returning const char * from the original std::string object

passphrase_cache.cc

@@ -1,7 +1,7 @@
 #include "passphrase_cache.hh"
 
 namespace pEp {
-    const char *PassphraseCache::add(std::string passphrase)
+    const char *PassphraseCache::add(std::string&& passphrase)
     {
         std::lock_guard<std::mutex> lock(_mtx);
 

passphrase_cache.hh

@@ -38,7 +38,7 @@ namespace pEp {
 
         // adding a passphrase to the cache, which will timeout
 
-        const char *add(std::string passphrase);
+        const char *add(std::string&& passphrase);
 
         // for each passphrase call the callee until it returns true for a
         // matching passphrase or no passphrases are left