JSONWriter deals in StringRefs instead of std::strings

Together with liberal use of `_sr` UDL to compile-time convert
string literals into StringRefs, this will reduce the number of
allocation and remove most of the strcpy calls inherent in
converting string lits into `std::string`s.

src/catch2/internal/catch_jsonwriter.cpp

@@ -29,7 +29,7 @@ namespace Catch {
     JsonObjectWriter::JsonObjectWriter( std::ostream& os,
                                         std::uint64_t indent_level ):
         m_os{ os }, m_indent_level{ indent_level } {
-        m_os << "{";
+        m_os << '{';
     }
     JsonObjectWriter::JsonObjectWriter( JsonObjectWriter&& source ):
         m_os{ source.m_os },
@@ -47,11 +47,11 @@ namespace Catch {
         m_os << '}';
     }
 
-    JsonValueWriter JsonObjectWriter::write( std::string const& key ) {
+    JsonValueWriter JsonObjectWriter::write( StringRef key ) {
         JsonUtils::appendCommaNewline(
             m_os, m_should_comma, m_indent_level + 1 );
 
-        m_os << '"' << key << '"' << ": ";
+        m_os << '"' << key << "\": ";
         return JsonValueWriter{ m_os, m_indent_level + 1 };
     }
 
@@ -60,7 +60,7 @@ namespace Catch {
     JsonArrayWriter::JsonArrayWriter( std::ostream& os,
                                       std::uint64_t indent_level ):
         m_os{ os }, m_indent_level{ indent_level } {
-        m_os << "[";
+        m_os << '[';
     }
     JsonArrayWriter::JsonArrayWriter( JsonArrayWriter&& source ):
         m_os{ source.m_os },
@@ -108,8 +108,41 @@ namespace Catch {
         return JsonArrayWriter{ m_os, m_indent_level };
     }
 
+    void JsonValueWriter::write( Catch::StringRef value ) && {
+        writeImpl( value, true );
+    }
+
     void JsonValueWriter::write( bool value ) && {
-        writeImpl( value ? "true" : "false", false );
+        writeImpl( value ? "true"_sr : "false"_sr, false );
+    }
+
+    void JsonValueWriter::writeImpl( Catch::StringRef value, bool quote ) {
+        if ( quote ) { m_os << '"'; }
+        for (char c : value) {
+            // Escape list taken from https://www.json.org/json-en.html,
+            // string definition.
+            // Note that while forward slash _can_ be escaped, it does
+            // not have to be, if JSON is not further embedded somewhere
+            // where forward slash is meaningful.
+            if ( c == '"' ) {
+                m_os << "\\\"";
+            } else if ( c == '\\' ) {
+                m_os << "\\\\";
+            } else if ( c == '\b' ) {
+                m_os << "\\b";
+            } else if ( c == '\f' ) {
+                m_os << "\\f";
+            } else if ( c == '\n' ) {
+                m_os << "\\n";
+            } else if ( c == '\r' ) {
+                m_os << "\\r";
+            } else if ( c == '\t' ) {
+                m_os << "\\t";
+            } else {
+                m_os << c;
+            }
+        }
+        if ( quote ) { m_os << '"'; }
     }
 
 } // namespace Catch

src/catch2/internal/catch_jsonwriter.hpp

@@ -37,45 +37,26 @@ namespace Catch {
         void write( T const& value ) && {
             writeImpl( value, !std::is_arithmetic<T>::value );
         }
-
+        void write( StringRef value ) &&;
         void write( bool value ) &&;
 
     private:
-        template <typename T>
+        void writeImpl( StringRef value, bool quote );
+
+        // Without this SFINAE, this overload is a better match
+        // for `std::string`, `char const*`, `char const[N]` args.
+        // While it would still work, it would cause code bloat
+        // and multiple iteration over the strings
+        template <typename T,
+                  typename = typename std::enable_if_t<
+                      !std::is_convertible<T, StringRef>::value>>
         void writeImpl( T const& value, bool quote_value ) {
-            if ( quote_value ) { m_os << '"'; }
             m_sstream << value;
-            while ( true ) {
-                char c = m_sstream.get();
-                if ( m_sstream.eof() ) { break; }
-
-                // see https://www.json.org/json-en.html, string definition for the escape list
-                if ( c == '"' ) {
-                    m_os << "\\\"";
-                } else if ( c == '\\' ) {
-                    m_os << "\\\\";
-                // Note that while forward slash _can_ be escaped, it
-                // does not have to be, if JSON is not further embedded
-                // somewhere where forward slash is meaningful.
-                } else if ( c == '\b' ) {
-                    m_os << "\\b";
-                } else if ( c == '\f' ) {
-                    m_os << "\\f";
-                } else if ( c == '\n' ) {
-                    m_os << "\\n";
-                } else if ( c == '\r' ) {
-                    m_os << "\\r";
-                } else if ( c == '\t') {
-                    m_os << "\\t";
-                } else {
-                    m_os << c;
-                }
-            }
-            if ( quote_value ) { m_os << '"'; }
+            writeImpl( m_sstream.str(), quote_value );
         }
 
         std::ostream& m_os;
-        std::stringstream m_sstream{};
+        std::stringstream m_sstream;
         std::uint64_t m_indent_level;
     };
 
@@ -89,7 +70,7 @@ namespace Catch {
 
         ~JsonObjectWriter();
 
-        JsonValueWriter write( std::string const& key );
+        JsonValueWriter write( StringRef key );
 
     private:
         std::ostream& m_os;