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# Parameter Passing - PowerPoint PPT Presentation

Parameter Passing. CMSC 331. Extracted with only very minor changes from http://www.itk.ilstu.edu/faculty/chungli/ITK327/Parameters.ppt. Parameter Passing . How are parameters passed? Looks simple enough… We will see seven techniques. formal parameters. int plus (int a, int b) {

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### Parameter Passing

CMSC 331

Extracted with only very minor changes from

http://www.itk.ilstu.edu/faculty/chungli/ITK327/Parameters.ppt

How are parameters passed?

Looks simple enough…

We will see seven techniques

formal parameters

int plus (int a, int b)

{

return a+b;

}

int x = plus(1,2);

function

body

actual parameters

(arguments)

function call

• By value

• By result

• By value-result

• By reference

• By macro expansion

• By name

• By need

int * sum (int & a, int & b)

{

a += b;

return & a;

}

int * sum (int & a, int & b)

{

int sum;

sum = a + b;

return & sum;

}

int a, b, *c;

a=1;

b=2;

c = sum(a,b);

int * sum (int & a, int & b)

{

a += b;;

return a;

}

Which actual parameters go to which formal parameters?

Most common case: positional parameters

Correspondence determined by positions

nth formal parameter matched with nth actual

Determined by matching parameter names

DIVIDE( DIVIDEND => X, DIVISOR => Y);

DIVIDE( DIVISOR => Y, DIVIDEND => X);

Parameter order is irrelevant here

Most languages that support keyword parameters allow both:

The first parameters in a list can be positional, and the remainder can be keyword parameters

Default values: formal parameter list includes default values to be used if the corresponding actual is missing

int f(int a=1, int b=2, int c=3) { body }

int x=f(), y=f(0), z=f(0,0), h=f(0,0,0);

int f() {f(1,2,3);}int f(int a) {f(a,2,3);}int f(int a, int b) {f(a,b,3);}int f(int a, int b, int c) { body }

Some languages allow actual parameter lists of unbounded length: C, C++, and scripting languages like JavaScript, Python, and Perl

Library routines must be used to access the excess actual parameters

A hole in static type systems, since the types of the excess parameters cannot be checked at compile time

int printf(char *format, ...) { body }

Simplest method

Widely used

The only method in Java!!!!

For by-value parameter passing, the formal parameter is just like a local variable in the activation record of the called method, with one important difference: it is initialized using the value of the corresponding actual parameter, before the called method begins executing.

Or not!

Java is Pass-By-Value, Dammit!

int plus(int a, int b) { a += b; return a; }

void f() { int x = 3; int y = 4; int z = plus(x, y);}

plus(3,4)

f()

When plus is starting

Also called copy-out

Actual must have an lvalue

Introduced in Algol 68; sometimes used for Ada

For by-result parameter passing, the formal parameter is just like a local variable in the activation record of the called method—it is uninitialized. After the called method finished executing, the final value of the formal parameter is assigned to the corresponding actual parameter.

void plus(int a, int b, by-result int c) { c = a+b;} void f() { int x = 3; int y = 4; int z; plus(x, y, z); }

plus(3,4,z)

f()

When plus is starting

void plus(int a, int b, by-result int c) { c = a+b;} void f() { int x = 3; int y = 4; int z; plus(x, y, z); }

plus(3,4,z)

f()

When plus is ready to return

void plus(int a, int b, by-result int c) { c = a+b;} void f() { int x = 3; int y = 4; int z; plus(x, y, z); }

f()

When plus has returned

Also called copy-in/copy-out

Actual must have an lvalue

For passing parameters by value-result, the formal parameter is just like a local variable in the activation record of the called method. It is initialized using the value of the corresponding actual parameter, before the called method begins executing. Then, after the called method finishes executing, the final value of the formal parameter is assigned to the actual parameter.

void plus(int a, by-value-result int b) { b += a;}void f() { int x = 3; plus(4, x); }

plus(4,x)

f()

When plus is starting

void plus(int a, by-value-result int b) { b += a;}void f() { int x = 3; plus(4, x); }

plus(4,x)

f()

When plus is ready to return

void plus(int a, by-value-result int b) { b += a;}void f() { int x = 3; plus(4, x); }

f()

When plus has returned

f(): int x = 3; fork: {

check_d(4, x);

check_d(5, x);

}

check_d(int a,

int value-result b) b = ....;

}

8

7

One of the earliest methods: Fortran

Most efficient for large objects

Still frequently used

For passing parameters by reference, the lvalue of the actual parameter is computed before the called method executes. Inside the called method, that lvalue is used as the lvalue of the corresponding formal parameter. In effect, the formal parameter is an alias for the actual parameter—another name for the same memory location.

void plus(int a, by-reference int b) { b += a;}void f() { int x = 3; plus(4, x); }

Can this be 4?

plus(4,x)

f()

When plus is starting

void f() { int x = 3; plus(4, x); }

plus(4,x)

f()

void plus(int a, int & b) { b += a;}

void f() { int x = 3; plus(4, x); }

This is call by reference

void plus(int a, int *b) { *b += a;}

void f() { int x = 3; plus(4, &x); }

This is call by value

void F( int a[]) { a[1]=a[1]+1;...}

int x[5];F(x); ....

