数据结构与程序设计(17)String Operator overload

1、7/18/2022数据结构与程序设计 1课程内容安排C String 与 String的比较。讨论C+中String Class的实现方法。7/18/2022数据结构与程序设计 2C String 与 String Classstring class: string class in the standard library accessed by #include C strings: can be accessed by #include A String constant is stored as a char array. 即一个常量的string被认为是cstring e.g.: “

2、Hello everyone” / cstring string s = “Hello everyone” /String object 7/18/2022数据结构与程序设计 3What is a C String? Char arrayA C string is a char array terminated by the null character 0 ( with ASCII value 0 ).A C string variable can be initialized in its declaration in following ways.char message 8 = H,

3、e, l, l, o, 0 ;char message 8 = “Hello” ;Char *message = “hello”;message 0 1 2 3 4 5 6 7H e l l o 07/18/2022数据结构与程序设计 4字符串 (String)字符串是n ( 0 ) 个字符的有限序列，记作 S : “c1c2c3cn”其中，S 是串名字 “c1c2c3cn” 是串值 ci 是串中字符 n 是串的长度。 7/18/2022数据结构与程序设计 5functions that manipulate C-strings/字符串的拷贝操作。char *strcpy(char *to,

4、char *from);Pre: The string from has been initialized.Post: The function copies string from to string to, including 0; it returns a pointer to the beginning of the string to./将from指向的内容链接在to的内容之后char *strcat(char *to, char *from);Pre: The strings from and to have been initialized.Post: The function

5、copies string from to the end of string to,including 0; it returns a pointer to the beginning of the string to.7/18/2022数据结构与程序设计 6functions that manipulate C-strings/求字符串的长度int strlen(char *s);Pre: The string s has been initialized.Post: The function returns the length of the string s, not includin

6、g the null byte 0 at the end of the string s./字符串的比较操作。int strcmp(char *s1, char *s2);Pre: The strings s1 and s2 have been initialized.Post: The function compares string s1 to string s2; it returns 0 if s1 0 if s1 s2.7/18/2022数据结构与程序设计 7functions that manipulate C-strings/返回s2第一次在s1中出现的位置。char *strs

7、tr(char *s1, char *s2);Pre: The strings s1 and s2 have been initialized.Post: The function returns a pointer to the first occurrence of the string s2 in the string s1, or it returns NULL if the string s2 is not present in s1.7/18/2022数据结构与程序设计 8Strings in Cchar * p=hello world; /cant be writtenchar

8、* q=new charstrlen(p)+1;strcpy(q,p);coutqendl;/q=p;p=q;*p=H; coutqendl;coutpendl;7/18/2022数据结构与程序设计 9Strings in Cchar *strcpy(char *to, char *from) int i; for(i=0;fromi!=0;i+)toi=fromi; toi=0; return to;7/18/2022数据结构与程序设计 10Strings in Cint strlen(char * s) int i; for(i=0;si!=0;i+); return (i);7/18/2

9、022数据结构与程序设计 11ComparisonBOOK P234 Figure 6.5“ Important string”“ acquires an alias”S2 = s1S2S17/18/2022数据结构与程序设计 12ComparisonC-strings are widely available.C-strings are very efficient.C-string objects are not encapsulated.C-strings are easy to misuse, with consequences that can be disastrous.It is

10、 easy for a client to create either garbage or aliases for C-string data.7/18/2022数据结构与程序设计 13/ * / PrintName program/ This program prints a name in two different formats/ *#include / for cout and endl#include / for data type stringusing namespace std;const string FIRST = “Herman”; / Persons first n

11、ameconst string LAST = “Smith”; / Persons last nameconst char MIDDLE = G; / Persons middle initialC+ Program-STL STRING7/18/2022数据结构与程序设计 14C+ Code Continuedint main( ) string firstLast; / Name in first-last format string lastFirst; / Name in last-first format firstLast = FIRST + “ “ + LAST ; cout “

12、Name in first-last format is “ endl firstLast endl; lastFirst = LAST + “, “ + FIRST + ; cout “Name in first-last format is “ endl lastFirst MIDDLE . endl; return 0; 7/18/2022数据结构与程序设计 15Output of Program Name in first-last format is Herman Smith Name in last-first-initial format is Smith, Herman G.7

13、/18/2022数据结构与程序设计 16Implementation of String Classclass String public: / methods of the string ADTString( );String( );String (const String ©); / copy constructorString (const char * copy); / conversion from C-stringString (List ©); / conversion from Listvoid operator = (const String ©);c

14、onst char *c_str( ) const; / conversion to C-style stringprotected:char *entries;int length;7/18/2022数据结构与程序设计 17运算符重载，全局方法bool operator = (const String &rst, const String &second);bool operator (const String &rst, const String &second);bool operator = (const String &rst, const String &second);bool

15、operator = (const String &rst, const String &second);bool operator != (const String &rst, const String &second);7/18/2022数据结构与程序设计 18String constructorString : String ()entries = new char1;entries0 = 0;length = 0;7/18/2022数据结构与程序设计 19String constructorString : String (List &in_list)/* Post: The Stri

16、ng is initialized by the character List in_list . */length = in_list.size( );entries = new charlength + 1;for (int i = 0; i length; i+) in_list.retrieve(i, entriesi);entrieslength = 0;7/18/2022数据结构与程序设计 20Implementation of String Classconst char*String : c_str( ) const/* Post: A pointer to a legal C

17、-string object matching the String is returned. */return (const char *) entries;7/18/2022数据结构与程序设计 21全局函数的实现Operator Overloadbool operator = (const String &first, const String &second)/* Post: Return true if the String first agrees with String second . Else: Return false . */return strcmp(first.c_st

18、r( ), second.c_str( ) = 0;7/18/2022数据结构与程序设计 22Function Overloadvoid strcat(String &add_to, const String &add_on)/* Post: The function concatenates String add_on onto the end of String add_to .*/const char *cfirst = add_to.c_str( );const char *csecond = add_on.c_str( );char *copy = new charstrlen(cf

19、irst) + strlen(csecond) + 1;strcpy(copy, cfirst);strcat(copy, csecond);add_to = copy; /?delete copy;7/18/2022数据结构与程序设计 23Main ProgramString read_in(istream &input)/* Post: Return a String read (as characters terminated by a new line or an end-of-file character) from an istream parameter. */List temp

20、;int size = 0;char c;while (c = input.peek( ) != EOF & (c = input.get( ) != n)temp.insert(size+, c);String answer(temp);return answer;7/18/2022数据结构与程序设计 24Main Programvoid write(String &s)/* Post: The String parameters is written to cout . */cout s.c_str( ) endl;注意：Char *p = “hello”;Coutp;/将输出字符串hel

21、lo的值。7/18/2022数据结构与程序设计 25Main Programostream & operator (ostream & output, String s1)output s1.c_str( ) endl;return output;7/18/2022数据结构与程序设计 26Main Programvoid main() String s1=s1 string;couts1.c_str()endl;coutstrlen(s1.c_str()endl;String s2(s2 string);couts2.c_str()endl;coutstrlen(s2.c_str()=s2)

22、cout=s2endl;else couts1s2endl;s2=s1;if(s1=s2) couts1=s2endl;else couts1!=s2endl;7/18/2022数据结构与程序设计 29Results1s2s1=s27/18/2022数据结构与程序设计 30Main ProgramString s3=read_in(cin);strcat(s3,s1);write(s3);7/18/2022数据结构与程序设计 31Resultheadheads1 string7/18/2022数据结构与程序设计 32Implementation of String Class目录String下例