simplify interface for usage with cache

examples/examples3.cpp

@@ -23,7 +23,7 @@ int main(int argc, char *argv[])
     qInfo() << "Default difficulty:" << ZeroStorageCaptcha::defaultDifficulty();
     qInfo() << "Default answer length:" << ZeroStorageCaptcha::defaultAnswerLength();
 
-    auto c = ZeroStorageCaptchaService::Cache::get();
+    auto c = ZeroStorageCaptcha::cached();
     QFile pic("c.png");
     if (not pic.open(QIODevice::WriteOnly)) return 1;
     pic.write(c->picturePng());

zerostoragecaptcha.cpp

@@ -80,6 +80,11 @@ ZeroStorageCaptcha::ZeroStorageCaptcha(const QString &answer, int difficulty)
     render();
 }
 
+QSharedPointer<ZeroStorageCaptcha> ZeroStorageCaptcha::cached()
+{
+    return ZeroStorageCaptchaService::Cache::get();
+}
+
 bool ZeroStorageCaptcha::validate(const QString &answer, const QString &token)
 {
     return ZeroStorageCaptchaService::TokenManager::validateAnswer(answer, token);

zerostoragecaptcha.h

@@ -119,10 +119,11 @@ private:
 
 class ZeroStorageCaptcha
 {
-    friend ZeroStorageCaptchaService::Cache;
+    friend ZeroStorageCaptchaService::Cache; // for dropToken()
 public:
     ZeroStorageCaptcha();
     ZeroStorageCaptcha(const QString& answer, int difficulty = ZeroStorageCaptchaService::Cache::difficulty());
+    static QSharedPointer<ZeroStorageCaptcha> cached();
     static bool validate(const QString& answer, const QString& token);
     static void setCacheMaxCapacity(qsizetype value);
     static qsizetype cacheMaxCapacity();