cpp: Add internal adapter for passing a vector of strings to gpgme

* lang/cpp/src/util.h (class StringsToCStrings): New.
* lang/cpp/src/util.cpp: New.
* lang/cpp/src/Makefile.am: Add new file.
--

This adapter simplifies passing a vector of strings as NULL-terminated
array of const char* to the C-interface of gpgme.

GnuPG-bug-id: 5808

lang/cpp/src/Makefile.am

@@ -37,7 +37,8 @@ main_sources = \
     defaultassuantransaction.cpp \
     scdgetinfoassuantransaction.cpp gpgagentgetinfoassuantransaction.cpp \
     statusconsumerassuantransaction.cpp \
-    vfsmountresult.cpp configuration.cpp tofuinfo.cpp swdbresult.cpp
+    vfsmountresult.cpp configuration.cpp tofuinfo.cpp swdbresult.cpp \
+    util.cpp
 
 gpgmepp_headers = \
     configuration.h context.h data.h decryptionresult.h \

lang/cpp/src/util.cpp

@@ -0,0 +1,47 @@
+/*
+  util.cpp - some internal helpers
+  Copyright (c) 2022 g10 Code GmbH
+  Software engineering by Ingo Klöcker <dev@ingo-kloecker.de>
+
+  This file is part of GPGME++.
+
+  GPGME++ is free software; you can redistribute it and/or
+  modify it under the terms of the GNU Library General Public
+  License as published by the Free Software Foundation; either
+  version 2 of the License, or (at your option) any later version.
+
+  GPGME++ is distributed in the hope that it will be useful,
+  but WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+  GNU Library General Public License for more details.
+
+  You should have received a copy of the GNU Library General Public License
+  along with GPGME++; see the file COPYING.LIB.  If not, write to the
+  Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
+  Boston, MA 02110-1301, USA.
+*/
+
+#ifdef HAVE_CONFIG_H
+ #include "config.h"
+#endif
+
+#include "util.h"
+
+#include <functional>
+
+StringsToCStrings::StringsToCStrings(const std::vector<std::string>& v)
+    : m_strings{v}
+{
+}
+
+const char **StringsToCStrings::c_strs() const
+{
+    if (m_cstrings.empty()) {
+        m_cstrings.reserve(m_strings.size() + 1);
+        std::transform(std::begin(m_strings), std::end(m_strings),
+                       std::back_inserter(m_cstrings),
+                       std::mem_fn(&std::string::c_str));
+        m_cstrings.push_back(nullptr);
+    }
+    return m_cstrings.data();
+}

lang/cpp/src/util.h

@@ -1,8 +1,10 @@
 /*
-  util.h - some inline helper functions
+  util.h - some internal helpers
   Copyright (C) 2004 Klarälvdalens Datakonsult AB
   2016 Bundesamt für Sicherheit in der Informationstechnik
   Software engineering by Intevation GmbH
+  Copyright (c) 2022 g10 Code GmbH
+  Software engineering by Ingo Klöcker <dev@ingo-kloecker.de>
 
   This file is part of GPGME++.
 
@@ -175,4 +177,25 @@ static inline gpgme_sig_notation_flags_t  add_to_gpgme_sig_notation_flags_t(unsi
     return static_cast<gpgme_sig_notation_flags_t>(result);
 }
 
+/**
+ * Adapter for passing a vector of strings as NULL-terminated array of
+ * const char* to the C-interface of gpgme.
+ */
+class StringsToCStrings
+{
+public:
+    explicit StringsToCStrings(const std::vector<std::string> &v);
+    ~StringsToCStrings() = default;
+
+    StringsToCStrings(const StringsToCStrings &) = delete;
+    StringsToCStrings &operator=(const StringsToCStrings &) = delete;
+    StringsToCStrings(StringsToCStrings &&) = delete;
+    StringsToCStrings &operator=(StringsToCStrings &&) = delete;
+
+    const char **c_strs() const;
+private:
+    const std::vector<std::string> m_strings;
+    mutable std::vector<const char *> m_cstrings;
+};
+
 #endif // __GPGMEPP_UTIL_H__