PERF: Allow compiler to choose best way to construct a copy

With move semantics added to the language and the standard library updated with
move constructors added for many types it is now interesting to take an
argument directly by value, instead of by const-reference, and then copy. This
check allows the compiler to take care of choosing the best way to construct
the copy.

The transformation is usually beneficial when the calling code passes an rvalue
and assumes the move construction is a cheap operation. This short example
illustrates how the construction of the value happens:

SRCDIR=/Users/johnsonhj/src/jsoncpp/ #My local SRC
BLDDIR=/Users/johnsonhj/src/jsoncpp/cmake-build-debug/ #My local BLD

cd /Users/johnsonhj/src/jsoncpp/cmake-build-debug/
run-clang-tidy.py -extra-arg=-D__clang__ -checks=-*,modernize-pass-by-value  -header-filter=.* -fix

include/json/value.h

@@ -54,7 +54,7 @@ namespace Json {
  */
 class JSON_API Exception : public std::exception {
 public:
-  Exception(JSONCPP_STRING const& msg);
+  Exception(JSONCPP_STRING msg);
   ~Exception() JSONCPP_NOEXCEPT override;
   char const* what() const JSONCPP_NOEXCEPT override;
 

include/json/writer.h

@@ -300,8 +300,8 @@ public:
   /**
    * \param indentation Each level will be indented by this amount extra.
    */
-  StyledStreamWriter(const JSONCPP_STRING& indentation = "\t");
+  StyledStreamWriter(JSONCPP_STRING indentation = "\t");
-  ~StyledStreamWriter() {}
+  ~StyledStreamWriter() = default;
 
 public:
   /** \brief Serialize a Value in <a HREF="http://www.json.org">JSON</a> format.

src/lib_json/json_reader.cpp

@@ -88,9 +88,8 @@ bool Reader::containsNewLine(Reader::Location begin, Reader::Location end) {
 // //////////////////////////////////////////////////////////////////
 
 Reader::Reader()
-    : errors_(), document_(), begin_(), end_(), current_(), lastValueEnd_(),
+    : errors_(), document_(), commentsBefore_(), features_(Features::all())
-      lastValue_(), commentsBefore_(), features_(Features::all()),
+      {}
-      collectComments_() {}
 
 Reader::Reader(const Features& features)
     : errors_(), document_(), begin_(), end_(), current_(), lastValueEnd_(),

src/lib_json/json_value.cpp

@@ -213,7 +213,7 @@ static inline void releaseStringValue(char* value, unsigned) { free(value); }
 
 namespace Json {
 
-Exception::Exception(JSONCPP_STRING const& msg) : msg_(msg) {}
+Exception::Exception(JSONCPP_STRING  msg) : msg_(std::move(msg)) {}
 Exception::~Exception() JSONCPP_NOEXCEPT {}
 char const* Exception::what() const JSONCPP_NOEXCEPT { return msg_.c_str(); }
 RuntimeError::RuntimeError(JSONCPP_STRING const& msg) : Exception(msg) {}

src/lib_json/json_writer.cpp

@@ -642,8 +642,8 @@ bool StyledWriter::hasCommentForValue(const Value& value) {
 // Class StyledStreamWriter
 // //////////////////////////////////////////////////////////////////
 
-StyledStreamWriter::StyledStreamWriter(const JSONCPP_STRING& indentation)
+StyledStreamWriter::StyledStreamWriter(JSONCPP_STRING indentation)
-    : document_(nullptr), rightMargin_(74), indentation_(indentation),
+    : document_(nullptr), indentation_(std::move(indentation)),
       addChildValues_(), indented_(false) {}
 
 void StyledStreamWriter::write(JSONCPP_OSTREAM& out, const Value& root) {
@@ -872,11 +872,11 @@ struct CommentStyle {
 };
 
 struct BuiltStyledStreamWriter : public StreamWriter {
-  BuiltStyledStreamWriter(JSONCPP_STRING const& indentation,
+  BuiltStyledStreamWriter(JSONCPP_STRING indentation,
                           CommentStyle::Enum cs,
-                          JSONCPP_STRING const& colonSymbol,
+                          JSONCPP_STRING colonSymbol,
-                          JSONCPP_STRING const& nullSymbol,
+                          JSONCPP_STRING nullSymbol,
-                          JSONCPP_STRING const& endingLineFeedSymbol,
+                          JSONCPP_STRING endingLineFeedSymbol,
                           bool useSpecialFloats,
                           unsigned int precision,
                           PrecisionType precisionType);
@@ -912,17 +912,17 @@ private:
   PrecisionType precisionType_;
 };
 BuiltStyledStreamWriter::BuiltStyledStreamWriter(
-    JSONCPP_STRING const& indentation,
+    JSONCPP_STRING indentation,
     CommentStyle::Enum cs,
-    JSONCPP_STRING const& colonSymbol,
+    JSONCPP_STRING colonSymbol,
-    JSONCPP_STRING const& nullSymbol,
+    JSONCPP_STRING nullSymbol,
-    JSONCPP_STRING const& endingLineFeedSymbol,
+    JSONCPP_STRING endingLineFeedSymbol,
     bool useSpecialFloats,
     unsigned int precision,
     PrecisionType precisionType)
-    : rightMargin_(74), indentation_(indentation), cs_(cs),
+    : rightMargin_(74), indentation_(std::move(indentation)), cs_(cs),
-      colonSymbol_(colonSymbol), nullSymbol_(nullSymbol),
+      colonSymbol_(std::move(colonSymbol)), nullSymbol_(std::move(nullSymbol)),
-      endingLineFeedSymbol_(endingLineFeedSymbol), addChildValues_(false),
+      endingLineFeedSymbol_(std::move(endingLineFeedSymbol)), addChildValues_(false),
       indented_(false), useSpecialFloats_(useSpecialFloats),
       precision_(precision), precisionType_(precisionType) {}
 int BuiltStyledStreamWriter::write(Value const& root, JSONCPP_OSTREAM* sout) {