https://bugs.gentoo.org/788301
--- a/include/osgDB/ObjectCache
+++ b/include/osgDB/ObjectCache
@@ -72,7 +72,7 @@ class OSGDB_EXPORT ObjectCache : public osg::Referenced
 
         class ClassComp {
         public:
-            bool operator() (const ObjectCache::FileNameOptionsPair& lhs, const ObjectCache::FileNameOptionsPair& rhs);
+            bool operator() (const ObjectCache::FileNameOptionsPair& lhs, const ObjectCache::FileNameOptionsPair& rhs) const;
         };
 
         typedef std::pair<osg::ref_ptr<osg::Object>, double >           ObjectTimeStampPair;
--- a/src/osgDB/ObjectCache.cpp
+++ b/src/osgDB/ObjectCache.cpp
@@ -16,7 +16,7 @@
 
 using namespace osgDB;
 
-bool ObjectCache::ClassComp::operator() (const ObjectCache::FileNameOptionsPair& lhs, const ObjectCache::FileNameOptionsPair& rhs)
+bool ObjectCache::ClassComp::operator() (const ObjectCache::FileNameOptionsPair& lhs, const ObjectCache::FileNameOptionsPair& rhs) const
 {
     // check if filename are the same
     if (lhs.first < rhs.first) return true;