Avoid unnecessary detaching of containers

If only a constant iterator is needed, make the container const if
possible, otherwise use cbegin() and cend() to get a constant iterator
without detaching the container.

These fixes are needed so that `auto` can deduce the correct
iterator type.

taglib/ogg/oggfile.cpp

@@ -95,7 +95,7 @@ ByteVector Ogg::File::packet(unsigned int i)
 
   // Look for the first page in which the requested packet starts.
 
-  List<Page *>::ConstIterator it = d->pages.begin();
+  List<Page *>::ConstIterator it = d->pages.cbegin();
   while((*it)->containsPacket(i) == Page::DoesNotContainPacket)
     ++it;
 
@@ -160,7 +160,7 @@ bool Ogg::File::save()
   }
 
   Map<unsigned int, ByteVector>::ConstIterator it;
-  for(it = d->dirtyPackets.begin(); it != d->dirtyPackets.end(); ++it)
+  for(it = d->dirtyPackets.cbegin(); it != d->dirtyPackets.cend(); ++it)
     writePacket(it->first, it->second);
 
   d->dirtyPackets.clear();
@@ -236,7 +236,7 @@ void Ogg::File::writePacket(unsigned int i, const ByteVector &packet)
 
   // Look for the pages where the requested packet should belong to.
 
-  List<Page *>::ConstIterator it = d->pages.begin();
+  List<Page *>::ConstIterator it = d->pages.cbegin();
   while((*it)->containsPacket(i) == Page::DoesNotContainPacket)
     ++it;
 

taglib/ogg/oggpage.cpp

@@ -211,7 +211,7 @@ ByteVectorList Ogg::Page::packets() const
 
     d->file->seek(d->fileOffset + d->header.size());
 
-    List<int> packetSizes = d->header.packetSizes();
+    const List<int> packetSizes = d->header.packetSizes();
 
     List<int>::ConstIterator it = packetSizes.begin();
     for(; it != packetSizes.end(); ++it)
@@ -243,8 +243,8 @@ ByteVector Ogg::Page::render() const
       debug("Ogg::Page::render() -- this page is empty!");
   }
   else {
-    ByteVectorList::ConstIterator it = d->packets.begin();
-    for(; it != d->packets.end(); ++it)
+    ByteVectorList::ConstIterator it = d->packets.cbegin();
+    for(; it != d->packets.cend(); ++it)
       data.append(*it);
   }
 

taglib/ogg/oggpageheader.cpp

@@ -295,7 +295,7 @@ ByteVector Ogg::PageHeader::lacingValues() const
 {
   ByteVector data;
 
-  for(List<int>::ConstIterator it = d->packetSizes.begin(); it != d->packetSizes.end(); ++it) {
+  for(List<int>::ConstIterator it = d->packetSizes.cbegin(); it != d->packetSizes.cend(); ++it) {
 
     // The size of a packet in an Ogg page is indicated by a series of "lacing
     // values" where the sum of the values is the packet size in bytes.  Each of
@@ -304,7 +304,7 @@ ByteVector Ogg::PageHeader::lacingValues() const
 
     data.resize(data.size() + (*it / 255), '\xff');
 
-    if(it != --d->packetSizes.end() || d->lastPacketCompleted)
+    if(it != --d->packetSizes.cend() || d->lastPacketCompleted)
       data.append(static_cast<unsigned char>(*it % 255));
   }
 

taglib/ogg/xiphcomment.cpp

@@ -39,7 +39,7 @@ namespace
 
   typedef List<FLAC::Picture *> PictureList;
   typedef PictureList::Iterator PictureIterator;
-  typedef PictureList::Iterator PictureConstIterator;
+  typedef PictureList::ConstIterator PictureConstIterator;
 } // namespace
 
 class Ogg::XiphComment::XiphCommentPrivate
@@ -191,7 +191,7 @@ void Ogg::XiphComment::setTrack(unsigned int i)
 
 bool Ogg::XiphComment::isEmpty() const
 {
-  for(FieldConstIterator it = d->fieldListMap.begin(); it != d->fieldListMap.end(); ++it) {
+  for(FieldConstIterator it = d->fieldListMap.cbegin(); it != d->fieldListMap.cend(); ++it) {
     if(!(*it).second.isEmpty())
       return false;
   }
@@ -203,7 +203,7 @@ unsigned int Ogg::XiphComment::fieldCount() const
 {
   unsigned int count = 0;
 
-  for(FieldConstIterator it = d->fieldListMap.begin(); it != d->fieldListMap.end(); ++it)
+  for(FieldConstIterator it = d->fieldListMap.cbegin(); it != d->fieldListMap.cend(); ++it)
     count += (*it).second.size();
 
   count += d->pictureList.size();
@@ -225,11 +225,11 @@ PropertyMap Ogg::XiphComment::setProperties(const PropertyMap &properties)
 {
   // check which keys are to be deleted
   StringList toRemove;
-  for(FieldConstIterator it = d->fieldListMap.begin(); it != d->fieldListMap.end(); ++it)
+  for(FieldConstIterator it = d->fieldListMap.cbegin(); it != d->fieldListMap.cend(); ++it)
     if (!properties.contains(it->first))
       toRemove.append(it->first);
 
-  for(StringList::ConstIterator it = toRemove.begin(); it != toRemove.end(); ++it)
+  for(StringList::ConstIterator it = toRemove.cbegin(); it != toRemove.cend(); ++it)
       removeFields(*it);
 
   // now go through keys in \a properties and check that the values match those in the xiph comment
@@ -366,13 +366,13 @@ ByteVector Ogg::XiphComment::render(bool addFramingBit) const
   // std::pair<String, StringList> where the first String is the field name and
   // the StringList is the values associated with that field.
 
-  FieldListMap::ConstIterator it = d->fieldListMap.begin();
-  for(; it != d->fieldListMap.end(); ++it) {
+  FieldListMap::ConstIterator it = d->fieldListMap.cbegin();
+  for(; it != d->fieldListMap.cend(); ++it) {
 
     // And now iterate over the values of the current list.
 
     String fieldName = (*it).first;
-    StringList values = (*it).second;
+    const StringList values = (*it).second;
 
     StringList::ConstIterator valuesIt = values.begin();
     for(; valuesIt != values.end(); ++valuesIt) {
@@ -385,7 +385,7 @@ ByteVector Ogg::XiphComment::render(bool addFramingBit) const
     }
   }
 
-  for(PictureConstIterator it = d->pictureList.begin(); it != d->pictureList.end(); ++it) {
+  for(PictureConstIterator it = d->pictureList.cbegin(); it != d->pictureList.cend(); ++it) {
     ByteVector picture = (*it)->render().toBase64();
     data.append(ByteVector::fromUInt(picture.size() + 23, false));
     data.append("METADATA_BLOCK_PICTURE=");