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From http://www.yolinux.com/TUTORIALS/LinuxTutorialC++StringClass.html

Simple example of a program using string class and comparison with C char:

view source print ? 01 #include <string> 02 #include <iostream> 03 #include <string.h>   // Required by strcpy() 04 #include <stdlib.h>   // Required by malloc() 05   06 using namespace std; 07   08 main() 09 { 10     string SS;     // C++ STL string 11     char CC[17];   // C character string (16 characters + NULL termination) 12                    // Storage pre-allocated 13     char *CC2;     // C character string. No storage allocated. 14   15     SS = "This is a string" ; 16     strcpy (CC, "This is a string" ); 17   18     CC2 = ( char *) malloc (17);    // Allocate memory for storage of string. 19     strcpy (CC2, "This is a string" ); 20     21     cout << SS << endl; 22     cout << CC << endl; 23     cout << CC2 << endl; 24 }

Results:

This is a string This is a string This is a string

The C and C++ methods of managing a character data type are both valid
but we will see that the C++ string class offers more functionality and
convenience.
The STL string does not require memory to be pre-allocated nor allocated
manually.
The STL string class also provides many methods of string assignment.

Example of a program using many of the build-in functions of the string class:

view source print ? 01 #include <string> 02 #include <iostream> 03   04 using namespace std; 05   06 main() 07 { 08     string a( "abcd efg" ); 09     string b( "xyz ijk" ); 10     string c; 11   12     cout << a << " " << b << endl;                        // Output: abcd efg xyz ijk 13   14     cout << "String empty: "     << c.empty()    << endl;  // String empty: 1 15                                                           // Is string empty? Yes it is empty. (TRUE) 16     c = a + b;                                            // concatenation 17     cout << c << endl;                                    // abcd efgxyz ijk 18     cout << "String length: "    << c.length()   << endl;  // String length: 15 19     cout << "String size: "      << c.size()     << endl;  // String size: 15 20     cout << "String capacity: " << c.capacity() << endl;  // String capacity: 15 21     cout << "String empty: "     << c.empty()    << endl;  // String empty: 0 22                                                           // Is string empty? No it is NOT empty. (FALSE) 23     string d = c; 24     cout << d << endl;                                    // abcd efgxyz ijk 25   26                                                           // First character: a 27     cout << "First character: " << c[0] << endl;          // Strings start with index 0 just like C. 28   29     string f( "    Leading and trailing blanks      " ); 30     cout << "String f:" << f << endl; 31     cout << "String length: " << f.length() << endl;      // String length: 37 32     cout << "String f:" << f.append( "ZZZ" ) << endl;       // String f:    Leading and trailing blanks      ZZZ 33     cout << "String length: " << f.length() << endl;      // String length: 40 34   35     string g( "abc abc abd abc" ); 36     cout << "String g: " << g << endl;                    // String g: abc abc abd abc 37     cout << "Replace 12,1,/"xyz/",3: " << g.replace(12,1, "xyz" ,3) << endl;  // Replace 12,1,"xyz",3: abc abc abd xyzbc 38     cout << g.replace(0,3, "xyz" ,3) << endl;               // xyz abc abd xyzbc 39     cout << g.replace(4,3, "xyz" ,3) << endl;               // xyz xyz abd xyzbc 40     cout << g.replace(4,3, "ijk" ,1) << endl;               // xyz i abd xyzbc 41     cout << "Find: " << g.find( "abd" ,1) << endl;          // Find: 6 42     cout << g.find( "qrs" ,1) << endl; 43   44     string h( "abc abc abd abc" ); 45     cout << "String h: " << h << endl; 46     cout << "Find /"abc/",0: " << h.find( "abc" ,0) << endl; // Find "abc",0: 0 47     cout << "Find /"abc/",1: " << h.find( "abc" ,1) << endl; // Find "abc",1: 4 48     cout << "Find_first_of /"abc/",0: " << h.find_first_of( "abc" ,0) << endl; // Find_first_of "abc",0: 0 49     cout << "Find_last_of /"abc/",0: " << h.find_last_of( "abc" ,0) << endl;   // Find_last_of "abc",0: 0 50     cout << "Find_first_not_of /"abc/",0: " << h.find_first_not_of( "abc" ,0) << endl;  // Find_first_not_of "abc",0: 3 51     cout << "Find_first_not_of /" /": " << h.find_first_not_of( " " ) << endl;  // Find_first_not_of " ": 0 52     cout << "Substr 5,9: " << h.substr(5,9) << endl;       // Substr 5,9: bc abd ab 53     cout << "Compare 0,3,/"abc/": " << h.compare(0,3, "abc" ) << endl;  // Compare 0,3,"abc": 0 54     cout << "Compare 0,3,/"abd/": " << h.compare(0,3, "abd" ) << endl;  // Compare 0,3,"abd": -1 55     cout << h.assign( "xyz" ,0,3) << endl;                   // xyz 56     cout << "First character: " << h[0] << endl; // Strings start with 0 // First character: x 57   58   59 }

