-    GroupData::Opts::iterator o=i->second.opts.begin();
+    GroupData::Opts::const_iterator o=i->second.opts.begin();
     while(o!=i->second.opts.end()) {
       show(os,_opts.find(*o));
       ++o;
@@ -271,7 +271,7 @@
     }
   }
 
-  void ArgParser::showHelp(Opts::iterator i)
+  void ArgParser::showHelp(Opts::const_iterator i) const
   {
     if(i->second.help.size()==0||i->second.syn) return;
     std::cerr << "  ";
@@ -279,20 +279,21 @@
     std::cerr << std::endl;
     std::cerr << "     " << i->second.help << std::endl;
   }
-  void ArgParser::showHelp(std::vector<ArgParser::OtherArg>::iterator i)
+  void ArgParser::showHelp(std::vector<ArgParser::OtherArg>::const_iterator i)
+    const
   {
     if(i->help.size()==0) return;
     std::cerr << "  " << i->name << std::endl
               << "     " << i->help << std::endl;
   }
 
-  void ArgParser::shortHelp()
+  void ArgParser::shortHelp() const
   {
     const unsigned int LINE_LEN=77;
     const std::string indent("    ");
     std::cerr << "Usage:\n  " << _command_name;
     int pos=_command_name.size()+2;
-    for(Groups::iterator g=_groups.begin();g!=_groups.end();++g) {
+    for(Groups::const_iterator g=_groups.begin();g!=_groups.end();++g) {
       std::ostringstream cstr;
       cstr << ' ';
       if(!g->second.mandatory) cstr << '[';
@@ -305,7 +306,7 @@
       std::cerr << cstr.str();
       pos+=cstr.str().size();
     }
-    for(Opts::iterator i=_opts.begin();i!=_opts.end();++i)
+    for(Opts::const_iterator i=_opts.begin();i!=_opts.end();++i)
       if(!i->second.ingroup&&!i->second.syn) {
         std::ostringstream cstr;
         cstr << ' ';
@@ -319,7 +320,7 @@
         std::cerr << cstr.str();
         pos+=cstr.str().size();
       }
-    for(std::vector<OtherArg>::iterator i=_others_help.begin();
+    for(std::vector<OtherArg>::const_iterator i=_others_help.begin();
         i!=_others_help.end();++i)
       {
         std::ostringstream cstr;
@@ -335,18 +336,18 @@
     std::cerr << std::endl;
   }
 
-  void ArgParser::showHelp()
+  void ArgParser::showHelp() const
   {
     shortHelp();
     std::cerr << "Where:\n";
-    for(std::vector<OtherArg>::iterator i=_others_help.begin();
+    for(std::vector<OtherArg>::const_iterator i=_others_help.begin();
         i!=_others_help.end();++i) showHelp(i);
-    for(Opts::iterator i=_opts.begin();i!=_opts.end();++i) showHelp(i);
+    for(Opts::const_iterator i=_opts.begin();i!=_opts.end();++i) showHelp(i);
     exit(1);
   }
 
 
-  void ArgParser::unknownOpt(std::string arg)
+  void ArgParser::unknownOpt(std::string arg) const
   {
     std::cerr << "\nUnknown option: " << arg << "\n";
     std::cerr << "\nType '" << _command_name <<
@@ -354,7 +355,7 @@
     exit(1);
   }
 
-  void ArgParser::requiresValue(std::string arg, OptType t)
+  void ArgParser::requiresValue(std::string arg, OptType t) const
   {
     std::cerr << "Argument '" << arg << "' requires a";
     switch(t) {
@@ -375,10 +376,10 @@
   }
 
 
-  void ArgParser::checkMandatories()
+  void ArgParser::checkMandatories() const
   {
     bool ok=true;
-    for(Opts::iterator i=_opts.begin();i!=_opts.end();++i)
+    for(Opts::const_iterator i=_opts.begin();i!=_opts.end();++i)
       if(i->second.mandatory&&!i->second.set)
         {
           if(ok)
@@ -387,18 +388,18 @@
           ok=false;
           showHelp(i);
         }
-    for(Groups::iterator i=_groups.begin();i!=_groups.end();++i)
+    for(Groups::const_iterator i=_groups.begin();i!=_groups.end();++i)
       if(i->second.mandatory||i->second.only_one)
         {
           int set=0;
-          for(GroupData::Opts::iterator o=i->second.opts.begin();
+          for(GroupData::Opts::const_iterator o=i->second.opts.begin();
               o!=i->second.opts.end();++o)
             if(_opts.find(*o)->second.set) ++set;
           if(i->second.mandatory&&!set) {
             std::cerr << _command_name <<
               ": At least one of the following arguments is mandatory.\n";
             ok=false;
-            for(GroupData::Opts::iterator o=i->second.opts.begin();
+            for(GroupData::Opts::const_iterator o=i->second.opts.begin();
                 o!=i->second.opts.end();++o)
               showHelp(_opts.find(*o));
           }
@@ -406,7 +407,7 @@
             std::cerr << _command_name <<
               ": At most one of the following arguments can be given.\n";
             ok=false;
-            for(GroupData::Opts::iterator o=i->second.opts.begin();
+            for(GroupData::Opts::const_iterator o=i->second.opts.begin();
                 o!=i->second.opts.end();++o)
               showHelp(_opts.find(*o));
           }
diff --git a/lemon/arg_parser.h b/lemon/arg_parser.h
--- a/lemon/arg_parser.h
+++ b/lemon/arg_parser.h
@@ -271,17 +271,20 @@
 
