Parameter Passing Parameter Passing MechanismsMechanisms

Reference ParametersReference Parameters

Read §10.1 - §10.3Read §10.1 - §10.3

1

ProblemProblem

2

Using OCD, design and implement a function that, given a string containing an IP address, decomposes it into its four network/host information blocks (aaa, bbb, ccc, ddd) and returns these values.

IPv4 (Internet Protocol) addresses are expressed using "dotted decimal" notation:

aaa.bbb.ccc.dddwhere aaa, bbb, ccc, and ddd are 1-byte (8-bit) integers that provide network or host information.

ipv616 bytes

Find/locate IP address

Preliminary AnalysisPreliminary Analysis

Our function can receive the IP address through a string parameter.

This problem requires that our function somehow communicate four values (the network/host information blocks ) back to its caller.

A function cannot return multiple values — the return statement only returns one value:

return expression; 3

BehaviorBehavior

Our subprogram should receive from its caller an IP address (a string). It should compute and pass back its four network/host information blocks or halt execution if it did not receive a valid IP address.

4

ObjectsObjects

5

Description Type NameIP address string ipAddr

first block string info1

second block string info2

third block string info3

fourth block string info4

Movement______

______

______

______

______

OperationsOperations

Description Predefined? Library? Name

6

receive a string yes built-in none

select part of a yes string substr()string

search a string yes string find()

halt if error yes cassert assert()

pass back 4strings

yes built-in ??

AlgorithmAlgorithm

7

0. Receive ipAddress from caller and declare four variables info1, info2,info3, and info4.

1. Fill info1 with appropriate substring of ipAddr or halt if it can't be found.

2. Fill info2 with appropriate substring of ipAddr or halt if it can't be found.

3. Fill info3 with appropriate substring of ipAddr or halt if it can't be found.

4. Fill info4 with appropriate substring of ipAddr or halt if it can't be found.

DiscussionDiscussion

Since a function cannot return 4 strings, we will

instead require the caller to pass it _______________

______________ as arguments, which our function will

then _______________________.

Parameters used up to now are called _________ parameters; they are built as __________ of their arguments.

Changing a value parameter changes thenot its corresponding .

So, we need a different kind of parameter.

8

Solution: Reference Parameters

A reference parameter is an _________ (i.e., another name) for its corresponding argument. They share _______________________

Changing the value of a reference parameter ____ the value of its corresponding argument.

In C++, reference parameters are declared with an ____________________(__) following the parameter’s type (and before its name).

9

& is also the"address of"

operator

CodingCoding

void chopIPAddress(string ipAddr, // value: TO

_____________ info1, // reference: TO-BACK

_____________ info2, // reference: TO-BACK

_____________ info3, // reference: TO-BACK

_____________ info4, // reference: TO-BACK{ int dot1 = ipAddr.find(".", 0); assert(dot1 != string::npos); info1 = ipAddr.substr(0, dot1);

int dot2 = ipAddr.find(".", dot1 + 1); assert(dot2 != string::npos); info2 = ipAddr.substr(dot1 + 1, dot2 - dot1 - 1);

int dot3 = ipAddr.find(".", dot2 + 1); assert(dot3 != string::npos); info3 = ipAddr.substr(dot2 + 1, dot3 - dot2 - 1);

assert(ipAddr.find(".", dot3 + 1) == string::npos); info4 = ipAddr.substr(dot3 + 1, ipAddr.size() - dot3 - 1);}

10

TestingTesting

11

The caller must now supply a variable for each reference parameter, to be "filled in” by the function.

cout << "Enter an IP address: ";string ipAddress, part1, part2, part3, part4;cin >> ipAddress;

chopIPAddress(ipAddress,___________________________ ___________________________);

cout << "Network/Host blocks are:\n" << part1 << endl << part2 << endl << part3 << endl << part4 << endl;

NotesNotesWhen function chopIDAddress() is called:

– ipAddr is allocated a memory location and a _________ of the argument ipAddress is stored there

– Each of the parameters info1, info2, info3, info4 is an ________ of the corresponding argument — part1, part2, part3, part4; they ____________the same memory location; that is,info1 and part1 are names of the same memory location, as are info2 and part2, info3 and part3,info4 and part4.

