Transcript Lecture 22 - Computer Science at UVA

Lecture 22:
C#: “Sharp”,
“Hash” or
“Pound”?
CS201j: Engineering Software
University of Virginia
Computer Science
David Evans
http://www.cs.virginia.edu/evans
Menu
• Db and C#
• CLU
• What does the “J” in CS201J
really stand for?
Today’s notes: web only
Lots of links to Java history and C# info
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Java History
• 1991: Green Project starts, developing software
for interactive TV boxes
• 1992: Develop a language, “Oak” for
programming them
• Renamed “Java” (by Kim Polese) in a meeting in
a coffee shop
• March 1995: Posted on the Sun web site
• May 1995: Sun announces Java, HotJava
browser, Netscape licenses
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Microsoft and JavaTM
• Dec 1995: Microsoft licenses Java
• Microsoft replaces Sun’s JNI native interface
with their own “improved”, incompatible version
• Oct 1997: Sun sues Microsoft – breach of
contract to provide “Java compatible” products
• Microsoft countersues Sun for failing to deliver
an implementation that passes Sun’s test suite
and failing to provide public test cases
• Jan 2001: Sun wins lawsuit (Microsoft pays
$20M, accepts termination of Java license, and
agrees not to us Java trademarks)
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Exam Question 10:
“…attempt to implement a new
language that will offer the
performance advantages of C
with the safety and ease of
development advantages of
Java…”
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C#
C# is a simple, modern, object oriented,
and type-safe programming language
derived from C and C++. It will
immediately be familiar to C and C++
programmers. C# aims to combine the
high productivity of Visual Basic and the
raw power of C++.
C# Language Specification (p. 15 of 403)
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“C# aims to combine the high
productivity of Visual Basic (Java) and
the raw power of C++.”
Why is this hard?
• Garbage collection depends on:
– Knowing which values are addresses
– Knowing that objects without references cannot
be reached
• If your language allows direct manipulation
of addresses these are impossible
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Possible Solution
• Require type safety
– No unchecked conversions between types, no
conversions between numeric and pointer
types
• Restrict what can be done with addresses:
statically check that only in-object
manipulations are permitted
• Microsoft’s solution:
– Give up! Let programmers make sections of
code “unsafe” then they can
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Unsafe in C#
While practically every pointer type construct in C or C++ has a
reference type counterpart in C#, there are nonetheless situations where
access to pointer types becomes a necessity. For example, interfacing
with the underlying operating system, …, or implementing a time-critical
algorithm may not be possible or practical without access to pointers. To
address this need, C# provides the ability to write unsafe code.
In unsafe code it is possible to declare and operate on pointers, to
perform conversions between pointers and integral types, to take the
address of variables, and so forth. In a sense, writing unsafe code is
much like writing C code within a C# program.
Unsafe code is in fact a “safe” feature from the perspective of both
developers and users. Unsafe code must be clearly marked with the
modifier unsafe, so developers can’t possibly use unsafe features
accidentally, and the execution engine works to ensure that unsafe code
cannot be executed in an untrusted environment.
C# Language Specification, Appendix B
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Unsafe
static unsafe void
copy (byte[] src, byte [] dst, int count) {
fixed (byte* pSrc = src, pDst = dst) {
byte *ps = pSrc; byte *pd = pDst;
for (int n = count; n != 0; n--) {
*pd = *ps;
pd++; ps++;
Within an unsafe block, we
can manipulate pointers
}
similarly C.
}
}
What if stop-and-copy garbage collector
runs while inside the loop?
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Fixed
static unsafe void
copy (byte[] src, byte [] dst, int count) {
fixed (byte* pSrc = src, pDst = dst) {
byte *ps = pSrc; byte *pd = pDst;
for (int n = count; n != 0; n--) {
*pd = *ps;
pd++; ps++;
}
Fixed pins the object where it is; within the
}
fixed block, garbage collector may not
move src or dst. C# compiler will disallow
}
assignments to pointers without the fixed.
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C# - Java
• Java compiles to Java • C# compiles to Microsoft
Intermediate Language
byte codes (JVML)
(MSIL)
• Java VM runs JVML • Microsoft Common
Language Runtime
code
(CLR) runs MSIL code
• Java VM (bytecode
• CLR verifies safety
verifier) verifies safety
properties of JVML code
properties of MSIL
• Designed around
• Designed around .NET
Internet
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Types
• In Java:
– Primitive Types: int, char, etc. (on the stack)
– Object Types: objects, arrays (on the heap)
• In C#:
– Value Types: int, char, struct, etc. (on the
stack)
– Reference Types: objects, arrays (on the
heap)
– All types are subtypes of object (including
value types)
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struct types
• Programmers can define their own types
that will be stored on the stack
public struct Point {
public int x, y;
}
Point points[1000];
In fact, the built-in primitive
types (e.g., int) in C# are just
struct types!
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Boxing and Unboxing
• Value types need to be “boxed” before
they can be used as subtypes of object:
int i = 123;
int i = 123;
object box = i; object box = new int_Box(i);
int j = (int) box;
int i = 123;
Object box = new Integer (i);
Boxing makes a copy of
a value type on the heap.
Cast “unboxes”, can fail run-time type check.
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Implicit Boxing and Unboxing
Vector v;
int i = 23;
Implicit boxing to put i value in heap object
v.add (i);
int el = (int) v.elementAt (0);
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C# Example
static void Main(string[] args)
{
int i = 20;
object o = i;
MyInt mi = new MyInt (20);
MyInt mi2 = mi;
i++;
mi++;
Console.WriteLine (“Values: " + i + " / " + o + " / " + mi + " / " + mi2);
}
Values: 21 / 20 / 21 / 21
int may be a subtype of object, but assignment means
something different for objects an ints!
