Linux Tutorial: - C++ String Class Tutorial

String class example:

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

C++ String Class: C character:

01#include <string>
02#include <iostream>
03 
04using namespace std;
05 
06main()
07{
08   string SS;     // C++ STL string
09   string SS2;    // C++ STL string
10 
11   SS = "This is a string";
12   SS2 = SS;
13 
14   cout << SS << endl;
15   cout << SS2 << endl;
16}

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

Compile: g++ stringtest.cpp -o stringtest
Run: ./stringtest
Results for both examples:

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.

String class functions:

Constructors:

view source print ?
1 string sVar1("abc");
2 string sVar2(C-string-variable); // Null terminated char
3 string sVar3(10," "); // Generate string initialized to 10 blanks.
4 string sVar4(Var1, string-index); // Initialize with characters from string starting with index string-index.
5 string sVar5(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 (operator=). 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 to the end of the string. For string `Var("abc");` the call to `Var.append("xyz")` will result in the string "abcxyz".
Var.insert(size_t position, string) Var.insert(size_t position, char ) Var.insert(size_t position, string, size_t pos1, size_t len) Var.insert(size_t position, char , size_t pos1, size_t len)	Insert characters. position: insert before this character position. If 0 then insert before the string pos1: position of the first char of the string being inserted len: length of string to be inserted
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 memory allocated to storage of the 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). For `Var("abc");`, Var[2] returns "c".
Var.copy(char *str,size_t len, size_t index)	str: allocated char storage to which the copy ismade len: the number of characters to copy index: the starting place in the string from which to start the copy. Counting starts from 0 returns the number of characters copied
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, size_t position) Var.find_first_of( char str, size_t position) Var.find_first_of( char str, size_t position, size_t len )	Find strings and substrings. Where string is another STL string and `str` is a null terminated C string. If `position` = 0, than start at beginning of string.
Var.find_last_of(string, size_t position) Var.find_last_of( char str, size_t position) Var.find_last_of( char str, size_t position, size_t len )	Find strings and substrings. position: the last character considered. If equal to the string length, then the entire string is searched len: number of characters you are searching for
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.

ANSI C++ string class iterators:

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

01#include <iostream>
02#include <string>
03using namespace std;
04    
05int 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

ANSI C++ string class and the C standard library:

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

01#include <strings.h>
02#include <string>
03#include <stdio.h>
04using namespace std;
05    
06int 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

Using ostringstream and an internal write:

In memory I/O string processing used as a data type conversion. This can also be used to make use of formatting of output in memory.

File: int2string.cpp

01#include <iostream>
02#include <sstream>
03#include <string>
04using namespace std;
05  
06string int2string(const int& number)
07{
08   ostringstream oss;
09   oss << number;
10   return oss.str();
11}
12  
13main()
14{
15   int number=7878;
16   string test="SSSSS";
17   test += int2string(number);
18   cout << test << endl;
19}

Compile and run:

    [prompt]$ g++ int2string.cpp
    [prompt]$ a.out
    SSSSS7878

[Potential Pitfall]: Returned string value must be used right away without other memory being set as string destructor will free the memory associated with its contents. It is much safer for the function to return a char data type or pass the string reference as an argument.

Using istringstream and an internal read:

This is used to make use of reading and parsing a string in memory. It will also allow data type conversion from a string to the type read.

File: test.cpp

01#include <iostream>
02#include <sstream>
03#include <string>
04using namespace std;
05  
06main()
07{
08   string test="AAA 123 SSSSS 3.141592654";
09   istringstream totalSString( test );
10   string string1, string2;
11   int    integer1;
12   double PI;
13 
14   totalSString >> string1 >> integer1 >> string2 >> PI;
15    
16   cout << "Individual parsed variables:" << endl;
17   cout << "First string:  " << string1   << endl;
18   cout << "First integer: " << integer1  << endl;
19   cout << "Value of PI:   " << PI        << endl;
20}

Compile and run:

    [prompt]$ g++ test.cpp
    [prompt]$ a.out
    Individual parsed variables:
    First string:  AAA
    First integer: 123
    Value of PI:   3.14159

String conversion routine: std::to_string()

The string function std::to_string() is part of the C++ 11 standard and converts integer and floating point values to a string. The g++ compiler will require the compile flag -std=c++11 to support C++ 11.

