new ExifKey constructor based on TagInfo container. Factoring duplicate code with other ExifKey constructor

src/tags.cpp

@@ -2801,8 +2801,7 @@ namespace Exiv2 {
         key_ = std::string(familyName_) + "." + groupName_ + "." + tagName();
     }
 
-    ExifKey::ExifKey(uint16_t tag, const std::string& groupName)
-        : p_(new Impl)
+    void ExifKey::create_internal(uint16_t tag, const std::string& groupName)
     {
         IfdId ifdId = groupId(groupName);
         // Todo: Test if this condition can be removed
@@ -2817,6 +2816,18 @@ namespace Exiv2 {
         p_->makeKey(tag, ifdId, ti);
     }
 
+    ExifKey::ExifKey(uint16_t tag, const std::string& groupName)
+        : p_(new Impl)
+    {
+        create_internal(tag, groupName);
+    }
+
+    ExifKey::ExifKey(const TagInfo& ti)
+        : p_(new Impl)
+    {
+        create_internal(ti.tag_, Internal::groupName(static_cast<Internal::IfdId>(ti.ifdId_)));
+    }
+
     ExifKey::ExifKey(const std::string& key)
         : p_(new Impl)
     {

src/tags.hpp

@@ -173,6 +173,13 @@ namespace Exiv2 {
                  and group name.
          */
         ExifKey(uint16_t tag, const std::string& groupName);
+        /*!
+          @brief Constructor to create an Exif key from a TagInfo instance.
+          @param ti The TagInfo instance
+          @throw Error if the key cannot be constructed from the tag number
+                 and group name.
+         */
+        ExifKey(const TagInfo& ti);
         //! Copy constructor
         ExifKey(const ExifKey& rhs);
         //! Destructor
@@ -212,6 +219,8 @@ namespace Exiv2 {
     private:
         //! Internal virtual copy constructor.
         EXV_DLLLOCAL virtual ExifKey* clone_() const;
+        //! Internal construction method.
+        void create_internal(uint16_t tag, const std::string& groupName);
 
     private:
         // Pimpl idiom