Application - defined data

To add to what Cilu said the application defined data can be defined recursivelly as "a data type composed of 1 or more application defined application data or basic types". Example:

// aplication defined data containing only basic types
struct A
{
int x;
};

// aplication defined data containing basic types and library types
struct B
{
int y;
bool b;
std::string s;
};

// aplication defined data containing basic types, library types and other application defined data
struct C
{
A a;
B b;

float f;
std::string s;
};

An application data can also be an alias of a built-in type, except that in this case there is an inplicit conversion available to the built-in type.

typedef int Code;
typedef is a syntactical feature. It should resolve to the type it's aliasing at compile time. No conversions, implicit or explicit, should be required unless you meant conversion to the type name it's an alias for, at compile time.