Function	Description
std::string to_string( int value );	Returns a string given an integer
std::string to_string( long long value );	Returns a string given a long long
std::string to_string( unsigned value );	Returns a string given an unsigned
std::string to_string( unsigned long value );	Returns a string given an unsigned long
std::string to_string( unsigned long long value );	Returns a string given an unsigned long long
std::string to_string( float value );	Returns a string given a float
std::string to_string( double value );	Returns a string given a double
std::string to_string( long double value );	Returns a string given an long double

Example:

01#include <iostream>
02#include <string>
03#include <stdio.h>
04#include <string.h>
05#include <stdlib.h>
06 
07using namespace std;
08  
09main()
10{
11   string test  ="AAA ";
12   int    integerx = 5;
13   double Pi = 3.14159;
14   char ccc[5] = "CCC";
15   char *testc = (char *)malloc(32);
16 
17   // The C++ way
18   test.append(std::to_string(integerx));
19   cout << test << endl;
20 
21   // The old C way
22   sprintf(testc, "%s %d %s",ccc, integerx, std::to_string(Pi).c_str()); // Could have printed directly using printf()
23   printf("%s\n",testc);
24 
25   // or
26   printf("%s %d %7.5f\n",ccc, integerx, Pi);
27}

Compile and run:

[prompt]$ g++ -std=c++11 test.cpp
[prompt]$ a.out
AAA 5
CCC 5 3.141590    
CCC 5 3.14159

Examples and Code snipets:

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

01#include <string>
02#include <iostream>
03 
04using namespace std;
05 
06main()
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, ...)

Output: ./a.out

abcd efg xyz ijk
String empty: 1
abcd efgxyz ijk
String length: 15
String size: 15
String capacity: 15
String empty: 0
abcd efgxyz ijk
First character: a
String f:    Leading and trailing blanks      
String length: 37
String f:    Leading and trailing blanks      ZZZ
String length: 40
String g: abc abc abd abc
Replace 12,1,"xyz",3: abc abc abd xyzbc
xyz abc abd xyzbc
xyz xyz abd xyzbc
xyz i abd xyzbc
Find: 6
4294967295
String h: abc abc abd abc
Find "abc",0: 0
Find "abc",1: 4
Find_first_of "abc",0: 0
Find_last_of "abc",0: 0
Find_first_not_of "abc",0: 3
Find_first_not_of " ": 0
Substr 5,9: bc abd ab
Compare 0,3,"abc": 0
Compare 0,3,"abd": -1
xyz
First character: x

[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.

Code Snipets:

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 String Class and Debugging in GDB:

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.

01#include <string>
02#include <iostream>
03 
04using namespace std;
05 
06// Helper routine ps to print a string class variable.
07 
08void ps(string& s){ cout << s << endl; }
09 
10int 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.

Tips:

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 )

Links/Information:

Books:

	The C++ Standard Library: A Tutorial Reference Nicolai M. Josuttis ISBN #0201379260, Addison Wesley Longman This book is the only book I have seen which covers string classes as implemented by current Linux distributions. It offers extensive coverage of the C++ string classes as well as fairly complete coverage of the C++ Standard Template Library (STL).
	C++ How to Program by Harvey M. Deitel, Paul J. Deitel ISBN #0131857576, Prentice Hall Fifth edition. The first edition of this book (and Professor Sheely at UTA) taught me to program C++. It is complete and covers all the nuances of the C++ language. It also has good code examples. Good for both learning and reference.

C++ String class Examples and Tutorial

STL C++ string functions:

Iterator types:

Code Snipets: