Thread-safe LRU cache
Not repeating what 1201ProgramAlarm reported.

std::mutex    m_lock;   // reader writer lock, multiple getters are allowed

I see a regular exclusive lock, not a reader/writer lock.
std::lock_guard&lt;std::mutex&gt; gaurd(m_lock);

Not taking advantage of class template argument deduction?  This is a case where I really want it.  (If not, use an alias for the lock type so it can be easily changed.)
std::lock_guard guard(m_lock);


    m_lruList{},
    m_lruMap{},
    m_lock{}

Just adds noise — is that a thing someone actually recommends?  I would say don’t list all the things that are taken care of on their own.
   for (const auto &amp;node : m_lruList)
        std::cout &lt;&lt; node._key &lt;&lt; std::endl;

The style in C++ is to put the * or &amp; with the type, not the identifier.  This is called out specifically near the beginning of Stroustrup’s first book, and is an intentional difference from C style.
Some people criticize the leading underscore of identifiers as a style.  I point out that it is easy to clash with compiler-supplied identifiers and you must remember to never use a capital letter after the score, so it is a hazard.

Update
#ifndef LRUCache_h__

Any name containing a double underscore is reserved for use by the implementation.

/***
 * Method:        objcache::LRUCache::LRUCache
 * Access:        public 
 * Description: This creates a cache of given size. 
 */

Banner comments like this just get in the way of reading the code.  Repeating the function name in a comment just before its definition does what exactly?  (Oh, and a point of nomenclature:  C++ does not have “methods”.)  Nobody comments each function as being public or private.  For the description, don’t prefix it with “Description:”; just describe it.
Since I can see name of the argument is size and see how it’s used on the on the following line
LRUCache(size_t size) :
    m_cacheSize{ size },

a comment saying
// This creates a cache of given size

is useless.  It only repeats what is in the signature:  A constructor creates an instance of this class, and this one takes a size.
See (⧺P.1).  You might also find this useful (Comments starts on page 17; material originally published in 1992).