The general technique is to define a function named "serialize" in the ccs::serialization namespace, and that function must take a "serializer object" that can be of any type, and a reference to the class that is to be serialized. The function then passes to the "serializer object" the members of the class instance that must be serialized. For example, a banking program manages instances of this bank_account class:

class bank_account {
public:
  bank_account(int id, float balance) : id_(id), balance_(balance) {}

  int id_;        // this is a PUBLIC member
  float balance_; // this is a PUBLIC member
};

The following code serializes instances of the bank_account class:

namespace ccs {
  namespace serialization {
    template <typename S>  void  serialize(S& s, bank_account& ba) {
      s & ba.id_ & ba.balance_;
    }
  }
}

Notice that because the function is outside the class, the members must be declared "public:" otherwise the serialization function could not write to the members, which would make loading impossible. Also, the constructor of the bank account class must be default-constructible, otherwise it will not be possible to load a vector, say, of bank_account instances. Thus:

class bank_account {
public:
  bank_account(int id = 0, float balance = 0) :
    id_(id), balance_(balance) {}

  ...
};