Compile: g++ program.cpp

[Potential Pitfall]
: In Red Hat Linux versions 7.x
one could omit the "using namespace std;
" statement. Use of this
statement is good programming practice and is required in Red Hat 8.0.

[Potential Pitfall]
: Red Hat 8.0 requires
the reference to "#include <iostream>
". Red Hat versions 7.x
used "#include <iostream.h>
". (Also fstream, ...)

[Potential Pitfall]
: There have been some changes in the behavior of the string class from Red Hat 7.x to Red Hat 8.0:

• The compare function arguments have changed from X.compare("string",int-1, int-2);
  to X.compare(int-1, int-2, "string");
• The return value of the compare function call h.compare("abc",0,3)
  in 7.x was 12. In Red Hat 8.0 h.compare(0,3,"abc")
  it is 0.
• String capacity function call "c.capacity()
  " is 15. The returned value in Red Hat 7.x was 16.

• Constructors:

  view source

  print
  ?

  1	string sVar1( "abc" );

  2	string sVar1(C-string-variable);

  3	string sVar2(10, " " );  // Generate string initialized to 10 blanks.

  4	string sVar3(Var1, string-index);  // Initialize with characters from string starting with index string-index.

  5	string sVar4(iterator-index-begin, iterator-index-end)

• Destructor:

  view source

  print
  ?

  1	Var.~string();         // Destructor

• Replace:
  • Var.replace(beginning,end-position,string-class-variable
    )
  • Var.replace(beginning,end-position,C-char-variable
    )
  • Var.replace(beginning,end-position,string-class-variable,length
    )
  • Var.replace(beginning,end-position,integer-number,single-char
    )
  • Var.replace(beginning,end-position,new-beginning-porition,new-end-position
    )

  Code samples:

  view source

  print
  ?

  01	string g( "abc abc abd abc" );

  02	cout << g.replace(4,1, "ijk" ,3) << endl;

  03

  04	string h( "abc abc abd abc" );

  05	cout << h.replace(4,6, "ijk" ,3) << endl;

  06

  07	string k( "abc abc abd abc" );

  08	cout << k.replace(4,3, "ijk" ,3) << endl;

  09

  10	string l( "abc abc abd abc" );

  11	cout << k.replace(12,1, "xyz" ,3) << endl;

  Output:

  
  abc ijkbc abd abc - Beginning with the 4th index (character number 5) replace one character with 3 characters from string "ijk"
  
  
  abc ijkd abc
  
  abc ijk abd abc
  
  abc abc abd xyzbc
  

   

• Find: (also rfind(), find_first_of(), find_last_of(), find_first_not_of(), find_last_not_of())

  Arguments/parameters:

  • Val.find(const string& argument
    )
    
    Find first occurence of argument
    within string Val
  • find(const string& argument
    , size_type index
    )
    
    Find first occurence of argument
    within string Val
    starting search from position index
    .
  • find(const char* argument
    )
  • find(const char* argument
    , size_type index
    )
  • find(const char* argument
    , size_type index
    , size_type length
    )
    
    Find first occurence of argument
    within string Val
    starting search from position index
    and search for length
    number of characters.

STL C++ string functions:

Assuming declaration: string Var;