?

call by value

call by reference

Will array x be modified?

call by value

void F(int * a) { a = new int[10];...}

int x[5];F(x); ....

call by value

void F(int a[]) { a = new int[10];...}

int x[5];F(x); ....

This is what java does!!

When two expressions have the same lvalue, they are aliases of each other

Obvious cases:

ConsCell x = new ConsCell(0,null);ConsCell y = x;

Aliases: expressions that have the same lvalue

void sigsum(by-reference int n, by-reference int ans) { ans = 0; int i = 1; while (i <= n) ans += i++;}

int f() { int x,y; x = 10; sigsum(x,y); return y;}

int g() { int x; x = 10; sigsum(x,x); return x;}

void sigsum(by-reference int x, by-reference int bound) {

while (x <= bound) x++;

}

int a=10;

sigsum(a,a);

Like C macros

Natural implementation: textual substitutionbefore compiling

• The body of the macro is evaluated in the caller’s context.

• Each actual parameter is evaluated on every use in the body (in the caller’s context).

Macro Expansions In C

#define MIN(X,Y) ((X)<(Y)?(X):(Y))

....

....a = MIN(b,c);

source file:

....

....

a = ((b)<(c)?(b):(c))

expanded source:

Replace each use of the macro with a copy of the macro body, with actuals substituted for formals

An old technique, used in assemblers before the days of high-level languages

It has some odd effects…

#define M(X,Y) X = 2*Y

....

....M(a,b+c);

source file:

....

....

a = 2*b+c;

expanded source:

Each actual parameter is re-evaluated upon every use

#define MIN(X,Y) ((X)<(Y)?(X):(Y))

.....a = MIN(f(b),f(c));

.....

a = MIN(b++,c++);

a = ((f(b))<(f(c))?(f(b)):(f(c)))

a = ((b++)<(c++)?(b++):(c++))

Capture, used to be a big problem in logic

Txy(x=y+c+d)

free variables

capture c

Hc T

H  cxy(x=y+c+d)

free variable

Most PL’s macro expansions do that! Bad!!

g(a) cxy(x=y+c+a)

free variable

h g(c)

capture

h = cxy(x=y+c+c)

T(c)xy(x=y+c+d)

xxy(x=y+x+d)

Hx T(x)

Conversion:

x  a

incorrect

T(c)ay(a=y+c+d)

H  xay(a=y+x+d)

Most PL’s macro expansions don’t do that

In a program fragment, any occurrence of a variable that is not statically bound is free

When a fragment is moved to a different context, its free variables can become bound

This phenomenon is called capture:

Free variables in the actuals can be captured by definitions in the macro body

Also, free variables in the macro body can be captured by definitions in the caller

Logic

#define intswap(X,Y) {int temp=X; X=Y; Y=temp;}

int main() { int temp=1, b=2;intswap(temp,b); ……

}

Source code

Expanded code

int main() { int temp=1, b=2;{int temp = temp ; temp = b ; b =temp;}; ……}

x is captured by temp

By Name (a way to avoid previous bad capture)

Like macro expansion without capture

Algol 60 and others

Now unpopular

Each actual parameter is evaluated in the caller’s context on every use of the corresponding formal parameter.

The actual parameter is treated like a little anonymous function

Whenever the called method needs the value of the formal (either rvalue or lvalue) it calls the function to get it

The function must be passed with its nesting link, so it can be evaluated in the caller’s context

intswap(by-name int X, by-name int Y){ int temp=X; X=Y; Y=temp;}

temp()

int main() { int temp=1, b=2;intswap(temp,b); ……

}

b()

intswap(x, y)

Main()

Previous

Activation Record

Previous

Activation Record

local variable

:

temp:

local variable

:

temp:1

b:2

temp = X;

X = Y;

1

2

1

Y = temp;

void f(by-name int a, by-name int b) { b=5; b=a;}

int g() { int i = 3; f(i+1,i); return i;}

6

5

5

i = 5;

i = i+1;

i = 5;

i = i+1;

Like macro expansion, by-name parameters are re-evaluated every time they are used

(Can be useful, but more often this is merely wasteful)

Unlike macro expansion, there is no possibility of capture.

Used in lazy functional languages (Haskell and other Languages)

Avoids wasteful re-computations of by-name

Each actual parameter is evaluated in the caller’s context, on the first use of the corresponding formal parameter. The value of the actual parameter is then cached, so that subsequent uses of the corresponding formal parameter do not cause reevaluation.

void f(by-need int a, by-need int b) { b=a; b=a;}

void g() { int i = 3; f(i+1,i); return i;}

4

3

4

/* a = i+1; */

b = a;

b = a;

void f(by-name int a, by-name int b) { b=a; b=a;}

void g() { int i = 3; f(i+1,i); return i;}

5

4

5

4

3

5

4

i = i+1;

i = i+1;

Laziness: compute only when necessary

boolean andand(by-need boolean a, by-need boolean b) { if (a) return true; else return b;}

Compute b only when necessary

void f() { andand(sometest(),g());}

boolean g() { while (true) { } return true;}