Post on 03-Jan-2016
Parameter Passing Parameter Passing MechanismsMechanisms
Reference ParametersReference Parameters
Read §10.1 - §10.3Read §10.1 - §10.3
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ProblemProblem
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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.
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ObjectsObjects
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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
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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
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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.
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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).
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& 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);}
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TestingTesting
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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.
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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;
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????
Memory
ipAddress
part1
part2
part3
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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
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ipAddress 153.106.4.23
153.106.4.23
part4
ipAddr
________
________
________
________
chopIPAddress(ipAddress, part1, part2 part3, part4);
????
3. The function computesinfo1, changing part1
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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
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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
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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
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Memory
part1
part2
part3
ipAddress 153.106.4.23
153.106.4.23
part4
info1
info2
info3
info4
ipAddr
153
106
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assert(ipAddr.find(".", dot3 + 1) == string::npos); info4 = ipAddr.substr(dot3 + 1, ipAddr.size() - dot3 - 1);
7. The function returns, destroying all parameters
Memory
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part1
part2
part3
ipAddress 153.106.4.23
part4
153
106
4
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. . .
}
8. part1, ... , part4now contain the
information!
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Memory
part1
part2
part3
ipAddress 153.106.4.23
part4
153
106
4
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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.
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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.
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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.
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0. Before the function call
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Memory
ipAddress
part1
part2
part3
153.106.4.23
part4
????
1. ipAddr is created as aconst reference of original
ipAddr
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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).
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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