Parameter Passing Mechanisms Reference Parameters § §10.3 1

Problem 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. 2 IPv4 (Internet Protocol) addresses are expressed using "dotted decimal" notation: aaa.bbb.ccc.ddd where aaa, bbb, ccc, and ddd are 1-byte (8-bit) integers that provide network or host information. ipv6 16 bytes Find/locate IP address

Preliminary 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

Behavior 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

Objects Description Type Name IP address stringipAddr first block string info1 second block string info2 third block string info3 fourth block string info4 5 Movement to back Where? main() or in function

Operations Description Predefined? Library? Name 6 receive a stringyesbuilt-innone select part of a yes string substr() string search a stringyes string find() halt if erroryes cassert assert() pass back 4 strings yes built-in ??

Algorithm 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. 7

Discussion Since a function cannot return 4 strings, we will instead require the caller to pass it four string variables as arguments, which our function will then assign values. Parameters used up to now are called value parameters; they are built as copies of their arguments. Changing a value parameter changes the copy, not its corresponding argument. So, we need a different kind of parameter. 8

Solution: Reference Parameters A reference parameter is an alias (i.e., another name) for its corresponding argument. They share memory locations. Changing the value of a reference parameter changes the value of its corresponding argument. In C++, reference parameters are declared with an ampersand (&) following the parameter’s type (and before its name). 9 & is also the "address of" operator

Coding void chopIPAddress(string ipAddr, // value: TO string & info1, // reference: BACK string & info2, // reference: BACK string & info3, // reference: BACK string & info4) // reference: 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

Testing The caller must now supply a variable for each reference parameter, to be "filled in" by the function. 11 cout << "Network/Host blocks are:\n" << part1 << endl << part2 << endl << part3 << endl << part4 << endl; cout > ipAddress; chopIPAddress(ipAddress, part1, part2, part3, part4);

Notes When function chopIDAddress() is called: – –ipAddr is allocated a memory location and a copy of the argument ipAddress is stored there – –Each of the parameters info1, info2, info3, info4 is an alias of the corresponding argument — part1, part2, part3, part4; they share 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 call Memory ipAddress part1 part2 part part4 cout > ipAddress; 13 ????????

1. ipAddr is created as a copy of ipAddress Memory ipAddress part1 part2 part3 ipAddr part4 chopIPAddress(ipAddress, part1, part2 part3, part4); ????????

2. info1,..., info4 are created as aliases for part1,..., part4 Memory part1 part2 part3 15 ipAddress part4 info1 info2 info3 info4 ipAddr chopIPAddress(ipAddress, part1, part2 part3, part4); ????????

3. The function computes info1, changing part1 16 Memory part1 part2 part3 ipAddress part4 info1 info3 info4 ipAddr 153 int dot1 = ipAddr.find(".", 0); assert(dot1 != string::npos); info1 = ipAddr.substr(0, dot1); info2 ??????

4. The function computes info2, changing part2 17 Memory part1 part2 part3 ipAddress part4 info1 info2 info3 info4 ipAddr int dot2 = ipAddr.find(".", dot1 + 1); assert(dot2 != string::npos); info2 = ipAddr.substr(dot1 + 1, dot2 - dot1 - 1); ????

5. The function computes info3, changing part3 18 Memory part1 part2 part3 ipAddress part4 info1 info2 info3 info4 ipAddr int dot3 = ipAddr.find(".", dot2 + 1); assert(dot3 != string::npos); info3 = ipAddr.substr(dot2 + 1, dot3 - dot2 - 1); ?

6. The function computes info4, changing part4 19 Memory part1 part2 part3 ipAddress part4 info1 info2 info3 info4 ipAddr 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 part } info1 info2 info3 info4 ipAddr

8. part1,..., part4 now contain the information! 21 Memory part1 part2 part3 ipAddress part cout << "Network/Host blocks are:\n" << part1 << endl << part2 << endl << part3 << endl << part4 << endl

Notes By default, parameters are value parameters. Reference parameters are specified by placing an ampersand after the parameters 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

An Alternative to Value Parameters 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

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 part part4 ????????

1. ipAddr is created as a const reference of original ipAddr 26 read-only Memory ipAddress part1 part2 part 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

Discussion 28 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.