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MyInt Class
class MyInt
{
private int val;
public MyInt (int value) { val = value; }
public static MyInt operator++ (MyInt mi) {
mi.val++; return mi;
}
You can overload operators (this
is why mi++ works)
}
public override string ToString () {
return val.ToString ();
}
You need the override keyword to indicate
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when a method is overridden.
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Without the override
class MyInt {
private int val;
public MyInt (int value) { val = value; }
public static MyInt operator++ (MyInt mi) { mi.val++; return mi; }
public string ToString () { return val.ToString (); }
}
static void Main(string[] args) {
MyInt mi = new MyInt (20);
Console.WriteLine (mi);
Calls object.ToString ()
string s = mi.ToString ();
Calls MyInt.ToString ()
Console.WriteLine (s);
class1.cs(10,38): warning CS0114:
}
ConsoleApplication1.MyInt
20
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'ConsoleApplication1.MyInt.ToString()' hides
inherited member 'object.ToString()'.
To make the current member override that
implementation, add the override keyword.
Otherwise add the new keyword.
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Overriding
• Java
– Overriding is automatic: lots of confusion between
overriding and overloading
• e.g., public boolean equals (Cell c)
– Methods declared with final cannot be overrode
• C#
– Methods declared with virtual can be overridden,
must explicitly use override keyword in method
header
– Methods declared without virtual can be overriden
but overriding method must use the new keyword in
method header
– Methods declared with sealed cannot be overriden
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C# Example
struct MyInt
{
private int val;
public MyInt (int value) { val = value; }
public static MyInt operator++ (MyInt mi) {
static void Main(string[] args)
mi.val++; return mi;
{
}
int i = 20;
object o = i;
public override string ToString () {
MyInt mi = new MyInt (20);
return val.ToString ();
MyInt mi2 = mi;
}
i++;
}
mi++;
Console.WriteLine (“Values: " + i + " / " + o + " / " + mi + " / " + mi2);
}
Values: 21 / 20 / 21 / 20
MyInt is a struct instead of a class, so it is now stored
on the stack, and assignment means copying!
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Other Differences
• synchronized → lock
• Meaning is identical (lock is a better name)
• import → using
• Meaning slightly different (more like C++ namespaces
than Java packages)
• extends → :
• Exceptions
implements → :
• C# can have catch without exception type (catches
any exception like: catch (Exception e) …
– C# has no throws clauses in declaration, and will
compile code without catch clauses
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Iteration Abstraction
In Java:
Vector v; //@v.elementType == \type(String)
…
for (Enumeration e = v.elements() ; e.hasMoreElements() ;) {
System.out.println ((String) e.nextElement());
}
In C#:
foreach (string s in namesList)
Console.WriteLine(s);
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Getters and Setters
• In Java:
– Need to define methods like getId (), getScore
(), etc. by hand
• In C#:
public int Score {
get { return score; }
set { score = value; } }
In client code:
x.score = x.score + 1; x.setScore (x.getScore () + 1);
Syntactic sugar for this in CLU.
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“Sharp”, “Hash” or “Pound”?
• Microsoft says: “pronounced C sharp”
– But, it is hard to call any language with unsafe and
fixed “sharp”
• C# is a “hash” of C, Java and C++
– But, people might think they made a “hash” of it
• The C# in a nutshell book weighs a few “pound”s
– 856 pages, Java 4th edition = 992 pages, Stephen
Hawking’s “Universe in a Nutshell” = 224 pages
– But, it will be awkward to rename the second edition
“Ckg in a Nutshell”
• Db = a half-tone short of the successor to C?
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CLU
(Liskov et. al., 1975)
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CLU
• Language designed by Liskov’s group in
the 1970s, focused on providing
abstraction mechanisms
• Your textbook was originally called
“Abstraction and Specification in Program
Development” and used CLU as the main
language
• CLU died due to lack of commercial
compiler support
– Only 2½ CLU programmers left
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CLU Datatype Implementation
set = cluster [t: type] is create, insert, elements
rep = array[t]
Datatype implementation can have type
parameters! Can’t do this in Java or C# (in
create = proc () returns (set)
C++, templates provide similar functionality
return up (rep$new ())
very awkwardly)
end create
insert = proc (s: set, el: t)
rep$addh (down (s), el)
end insert
No this object, need to explicitly pass in set.
Explict conversions between rep and
abstract type (up and down)
elements = iter (s: set) yields (t)
for el: t in rep$elements (down (s)) do
yield el
Simple and elegant way to define iteration
end
abstractions. Java has nothing (enumerations),
end elements
C# has foreach for builtin arrays and ArrayList
end set
type, but you can’t define your own.
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Good News / Bad News
• Good News
– You now know enough to list that you know
Java, C, C# and CLU on your resume!
• Bad News
– No one is hiring CLU programmers
– You shouldn’t want to work for anyone too
easily impressed by you knowing the others
Applicants must also have extensive knowledge of Unix,
although they should have sufficiently good programming taste
to not consider this an achievement.
Hal Abelson, MIT job advertisement
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What is the “J” for?
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“Jeffersonian”
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Snakes Tournament
• Thursday’s class will meet in Olsson 001 (usual
time): Open to anyone, bring your cheering
supporters to intimidate your opponents
• Qualification requirements:
– Pass basic functionality tests
– ESC/Java (don’t need to be warning-free to qualify, but
must have annotated important, checkable invariants)
• Tournaments:
– Human driven snakes
– Automatic snakes
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Returning Exam 2
• You have a circled star on your exam: read
the explanation on the exam comments
carefully to interpret it!
– Final cannot count against you, but it is more
“optional” for some of you than others!
• Final will be handed out Thursday, and
due Tuesday, 10 December
– A somewhat Jeffersonian essay question
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