c++ persisting data

Question

what I want to archive is a simple way to make some variables persistent. For this I wrote a PeristenceProvider Class which wraps the boost property tree functionality to store data in xml / ini files.

At the moment I need to do things like this:

ClassA::ClassA()
{
   m_valueI = PersistenceProvider::getInstance.get<int>("valueI");
}

ClassA::~ClassA()
{
    PeristenceProvider::getInstance.set<int>("valueI", m_valueI);
}

But is there a chance to hide this in a way like this:

class ClassA
{
     Persist<int, "valueI"> m_ValueI;
}

Answer 1

It's possible but not exactly that way. You cannot use string literals to instantiate template. String objects with external linkage are only allowed to be non-type arguments. So string constant must be defined as extern and be char[], not just char*.

See example (it will print "Hello" and "World", really cool, isn't it?):

extern const char hello[] = "Hello";
extern const char world[] = "World";

template<const char* s> struct X
{
   X()
   {
      std::cout << s << std::endl;
   }
};

X<hello> z1;
X<world> z2;