Binding
COMP360
“One Ring to rule them all,
One Ring to find them,
One Ring to bring them all,
And in the darkness bind them.”
inscription on the One Ring
J. R. R. Tolkien
Reading
Read chapter 10 of the textbook
Binding
• A binding is an association between two things, such
as a name and the thing it names
• A name is exactly what you think it is
• Most names are identifiers
• symbols (like '+') can also be names
• The scope of a binding is the part of the program in
which the binding is active
from Programming Language Pragmatics, 4th ed, by Michael Scott
What are we binding?
• At some point a variable name is bound to a memory
location
• In Java, the keyword this means this object. It is
bound to a specific object when the method is
executed
• Keywords (such as final, synchronized or long) are
bound to a specific concept at language design
Binding Time
• Binding Time is the point at which a binding is created
or, more generally, the point at which any
implementation decision is made
• Some bindings are made before the program is
executed
• Other bindings are made during program execution
from Programming Language Pragmatics, 4th ed, by Michael Scott
Binding Time Examples
• Language design time – meaning of keywords
• Language implementation time – implementation of
types
• Program writing time – names to concepts
• Compile time – variable names to relative addresses,
program statements to machine language
• Link time - layout of whole program in memory
Run Time Binding Examples
• Program load – variables and methods assigned to
physical addresses
• Method Entry – actual parameter values bound to
formal parameters, local variables bound to memory
locations
Static and Dynamic
• The terms STATIC and DYNAMIC are generally used to
refer to things bound before run time and at run time,
respectively
• The period of time from creation to destruction is
called the LIFETIME of a binding
• If an object outlives binding, it's garbage
• If a binding outlives an object, it's a dangling reference
from Programming Language Pragmatics, 4th ed, by Michael Scott
Pros and Cons
• In general, early binding times are associated with
greater efficiency
• Later binding times are associated with greater
flexibility
• Compiled languages tend to have early binding times
• Interpreted languages tend to have later binding times
• We generally talk about the binding of identifiers to
the variables they name
from Programming Language Pragmatics, 4th ed, by Michael Scott
When is a value bound to dog?
int myMethod( double dog )
A.
B.
C.
D.
E.
Writing of the program
Compile time
Program linking
Program load
Method entry
Storage Management
• Storage Allocation mechanisms
• Static
• Stack
• Heap
• Static allocation for
• code
• globals
• static or own variables
• explicit constants (including strings, sets, etc)
• scalars may be stored in the instructions
from Programming Language Pragmatics, 4th ed, by Michael Scott
Stack
• Call stack for
• parameters
• local variables
• temporaries
• Why a stack?
• allocate space for recursive routines
(not necessary in FORTRAN – no recursion)
• reuse space
(in all programming languages)
from Programming Language Pragmatics, 4th ed, by Michael Scott
Stack Contents
• Contents of a stack frame
• arguments and returns
• local variables
• temporaries
• bookkeeping (saved registers, line number static link, etc.)
