C String, Proper Type, and String Objects

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C String, Proper Type, and String Objects

• Andy Wang
• COP 4530 Data Structures, Algorithms, and
  Generic Programming

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Lecture Overview

• Goal
• Design a string class that overcomes the
  disadvantages of C strings
• Roadmap
• Critique of C strings
• The need for proper type
• String objects

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In-Class Exercise

• Write a C function that copies one string to
  another.
• void strcpy(char dest, char src)

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Assumptions of C Strings

• Allocated memory
• char str
• str malloc(sizeof(char)11)
• Null termination
• str10 \0
• strcpy(str, 0123456789)

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What can go wrong?

• void strcpy(char copy_cat, char cat)
• int j
• for (j 0 catj ! \0 j)
• copy_catj catj
• copy_catj \0

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Other String Functions

• Still assume null-terminated strings
• Still assume allocated memory
• Security holes
• Internet worms
• System break-ins

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The Concept of Proper Type

• A proper type should be
• Responsible for its own data
• Data protected from clients and other programs
• Responsible for its own behavior
• Make behavior available to clients
• Implementation hidden from the public view
• Responsible for its own existence
• Automatic memory management

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C Class Objects

• Responsible for their own data
• Protected or private data
• Controlled access through public methods
• Responsible for their own behavior
• Public methods
• Responsible for their own existence
• Constructors/deconstructors

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In-Class Exercise String Class

• namespace rcl
• class String
• // friend
• public
• //
• private
• //
• //

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String Class

• class String
• // friend iostream operators
• public
• // constructors/destructors
• // operators
• // builders
• // data accessors (const)
• // string comparison function
• private
• // data
• // helper methods
• // equality and order comparison operators

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Private Data

• char str
• unsigned int size // equal to C strlen()

class String // friend iostream
operators public // constructors/destructors
// operators // builders // data accessors
(const) // String comparison function private
// data // helper methods // equality and
order comparison operators
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Data Accessors

• unsigned int Size() const
• unsigned int Length() const
• char Element(unsigned int n) const

class String // friend iostream
operators public // constructors/destructors
// operators // builders // data accessors
(const) // String comparison function private
// data // helper methods // equality and
order comparison operators
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Data Accessors

• unsigned int StringSize() const
• return size
• unsigned int StringLength() const
• // note strlen takes char therefore, we need
  to implement an operator that converts String to
  char.
• return strlen(this)

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In-Class Exercise Element

• Return the nth character of the String. Watch
  out for boundary cases.
• char StringElement(unsigned int n) const

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Element

• char StringElement(unsigned int n) const
• char ch
• if ((size 0) (n gt size))
• return \0
• else
• return strn

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String Comparison Function

• static int StrCmp(const String, const String)

class String // friend iostream
operators public // constructors/destructors
// operators // builders // data accessors
(const) // String comparison function private
// data // helper methods // equality and
order comparison operators
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StrCmp

• int StringStrCmp(const String S1, const
  String S2)
• if ((S1.Size() 0) (S2.Size() 0))
• return 0
• else if ((S1.Size() 0) (S2.Size() ! 0))
  
• return 1
• else if ((S1.Size() ! 0) (S2.Size() 0))
  
• return 1
• else
• return (strcmp(S1.str, S2.str)

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Equality Operators

• int operator(const String S1, const String
  S2)
• int operator!(const String S1, const String
  S2)
• int operatorlt(const String S1, const String
  S2)
• int operatorlt(const String S1, const String
  S2)
• int operatorgt(const String S1, const String
  S2)
• int operatorgt(const String S1,
• const String S2)

class String // friend iostream
operators public // constructors/destructors
// operators // builders // data accessors
(const) // String comparison function private
// data // helper methods // equality and
order comparison operators
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Comparison Operators

• int operator(const String S1, const String
  S2)
• return (StringStrCmp(S1, S2) 0)
• int operator!(const String S1, const String
  S2)
• return (StringStrCmp(S1, S2) ! 0)
• int operatorgt(const String S1, const String S2)
  
• return (StringStrCmp(S1, S2) gt 0)

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Comparison Operators

• int operatorgt(const String S1, const String
  S2)
• return (StringStrCmp(S1, S2) gt 0)
• int operatorlt(const String S1, const String S2)
  
• return (StringStrCmp(S1, S2) lt 0)
• int operatorlt(const String S1, const String
  S2)
• return (StringStrCmp(S1, S2) lt 0)

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Helper Methods

• static void Error(const char)
• static char newstr(int n)
• static int xstrlen(const char)
• static void xstrcpy(char , const char)
• void Clear()
• void Clone(const String S)

class String // friend iostream
operators public // constructors/destructors
// operators // builders // data accessors
(const) // String comparison function private
// data // helper methods // equality and
order comparison operators
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Error

• void StringError(const char msg)
• cerr ltlt String error ltlt msg ltlt \n
• exit(EXIT_FAILURE)

