Creating a system wide resource C++

I am having an issue designing a software. I have a very big object(BO), let's say a map of dynamic arrays, that I need to access across various classes. Each time a class uses BO, it adds new data to it, or uses some of the data in BO to do computation.

As a newbie, I am lost on how to design such an object in C++ so that my code design and architecture is good. Making a global variable is not efficient enough, researched on Singleton but found out the rule is you can only access the object, not change it.

Anyone can point me in the right direction? what design pattern should I follow that is the most efficient and scalable?

Thanks