In .NET, there are two categories of types, reference types and value types.
Structs are value types and classes are reference types.
The general difference is that a reference type lives on the heap, and a value type lives inline, that is, wherever it is your variable or field is defined.
A variable containing a value type contains the entire value type value. For a struct, that means that the variable contains the entire struct, with all its fields.
A variable containing a reference type contains a pointer, or a reference to somewhere else in memory where the actual value resides.
This has one benefit, to begin with:
- value types always contains a value
- reference types can contain a null-reference, meaning that they don't refer to anything at all at the moment
Internally, reference types are implemented as pointers, and knowing that, and knowing how variable assignment works, there are other behavioral patterns:
- copying the contents of a value type variable into another variable, copies the entire contents into the new variable, making the two distinct. In other words, after the copy, changes to one won't affect the other
- copying the contents of a reference type variable into another variable, copies the reference, which means you now have two references to the same somewhere else storage of the actual data. In other words, after the copy, changing the data in one reference will appear to affect the other as well, but only because you're really just looking at the same data both places
When you declare variables or fields, here's how the two types differ:
- variable: value type lives on the stack, reference type lives on the stack as a pointer to somewhere in heap memory where the actual memory lives (though note Eric Lipperts article series: The Stack Is An Implementation Detail.)
- class/struct-field: value type lives completely inside the type, reference type lives inside the type as a pointer to somewhere in heap memory where the actual memory lives.
The differences between a class and a struct in C++ is:
struct members and base classes/structs are public by default.class members and base classes/struts are private by default.
Both classes and structs can have a mixture of public, protected and private members, can use inheritance and can have member functions.
I would recommend you:
- use
struct for plain-old-data structures without any class-like features;
- use
class when you make use of features such as private or protected members, non-default constructors and operators, etc.
Best Answer
The basic idea here is that your class only exists for the duration of the html request. After the request is processed, your class goes away.
State means that you "remember" things from one call to the next in class variables. Stateless means that you don't.
It is certainly possible to store a class in the session and have it around for every page, but I gather that is not the situation discussed in the book.
EDIT:
An example of a stateful class might be one that increments a "pages visited" counter each time a page is loaded. That would not work unless you persist your class in the session.