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In-Class Exercise newstr

• It creates a new String of size n (C String with
  an array size of n 1).
• void Stringnewstr(int n)

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newstr

• char Stringnewstr(int n)
• char Cptr 0
• if (n gt 0)
• Cptr new charn 1
• if (Cptr 0)
• Error(memory allocation failure)
• Cptrn \0
• return Cptr

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xstrlen

• int Stringxstrlen(const char s)
• int len
• if (s ! 0) // check for null pointer
• for (len 0 slen ! \0 len)
• return len

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xstrcpy

• void Stringxstrcpy(char dest, const char src)
• if (src ! 0) // check for null pointer
• if (dest ! 0) // check for null pointer
• int j
• for (j 0 srcj ! \0 j)
• destj srcj
• destj \0
• else
• Error(xstrcpy null destination)

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In-Class Exercise Clear

• If the String has storage allocated, delete it,
  and set the storage pointer and size to zero.
• void Clear()

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Clear

• void StringClear()
• if (str)
• delete str
• str 0
• size 0

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In-Class Exercise Clone

• If makes a copy of the parameter S.
• void StringClone(const String S)

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Clone

• void StringClone(const String S)
• size S.Size()
• if (size gt 0)
• str newstr(size)
• xstrcpy(str, S.str)
• else
• str 0

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Operators

• String operator(const String S)
• char operator (unsigned int I) const
• operator const char () const

class String // friend iostream
operators public // constructors/destructors
// operators // builders // data accessors
(const) // String comparison function private
// data // helper methods // equality and
order comparison operators
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Operator

• String Stringoperator(const String S)
• if (this ! S)
• Clear()
• Clone(S)
• return this

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Operator

• char Stringoperator (unsigned int n) const
• if ((size 0) (n gt size))
• Error(index out of range)
• return strn

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Operator const char

• Stringoperator const char () const
• return str

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Builder Functions

• void Wrap(const char Cptr)
• void GetLine(istream inl)
• int SetSize(unsigned int sz, char fill)

class String // friend iostream
operators public // constructors/destructors
// operators // builders // data accessors
(const) // String comparison function private
// data // helper methods // equality and
order comparison operators
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Wrap

• void StringWrap(const char Cptr)
• Clear()
• if (Cptr)
• size xstrlen(Cptr)
• str newstr(size)
• xstrcpy(str, Cptr)

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GetLine

• void StringGetLine(istream is)
• unsigned int curr_size 0, buff_size
  init_buff_size
• char buffer new charbuff_size 1
• for (char x is.get() ((x ! \n)
  (!is.eof()) x is.get())
• if (curr_size buff_size)
• buff_size 2
• char newbuffer new charbuff_size 1
• // copy buffer to newbuffer
• delete buffer
• buffer newbuffer
• buffercurr_size x
• buffercurr_size \0
• Wrap(buffer)
• delete buffer

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SetSize

• int StringSetSize(unsigned int sz, char fill)
• if (sz ! Size())
• char newdata newstr(sz)
• if (newdata 0) return 0
• unsigned int j
• if (sz lt Size())
• for (j 0 j lt sz j)
• newdataj strj
• else
• for (j 0 j lt Size() j)
• newdataj strj
• for (j Size() j lt sz j)
• newdataj fill
• delete str
• str newdata
• size sz

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Constructors/Destructors

• String()
• explicit String(const char Cptr)
• String()
• String (const String S)

class String // friend iostream
operators public // constructors/destructors
// operators // builders // data accessors
(const) // String comparison function private
// data // helper methods // equality and
order comparison operators
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Constructors /Deconstructors

• StringString() str(0), size(0)
• StringString(const char Cptr) str(0),
  size(0)
• Wrap(Cptr)
• StringString(const String S)
• Clone(S)
• StringString()
• if (str) delete str

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I/O Operators

• friend ostream operatorltlt(ostream os, const
  String S)
• friend istream operatorgtgt(istream is, String
  S)

class String // friend iostream
operators public // constructors/destructors
// operators // builders // data accessors
(const) // String comparison function private
// data // helper methods // equality and
order comparison operators
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Operatorltlt

• ostream operatorltlt(ostream os, const String S)
  
• os ltlt S.str
• return os

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Operatorgtgt

• istream operatorgtgt(istream is, String S)
• unsigned int curr_size 0 buff_size
  init_buff_size
• char x
• // skip clear space
• char buffer Stringnewstr(buff_size)
• buffercurr_size x
• for (x is.peek() (// x is not white space or
  eof) x is.peek())
• if (curr_size buff_size)
• buff_size 2
• char newbuffer Stringnewstr(buff_size)
• // copy buffer into newbuffer
• delete buffer
• buffer newbuffer
• buffercurr_size x
• is.get()
• buffercurr_size \0
• S.Wrap(buffer)