Function/Operation	Description
Var = string2 Var.assign("string-to-assign ")	Assignment of value to string. When assigning a C "char" data type, first check if NULL to avoid failure/crash. i.e.: if( szVar ) sVar.assign( szVar ); where szVar is a C "char *" data type and sVar is of type "string".
Var.swap(string2 ) swap(string1,string2 )	Swap with value held in string2. Function swap will exchange contents of two string class variables.
Var += string2 Var.append() Var.push_back()	Append string/characters.
Var.insert()	Insert characters
Var.erase() Var = ""	Clear string variable. No arguments necessary.
+	Concatenate
==, !=, <, <=, >, >=	Compare strings.
Var.compare(string ) Var.compare( size_t pos1, size_t len, string ) const; Var.compare( size_t pos1, size_t len1, const string , size_t pos2, size_t len2 ) const;	Compare strings. Returns int: 0: if equal. -1: Not equal. 1st non matching character in Var is less in value based on ASCII table than in compare string. +1: Not equal. 1st non matching character is greater in value based on ASCII table. Where string is another STL string or null terminated C string.
Var.length()	Return length of string. No arguments necessary. The methods length(), size() and capacity() all return the same value.
Var.size()	Return length of string. No arguments necessary.
Var.capacity()	Return length of string + 1. Red Hat 7.x. Red Hat 8.0+ returns the number of characters without the "+1". Number of characters that can be held without re-allocation. No arguments necessary.
Var.max_size()	Returns a very large number. No arguments necessary.
Var.empty()	Returns 1 if an empty string. Returns 0 if not empty.
<<	Output stream
>> getline()	Input stream
Var.c_str()	Returns C string pointer. C char string is null terminated. Do not free memory using this pointer!
Var.data()	Returns C string pointer. C char string is NOT null terminated. Do not free memory using this pointer!
Var[] Var.at(integer )	Access individual characters. Return single character at specified position (integer).
Var.find(string ) Var.find(string, positionFirstChar ) Var.find(string, positionFirstChar, len )	Find first occurance of string or substring. Returns int position of first occurance in string. Where len is the length of the sequence to search for. Returns string::npos if not found. i.e. if(Var.find("abc") == string::npos) cout << "Not found" << endl;
Var.rfind()	Find last occurance of string or substring.
Var.find_first_of(string , position) Var.find_first_of( string , size_t position, size_t len )	Find strings and substrings. Where string is another STL string or null terminated C string. If position = 0, than start at beginning of string.
Var.find_last_of()	Find strings and substrings.
Var.find_first_not_of() Var.find_last_not_of()	Find strings and substrings.
Var.replace(pos1, len1, string ) Var.replace(itterator1, itterator2, const string ) Var.replace(pos1, len1, string , pos2, len2)	Replace section of string with new characters. pos2 and len2 are given when using only a substring of string . Where string is another STL string or null terminated C string.
Var.substr(pos, len)	Return substring of text given a start position in string object and length.
Var.begin() Var.end()	Iterators
Var.rbegin() Var.rend()	Reverse iterators

Note that in most cases the string functions have been overloaded to accept
both string class arguments and C char variables.

Iterators provide the ability to access the individual characters in a string.

view source print ? 01 #include <iostream> 02 #include <string> 03 using namespace std; 04       05 int main() 06 { 07     string alphabetLC= "abcdefghijklmnopqrstuvwxyz" ; 08   09     string::const_iterator cii; 10     int ii; 11   12     for (cii=alphabetLC.begin(); cii!=alphabetLC.end(); cii++) 13     { 14        cout << ii++ << " " << *cii << endl; 15     } 16 }

This will print the integer position in the string followed by the letter
for all characters in the alphabet.

0 a 1 b 2 c 3 d 4 e 5 f 6 g 7 h ... ..

Iterator types:

• string::traits_type
• string::value_type
• string::size_type
• string::difference_type
• string::reference
• string::const_reference
• string::pointer
• string::const_pointer
• string::iterator
• string::const_iterator
• string::reverse_iterator
• string::const_reverse_iterator
• string::npos

The full use of the C standard library is available for use by utilizing the
".c_str" function return of the string class.

view source print ? 01 #include <strings.h> 02 #include <string> 03 #include <stdio.h> 04 using namespace std; 05       06 int main() 07 { 08     char *phrase1= "phrase" ; 09     string phrase2( "Second phrase" ); 10     char   phraseA[128]; 11     char   *phraseB; 12       13     strcpy (phraseA,phrase2.c_str()); 14     phraseB = strstr (phrase2.c_str(),phrase1); 15   16     printf ( "phraseA: %s/n" ,phraseA); 17     printf ( "phraseB: %s/n" ,phraseB); 18     printf ( "phrase2: %s/n" ,phrase2.c_str()); 19 }

Compile and run:

[prompt]$ g++ test.cpp [prompt]$ ./a.out phraseA: Second phrase phraseB: phrase phrase2: Second phrase

• Read lines from standard input:

  view source print ? 01 while ( getline(std::cin, sLine) ) 02 { 03     if ( sLine.empty() );                     // Ignore empty lines 04     else 05     { 06        cout << sLine[0] << sLine[1] << endl; 07        .... 08        ... 09     } 10 }