     ///@}
 
-    void show(std::ostream &os,Opts::iterator i);
-    void show(std::ostream &os,Groups::iterator i);
-    void showHelp(Opts::iterator i);
-    void showHelp(std::vector<OtherArg>::iterator i);
-    void shortHelp();
-    void showHelp();
+  private:
+    void show(std::ostream &os,Opts::const_iterator i) const;
+    void show(std::ostream &os,Groups::const_iterator i) const;
+    void showHelp(Opts::const_iterator i) const;
+    void showHelp(std::vector<OtherArg>::const_iterator i) const;
 
-    void unknownOpt(std::string arg);
+    void unknownOpt(std::string arg) const;
 
-    void requiresValue(std::string arg, OptType t);
-    void checkMandatories();
+    void requiresValue(std::string arg, OptType t) const;
+    void checkMandatories() const;
+
+    void shortHelp() const;
+    void showHelp() const;
+  public:
 
     ///Start the parsing process
     ArgParser &parse();
@@ -293,12 +296,12 @@
     }
 
     ///Give back the command name (the 0th argument)
-    const std::string &commandName() { return _command_name; }
+    const std::string &commandName() const { return _command_name; }
 
     ///Check if an opion has been given to the command.
-    bool given(std::string op)
+    bool given(std::string op) const
     {
-      Opts::iterator i = _opts.find(op);
+      Opts::const_iterator i = _opts.find(op);
       return i!=_opts.end()?i->second.set:false;
     }
 
@@ -311,15 +314,15 @@
     ///throws an exception (i.e. it performs runtime type checking).
     class RefType
     {
-      ArgParser &_parser;
+      const ArgParser &_parser;
       std::string _name;
     public:
       ///\e
-      RefType(ArgParser &p,const std::string &n) :_parser(p),_name(n) {}
+      RefType(const ArgParser &p,const std::string &n) :_parser(p),_name(n) {}
       ///\e
       operator bool()
       {
-        Opts::iterator i = _parser._opts.find(_name);
+        Opts::const_iterator i = _parser._opts.find(_name);
         LEMON_ASSERT(i!=_parser._opts.end(),
                      std::string()+"Unkown option: '"+_name+"'");
         LEMON_ASSERT(i->second.type==ArgParser::BOOL,
@@ -329,7 +332,7 @@
       ///\e
       operator std::string()
       {
-        Opts::iterator i = _parser._opts.find(_name);
+        Opts::const_iterator i = _parser._opts.find(_name);
         LEMON_ASSERT(i!=_parser._opts.end(),
                      std::string()+"Unkown option: '"+_name+"'");
         LEMON_ASSERT(i->second.type==ArgParser::STRING,
@@ -339,7 +342,7 @@
       ///\e
       operator double()
       {
-        Opts::iterator i = _parser._opts.find(_name);
+        Opts::const_iterator i = _parser._opts.find(_name);
         LEMON_ASSERT(i!=_parser._opts.end(),
                      std::string()+"Unkown option: '"+_name+"'");
         LEMON_ASSERT(i->second.type==ArgParser::DOUBLE ||
@@ -351,7 +354,7 @@
       ///\e
       operator int()
       {
-        Opts::iterator i = _parser._opts.find(_name);
+        Opts::const_iterator i = _parser._opts.find(_name);
         LEMON_ASSERT(i!=_parser._opts.end(),
                      std::string()+"Unkown option: '"+_name+"'");
         LEMON_ASSERT(i->second.type==ArgParser::INTEGER,
@@ -365,7 +368,7 @@
 
     ///Give back the value of an option.
     ///\sa RefType
-    RefType operator[](const std::string &n)
+    RefType operator[](const std::string &n) const
     {
       return RefType(*this, n);
     }
@@ -374,7 +377,7 @@
 
     ///Give back a reference to a vector consisting of the program arguments
     ///not starting with a '-' character.
-    std::vector<std::string> &files() { return _file_args; }
+    const std::vector<std::string> &files() const { return _file_args; }
 
   };
 }