cpp: Add safer member function returning text describing an error

* lang/cpp/src/error.h, lang/cpp/src/context.cpp (class Error): New
member function asStdString.
--

Unlike asString the new function doesn't cache the value returned by
gpgme_strerror_r. This ensures that on Windows the UTF-8 encoded text
is returned for the main thread even if the function was called before
for another thread. For other threads asStdString will still return the
natively encoded error description until UTF-8 encoded error
descriptions can be enabled for all threads (T7185).

GnuPG-bug-id: 7188

NEWS

@@ -14,6 +14,9 @@ Noteworthy changes in version 1.24.0 (unrelease)
  * cpp: Provide information about designated revocation keys for a Key.
    [T7118]
 
+ * cpp: Add safer member function returning text describing an error.
+   [T5960]
+
  * qt: Install headers for Qt 5 and Qt 6 in separate folders.  [T7161]
 
  * qt: Allow reading the data to decrypt/encrypt/sign/verify directly from
@@ -39,6 +42,7 @@ Noteworthy changes in version 1.24.0 (unrelease)
  cpp: Key::revocationKey                 NEW.
  cpp: Key::numRevocationKeys             NEW.
  cpp: Key::revocationKeys                NEW.
+ cpp: Error::asStdString                 NEW.
  qt: DecryptVerifyJob::setInputFile      NEW.
  qt: DecryptVerifyJob::inputFile         NEW.
  qt: DecryptVerifyJob::setOutputFile     NEW.

lang/cpp/src/context.cpp

@@ -130,6 +130,13 @@ const char *Error::asString() const
     return mMessage.c_str();
 }
 
+std::string Error::asStdString() const
+{
+    std::string message;
+    format_error(static_cast<gpgme_error_t>(mErr), message);
+    return message;
+}
+
 int Error::code() const
 {
     return gpgme_err_code(mErr);

lang/cpp/src/error.h

@@ -48,6 +48,7 @@ public:
 
     const char *source() const;
     const char *asString() const;
+    std::string asStdString() const;
 
     int code() const;
     int sourceID() const;