Pass three string parameters by address

Spoted by Coverity scan.

src/LyXVC.cpp

@@ -245,14 +245,13 @@ LyXVC::CommandResult LyXVC::checkIn(string & log)
 	LYXERR(Debug::LYXVC, "LyXVC: checkIn");
 	if (!vcs_)
 		return ErrorBefore;
-	docstring empty(_("(no log message)"));
 	docstring response;
 	bool ok = true;
 	if (vcs_->isCheckInWithConfirmation())
 		ok = Alert::askForText(response, _("LyX VC: Log Message"));
 	if (ok) {
 		if (response.empty())
-			response = empty;
+			response = _("(no log message)");
 		//shell collisions
 		response = subst(response, from_ascii("\""), from_ascii("\\\""));
 		return vcs_->checkIn(to_utf8(response), log);

src/frontends/qt/GuiCitation.cpp

@@ -835,7 +835,7 @@ void GuiCitation::init()
 
 void GuiCitation::findKey(BiblioInfo const & bi,
 	QString const & str, bool only_keys,
-	docstring field, docstring entry_type,
+	docstring const & field, docstring const & entry_type,
 	bool case_sensitive, bool reg_exp, bool reset)
 {
 	// FIXME THREAD

src/frontends/qt/GuiCitation.h

@@ -129,8 +129,8 @@ private:
 		BiblioInfo const & bi, //< optimize by passing this
 		QString const & str, //< string expression
 		bool only_keys, //< set to true if only keys shall be searched.
-		docstring field, //<field to search, empty for all fields
+		docstring const & field, //<field to search, empty for all fields
-		docstring entryType, //<entry type to display, empty for all
+		docstring const & entryType, //<entry type to display, empty for all
 		bool case_sensitive, //< set to true for case sensitive search.
 		bool reg_exp, //< set to true if \c str is a regular expression.
 		bool reset = false //< whether to reset and search all keys

src/insets/InsetCommandParams.cpp

@@ -99,7 +99,7 @@ static ParamInfo const & findInfo(InsetCode code, string const & cmdName)
 
 ParamInfo::ParamData::ParamData(std::string const & s, ParamType t,
                                 ParamHandling h, bool ignore,
-                                docstring default_value)
+                                docstring const & default_value)
 	: name_(s), type_(t), handling_(h), ignore_(ignore),
 	  default_value_(default_value)
 {}

src/insets/InsetCommandParams.h

@@ -55,7 +55,7 @@ public:
 		///
 		ParamData(std::string const &, ParamType, ParamHandling = HANDLING_NONE,
 		          bool ignore = false,
-		          docstring default_value = docstring());
+		          docstring const & default_value = docstring());
 		///
 		std::string name() const { return name_; }
 		///

src/lyxfind.cpp

@@ -1136,7 +1136,8 @@ public:
 private:
 	/// Auxiliary find method (does not account for opt.matchword)
 	MatchResult findAux(DocIterator const & cur, int len, matchType at_begin) const;
-	void CreateRegexp(FindAndReplaceOptions const & opt, string regexp_str, string regexp2_str, string par_as_string = "");
+	void CreateRegexp(FindAndReplaceOptions const & opt, string const & regexp_str,
+	                  string const & regexp2_str, string const & par_as_string = string());
 
 	/** Normalize a stringified or latexified LyX paragraph.
 	 **
@@ -3619,7 +3620,8 @@ static void identifyClosing(string & t, bool ignoreformat)
 static int num_replaced = 0;
 static bool previous_single_replace = true;
 
-void MatchStringAdv::CreateRegexp(FindAndReplaceOptions const & opt, string regexp_str, string regexp2_str, string par_as_string)
+void MatchStringAdv::CreateRegexp(FindAndReplaceOptions const & opt, string const & regexp_str,
+                                  string const & regexp2_str, string const & par_as_string)
 {
 #if QTSEARCH
 	if (regexp_str.empty() || regexp2_str.empty()) {