• Local variables and arguments are assigned fixed
OFFSETS from the stack pointer or frame pointer at
compile time
from Programming Language Pragmatics, 4th ed, by Michael Scott
Binding an Array Size
• Binding when program is written
• in myprog.c
int rabbit[123];
Binding an Array Size
• Binding based on a configuration file
• in config.h
#define SIZE 123
• in myprog.c
#include <config.h>
int rabbit[SIZE];
Binding an Array Size
• Binding at execution time
• in myprog.c
int arraysize;
// size of the array
cin >> arraysize;
// read size
int *rabbit;
// pointer to array
rabbit = new int[arraysize];
Binding an Array Size
• Dynamic size of array-like data structure
• in myprog.java
java.util.ArrayList<int> rabbit =
new java.util.ArrayList<>();
Variable Scope
• The “Scope” of a variable refers to where you can use
a variable
• Where you declare a variable determines where it
can be used
• The scope of a binding is the part of the program in
which the binding is active
Variable Range
• Variables defined in a block can only be used in that
block following the variable declaration
{
// you cannot use the variable moth here
double moth = 72.5;
// you may use the variable moth here
// this is the scope of moth
}
// The variable moth is not allowed here
Out of Scope
int cat;
if (whatever == 0) {
int dog = 1;
} else {
int dog = 2;
}
cat = dog * 3;
// Error, dog is out of scope
This works
int cat, dog;
if (whatever == 0) {
dog = 1;
} else {
dog = 2;
}
cat = dog * 3;
// Only one dog variable
Method Parameter Scope
• The parameters defined in a method header can
be used throughout that method
• Method parameter variables cannot be used in
another method
void aMethod( int dog, double cat ) {
You can use dog and cat throughout this method
}
Declarations First
• Java and C++ allow you to put variable declarations
anywhere in a program
• It is usually advantageous to declare a variable at the
beginning of a block
• Some older programming languages required you to
put all variable declarations at the beginning of a
block
• Fortran – All declarations are before executable
statements
• Cobol – declarations are in the “data division”
for Loop Scope
• A loop counter variable only has scope in the loop
for (int cow = 0; cow < 28; cow++) {
// loop body
// the loop counter, cow, can be used here
}
// You cannot use cow after the loop
Local Variables
public static void main( ) {
double
cat = 5, bird = 47;
cat = myfunc( bird );
System.out.print(cat);
}
double myfunc(double cow) {
double bull;
bull = cow * 2.0;
return bull;
}
Scope of
cat and bird
Scope of
cow and bull
Multiple Variable with the Same Name
• In different parts of a program, you can use the
same variable name, but it will mean a different
variable
• This can be confusing and should be avoided
Local Variables (Tricky)
public static void main( ) {
double
cat = 5, bird = 47;
cat = myfunc( bird );
System.out.print(cat);
}
int myfunc(double cow) {
int bird;
bird = (int)cow * 2.0;
return bird;
}
Scope of
cat and bird
Scope of
cow and bird
(different bird)
Variables
• A C++ or Java method can use three different types of
variables
• local variables defined in the method
• parameter variables
• object instance variables
• Object instance variables can be used in any of the
methods of the class
Name Collision
public class Collide {
int rat = 3;
public int square( int rat ) {
return rat * rat;
}
}
Collide thing = new Collide();
int turtle = thing.square( 2 );
• turtle is set to 4
Variable Priority
• A method cannot have a local variable the same name
as a parameter variable
• If a class instance variable has the same name as a
local variable or parameter, the method will always
use the local variable or parameter
More Name Collisions
public class Collide {
int rat = 3;
public int square( int mouse)
{
int rat = mouse * mouse;
return rat;
}
}
• The class instance variable rat is a different
memory location from the local variable rat
Name Collision
public class Collide {
int rat = 3;
public int square( int rat ) {
return this.rat * rat;
}
}
Collide thing = new Collide();
int turtle = thing.square( 2 );
• turtle gets the value 6
Separate Compilation
• Java, C++ and many languages allow modules to be
compiled separately
• Rules for how variables work with separate
compilation are messy
• In C++, static on a function or variable outside a
function or private in Java means it is usable only in
the current source file
• This static is a different notion from the static
variables inside a function
What is printed by this program?
public class Click {
static int crow= 5, raven= 7;
static void myfunc(int parrot) {
int crow;
crow = parrot + 1;
System.out.print( crow );
}
public static void main(…) {
Click ques = new Click();
ques.myfunc( 3 );
System.out.print( crow );
}
}
A.
B.
C.
D.
44
45
55
none of above
What is printed by this program?
public class Click {
static int crow= 5, raven= 7;
static void myfunc(int parrot) {
// changed here
crow = parrot + 1;
System.out.print( crow );
}
public static void main(…) {
Click ques = new Click();
ques.myfunc( 3 );
System.out.print(crow );
}
}
A.
B.
C.
D.
44
45
55
none of above
Created Elsewhere
• extern on a C++ variable or function means that it is
declared in another source file
• In Java and C++, method headers without bodies are
extern by default
No class next week
Spring break is
March 5 to March 11, 2017