thank you, Roker, for the idea with returning the string of the emplaced element ;-)

passphrase_cache.cc

@@ -1,11 +1,6 @@
 #include <cassert>
 #include "passphrase_cache.hh"
 
-namespace
-{
-    const char* const empty_string = "";
-}
-
 namespace pEp {
     PassphraseCache::cache_entry::cache_entry(const std::string& p, time_point t) :
             passphrase{p, 0, PassphraseCache::cache_entry::max_len}, tp{t}
@@ -42,11 +37,13 @@ namespace pEp {
                 _cache.pop_front();
             
             _cache.emplace_back(passphrase, clock::now());
-            auto back = _cache.back(); // FIXME: In C++17 list::emplace_back() returns the just inserted element already.
+            auto back = _cache.back();  // FIXME: In C++17 list::emplace_back()
+                                        // returns the just inserted element
+                                        // already.
             return back.passphrase.c_str();
         }
 
-        return empty_string;
+        return "";
     }
 
     bool PassphraseCache::for_each_passphrase(const passphrase_callee& callee)