12.3 Definite assignment

if the compiler can prove, by a particular static flow analysis (?2.3.3),
that the variable has been
automatically initialized or has been the target of at least one
assignment. [Note: Informally stated, the rules
of definite assignment are:
?An initially assigned variable (?2.3.1) is always considered definitely
assigned.
?An initially unassigned variable (?2.3.2) is considered definitely
assigned at a given location if all
possible execution paths leading to that location contain at least one of
the following:
A simple assignment (?4.13.1) in which the variable is the left operand.
An invocation expression (?4.5.5) or object creation expression (?4.5.10.1)
that passes the variable as an
output parameter.
For a local variable, a local variable declaration (?5.5) that includes a
variable initializer.
The formal specification underlying the above informal rules is described
in ?2.3.1, ?2.3.2, and ?2.3.3.
end note]
The definite assignment states of instance variables of a struct-type
variable are tracked individually as well
as collectively. In additional to the rules above, the following rules
apply to struct-type variables and their
instance variables:
?An instance variable is considered definitely assigned if its containing
struct-type variable is considered
definitely assigned.
?A struct-type variable is considered definitely assigned if each of its
instance variables is considered
definitely assigned.
Definite assignment is a requirement in the following contexts:
?A variable must be definitely assigned at each location where its value
is obtained. [Note: This ensures
that undefined values never occur. end note] The occurrence of a variable
in an expression is considered
to obtain the value of the variable, except when
the variable is the left operand of a simple assignment,
the variable is passed as an output parameter, or
the variable is a struct-type variable and occurs as the left operand of a
member access.
?A variable must be definitely assigned at each location where it is
passed as a reference parameter.
[Note: This ensures that the function member being invoked can consider the
reference parameter
initially assigned. end note]
?All output parameters of a function member must be definitely assigned at
each location where the
function member returns (through a return statement or through execution
reaching the end of the
function member body). [Note: This ensures that function members do not
return undefined values in
output parameters, thus enabling the compiler to consider a function member
invocation that takes a
variable as an output parameter equivalent to an assignment to the
variable. end note]
?The this variable of a struct-type instance constructor must be
definitely assigned at each location
where that instance constructor returns.