Differences between "in"/"out" and "nocopy"

By default, PL/SQL passes 

OUT

 and 

IN

 

OUT

 subprogram parameters by value. Before running the subprogram, PL/SQL copies each 

OUT

 and 

IN

 

OUT

 parameter
to a temporary variable, which holds the value of the parameter during subprogram execution. If the subprogram is exited normally, then PL/SQL copies the value of the temporary variable to the corresponding actual parameter. If the subprogram is exited with
an unhandled exception, then PL/SQL does not change the value of the actual parameter.

When 

OUT

 or 

IN

 

OUT

 parameters represent large data structures such as collections, records, and instances of ADTs, copying them slows execution and increases
memory use—especially for an instance of an ADT.

For each invocation of an ADT method, PL/SQL copies every attribute of the ADT. If the method is exited normally, then PL/SQL applies any changes that the method made to the attributes. If the method is exited with an unhandled exception, then PL/SQL does not
change the attributes.

Above are the describe in Oracle standard document. Now we use the below code to check this points:

declare
v_value1 number;
procedure add1(p_v1 in out number) as
v_t number := 1;
begin
p_v1 := p_v1 + 1;

v_t := v_t / 0;--throw out an exception
end;
procedure add2(p_v1 in out nocopy number) as
v_t number := 1;
begin
p_v1 := p_v1 + 1;

v_t := v_t / 0;--throw out an exception
end;
begin
v_value1 := 1;

dbms_output.put_line('check "in"/"out": ' || v_value1);

begin
add2(v_value1);

dbms_output.put_line(v_value1);
exception
when others then

dbms_output.put_line('error:' || v_value1);

end;

v_value1 := 1;

dbms_output.put_line('check "nocopy": ' || v_value1);

begin
add1(v_value1);

dbms_output.put_line(v_value1);
exception
when others then

dbms_output.put_line('error:' || v_value1);
end;
end;

Output:

check "in"/"out": 1
error:2
check "nocopy": 1
error:1

This feature is more like the differences between pass a variable by reference and by value in C.

The "nocopy" parameters use less memory and not need to copy the values when invoked, which is useful for performance tuning.