use string_view instead of string in to_utf8() to avoid copying.

src/to_utf8.cc

@@ -64,14 +64,14 @@ namespace
 
 static const size_t IconvBufSize = 64;
 
-std::string to_utf8_iconv(const std::string& charset, const std::string& s)
+std::string to_utf8_iconv(const sv& charset, const std::string& s)
 {
-    iconv_t ict = iconv_open("UTF-8", charset.c_str());
+    iconv_t ict = iconv_open("UTF-8", charset.data());
     if(ict == (iconv_t)-1)
     {
         if(errno==EINVAL)
         {
-            throw std::runtime_error("Cannot convert from charset \"" + charset + "\" to UTF-8.");
+            throw std::runtime_error("Cannot convert from charset \"" + std::string(charset) + "\" to UTF-8.");
         }else{
             throw std::runtime_error(std::string("Internal error: ") + strerror(errno) );
         }
@@ -113,7 +113,7 @@ std::string to_utf8_iconv(const std::string& charset, const std::string& s)
 } // end of anonymous namespace
 
 
-std::string to_utf8( const std::string& charset, const std::string& s)
+std::string to_utf8( const sv& charset, const std::string& s)
 {
     std::string charset_upper{charset};
     boost::algorithm::to_upper(charset_upper);

src/to_utf8.hh

@@ -1,8 +1,8 @@
 #ifndef PEP_MIME_TO_UTF8_HH
 #define PEP_MIME_TO_UTF8_HH
 
-#include <string>
+#include "pEpMIME_internal.hh"
 
-std::string to_utf8( const std::string& charset, const std::string& s);
+std::string to_utf8( const sv& charset, const std::string& s);
 
 #endif //  PEP_MIME_TO_UTF8_HH