12

0. Before the function call0. Before the function call

Memory

ipAddress

part1

part2

part3

153.106.4.23

part4

cout << "Enter an IP address: ";string ipAddress, part1, part2, part3, part4;cin >> ipAddress;

13

????

Memory

ipAddress

part1

part2

part3

14

153.106.4.23

153.106.4.23

part4

_________

chopIPAddress(ipAddress, part1, part2 part3, part4);

????

1. ipAddr is created as a copy of ipAddress

2. info1, ..., info4 are created as aliases for

part1, ..., part4

Memory

part1

part2

part3

15

ipAddress 153.106.4.23

153.106.4.23

part4

ipAddr

________

________

________

________

chopIPAddress(ipAddress, part1, part2 part3, part4);

????

3. The function computesinfo1, changing part1

16

Memory

part1

part2

part3

ipAddress 153.106.4.23

153.106.4.23

part4

info1

info2

info3

info4

ipAddr

int dot1 = ipAddr.find(".", 0); assert(dot1 != string::npos); info1 = ipAddr.substr(0, dot1);

???

4. The function computesinfo2, changing part2

17

Memory

part1

part2

part3

ipAddress 153.106.4.23

153.106.4.23

part4

info1

info2

info3

info4

ipAddr

153

int dot2 = ipAddr.find(".", dot1 + 1); assert(dot2 != string::npos); info2 = ipAddr.substr(dot1 + 1, dot2 - dot1 - 1);

??

5. The function computesinfo3, changing part3

18

Memory

part1

part2

part3

ipAddress 153.106.4.23

153.106.4.23

part4

info1

info2

info3

info4

ipAddr

153

106

int dot3 = ipAddr.find(".", dot2 + 1); assert(dot3 != string::npos); info3 = ipAddr.substr(dot2 + 1, dot3 - dot2 - 1);

?

6. The function computesinfo4, changing part4

19

Memory

part1

part2

part3

ipAddress 153.106.4.23

153.106.4.23

part4

info1

info2

info3

info4

ipAddr

153

106

4

assert(ipAddr.find(".", dot3 + 1) == string::npos); info4 = ipAddr.substr(dot3 + 1, ipAddr.size() - dot3 - 1);

7. The function returns, destroying all parameters

Memory

20

part1

part2

part3

ipAddress 153.106.4.23

part4

153

106

4

23

. . .

}

8. part1, ... , part4now contain the

information!

21

Memory

part1

part2

part3

ipAddress 153.106.4.23

part4

153

106

4

23

cout << "Network/Host blocks are:\n" << part1 << endl << part2 << endl << part3 << endl << part4 << endl

NotesNotes

By default, parameters are value parameters.

Reference parameters are specified by placing an ampersand after the parameter’s type.

Reference parameters must be specified in both a function’s prototype and its definition, or a linking error will occur.

Variables must be passed as arguments for reference parameters to fill, or a compiler error will occur.

22

ConsiderConsider

Copying argument ipAddress consumes time.

Creating an alias for an argument takes almost no time.

We could speed up calls to our function by making parameter ipAddr a reference parameter.

However, we then run the risk of changing ipAddress if we mistakenly change ipAddr.

23

SolutionSolution

Constant reference parameters are reference parameters whose declaration is preceded by the keyword const.

void chopIPAddress(const string & ipAddr, // TO string & info1, // BACK string & info2, // BACK string & info3, // BACK string & info4) // BACK// ...

Const reference parameters are read-only reference parameters -- aliases of their arguments -- but they cannot be changed.

24

0. Before the function call

25

Memory

ipAddress

part1

part2

part3

153.106.4.23

part4

????

1. ipAddr is created as aconst reference of original

ipAddr

26

read-only

Memory

ipAddress

part1

part2

part3

153.106.4.23

part4

2. The rest of the function proceeds as before,

except:• all accesses to ipAddr now access

ipAddress instead of the copy.

• Any attempt to change ipAddr will generate a compiler error (which makes sense, since its movement is IN, not OUT).

27

DiscussionDiscussion

Copying time is not significant for simple types (e.g., int, char, double, ...), but it is significant for class types (e.g., string, RandomInt, ...).So use value parameters to store simple type arguments whose movement is TO.

Use reference parameters for arguments whose movement is BACK or TO & BACK

Use const reference parameters to store class arguments whose movement is TO. 28