Unit type

In the area of mathematical logic and computer science known as type theory, a unit type is a type that allows only one value (and thus can hold no information). The carrier (underlying set) associated with a unit type can be any singleton set. There is an isomorphism between any two such sets, so it is customary to talk about "the" unit type and ignore the details of its value. One may also regard the unit type as the type of 0-tuples, i.e. the product of no types.
The unit type is the terminal object in the category of types and typed functions. It should not be confused with the "zero" or bottom type, which allows "no" values and is the initial object in this category. Similarly, the Boolean is the type with "two" values.
The unit type is implemented in most functional programming languages. The void type that is used in some imperative programming languages serves some of its functions, but because its carrier set is empty, it has some limitations (as detailed below).
In programming languages.
Several computer programming languages provide a unit type to specify the result type of a function with the sole purpose of causing a side effect, and the argument type of a function that does not require arguments. 
Void type as unit type.
In C, C++, C#, and D, codice_26 is used to designate a function that does not return anything useful, or a function that accepts no arguments. The unit type in C is conceptually similar to an empty codice_27, but a struct without members is not allowed in the C language specification (this is allowed in C++). Instead, 'codice_26' is used in a manner that simulates some, but not all, of the properties of the unit type, as detailed below. Like most imperative languages, C allows functions that do not return a value; these are specified as having the void return type. Such functions are called procedures in other imperative languages like Pascal, where a syntactic distinction, instead of type-system distinction, is made between functions and procedures.
Difference in calling convention.
The first notable difference between a true unit type and the void type is that the unit type may always be the type of the argument to a function, but the void type cannot be the type of an argument in C, despite the fact that it may appear as the sole argument in the list. This problem is best illustrated by the following program, which is a compile-time error in C:
int main(void)
 f(g()); // compile-time error here
 return 0;
This issue does not arise in most programming practice in C, because since the codice_26 type carries no information, it is useless to pass it anyway; but it may arise in generic programming, such as C++ templates, where codice_26 must be treated differently from other types. In C++ however, empty classes are allowed, so it is possible to implement a real unit type; the above example becomes compilable as:
class unit_type {};
const unit_type the_unit;
int main()
 f(g(the_unit));
 return 0;
Difference in storage.
The second notable difference is that the void type is special and can never be stored in a record type, i.e. in a struct or a class in C/C++. In contrast, the unit type can be stored in records in functional programming languages, i.e. it can appear as the type of a field; the above implementation of the unit type in C++ can also be stored. While this may seem a useless feature, it does allow one for instance to elegantly implement a set as a map to the unit type; in the absence of a unit type, one can still implement a set this way by storing some dummy value of another type for each key.
In Generics.
In Java Generics, type parameters must be reference types. The wrapper type codice_9 is often used when a unit type parameter is needed. Although the codice_9 type can never have any instances, it does have one value, codice_13 (like all other reference types), so it acts as a unit type. In practice, any other non-instantiable type, e.g. codice_35, can also be used for this purpose, since they also have exactly one value, codice_13.
public static void main(String[] args)
 f(g(null));