• Read lines from input file:
  

  view source print ? 01 #define SYS_CONFIG_FILE "/etc/file.conf" 02 #include <string> 03 #include <algorithm> 04 #include <vector> 05 #include <cctype> 06 #include <iostream> 07 #include <fstream> 08   09 using namespace std; 10   11 string::size_type posBeginIdx, posEndIdx; 12 string::size_type ipos=0; 13 string            sLine, sValue; 14 string            sKeyWord; 15 const string      sDelim( ":" ); 16   17 ifstream myInputFile(SYS_CONFIG_FILE, ios::in); 18 if ( !myInputFile ) 19 { 20     sError = "File SYS_CONFIG_FILE could not be opened" ; 21     return sError; // ERROR 22 } 23   24 while ( getline(myInputFile,sLine) ) 25 { 26     if ( sLine.empty() );                     // Ignore empty lines 27     else 28     { 29        posEndIdx = sLine.find_first_of( sDelim ); 30        sKeyWord  = sLine.substr( ipos, posEndIdx ); // Extract word 31        posBeginIdx = posEndIdx + 1;  // Beginning of next word (after ':') 32     .... 33     ... 34     } 35 }

• Strip blank characters:
  

  view source print ? 01 void 02 stripLeadingAndTrailingBlanks(string& StringToModify) 03 { 04     if (StringToModify.empty()) return ; 05   06     int startIndex = StringToModify.find_first_not_of( " " ); 07     int endIndex = StringToModify.find_last_not_of( " " ); 08     string tempString = StringToModify; 09     StringToModify.erase(); 10   11     StringToModify = tempString.substr(startIndex, (endIndex-startIndex+ 1) ); 12 }

The first thing you will notice when using the C++ string class is that you can't
de-reference any of the string class variables directly with GDB, ddd,...
One must create a helper routine (for older versions of gdb)
or use string class funtions (newer versions of gdb) to print out the
value of the string variable.

view source print ? 01 #include <string> 02 #include <iostream> 03   04 using namespace std; 05   06 // Helper routine ps to print a string class variable. 07   08 void ps(string& s){ cout << s << endl; } 09   10 int main() 11 { 12     string a( "String A" ); 13     string b; 14   15     b = "String B" ; 16   17     cout << "Hello!" << endl; 18 }

Compile program with symbolic code for the debugger: g++ -g testprog.cpp

Start gdb
debugger: gdb ./a.out

(gdb) l 1,18 - List lines 1 to 18 1 #include <string> 2 #include <iostream> 3 4 using namespace std; 5 6 // Helper routine ps to print a string class variable. 7 8 void ps(string& s){ cout << s << endl; } 9 10 int main() 11 { 12 string a("String A"); 13 string b; 14 15 b = "String B"; 16 17 cout << "Hello!" << endl; 18 } (gdb) break 17 Breakpoint 1 at 0x804893b: file testprog.cpp, line 17. (gdb) run Starting program: /home/user1/a.out Breakpoint 1, main () at testprog.cpp:17 17 cout << "Hello!" << endl; (gdb) p a - Gdb can't de-reference string class variable "a" $1 = {static npos = Cannot access memory at address 0x83a32d0 (gdb) call ps(a) String A - Call helper function ps to print string conents. (gdb) call ps(b) String B (gdb) c Continuing. Hello! Program exited normally. (gdb) quit

With newer versions of gdb, one may use built-in string class functions:

(gdb) p a.c_str() $1 = 0x8049e34 "String A" (gdb) p b.c_str() $3 = 0x8049e4c "String B" (gdb) p b.empty() $2 = false (gdb) p b.size() $4 = 8

Dereference string
and wstring
using GDB macro functions.
See YoLinux.com GDB tutorial on dereferencing STL strings and containers
.

• The string class is NOT a native data type, it is an object class and thus
  can not be handled like the traditional pointer to variable in gdb.
• One can pass strings by reference (i.e. argument declarations using (string& variable-name
  )
  ),
  by value (string variable-name
  )
  ,
  and by pointer (string *variable-name
  )
  .
• When using a reference, one may mimic the protection of a variable that
  passing by value enables by using
  (const string& variable-name
  )

• GNU C++ Library: String classes
• IBM AIX IString class example
• STL: basic_string
  - SGI STL home
• YoLinux.com: C++