Pass std::string by const ref (#749)

include/yaml-cpp/emittermanip.h

@@ -74,14 +74,14 @@ struct _Alias {
   std::string content;
 };
 
-inline _Alias Alias(const std::string content) { return _Alias(content); }
+inline _Alias Alias(const std::string& content) { return _Alias(content); }
 
 struct _Anchor {
   _Anchor(const std::string& content_) : content(content_) {}
   std::string content;
 };
 
-inline _Anchor Anchor(const std::string content) { return _Anchor(content); }
+inline _Anchor Anchor(const std::string& content) { return _Anchor(content); }
 
 struct _Tag {
   struct Type {
@@ -96,11 +96,11 @@ struct _Tag {
   Type::value type;
 };
 
-inline _Tag VerbatimTag(const std::string content) {
+inline _Tag VerbatimTag(const std::string& content) {
   return _Tag("", content, _Tag::Type::Verbatim);
 }
 
-inline _Tag LocalTag(const std::string content) {
+inline _Tag LocalTag(const std::string& content) {
   return _Tag("", content, _Tag::Type::PrimaryHandle);
 }
 
@@ -108,7 +108,7 @@ inline _Tag LocalTag(const std::string& prefix, const std::string content) {
   return _Tag(prefix, content, _Tag::Type::NamedHandle);
 }
 
-inline _Tag SecondaryTag(const std::string content) {
+inline _Tag SecondaryTag(const std::string& content) {
   return _Tag("", content, _Tag::Type::NamedHandle);
 }
 
@@ -117,7 +117,7 @@ struct _Comment {
   std::string content;
 };
 
-inline _Comment Comment(const std::string content) { return _Comment(content); }
+inline _Comment Comment(const std::string& content) { return _Comment(content); }
 
 struct _Precision {
   _Precision(int floatPrecision_, int doublePrecision_)

include/yaml-cpp/exceptions.h

@@ -223,7 +223,7 @@ inline TypedKeyNotFound<T> MakeTypedKeyNotFound(const Mark& mark,
 
 class YAML_CPP_API InvalidNode : public RepresentationException {
  public:
-  InvalidNode(std::string key)
+  InvalidNode(const std::string& key)
       : RepresentationException(Mark::null_mark(),
                                 ErrorMsg::INVALID_NODE_WITH_KEY(key)) {}
   InvalidNode(const InvalidNode&) = default;