#include <string>
#include <iostream>

using namespace std;

main()
{
   string a("abcd efg");
   string b("xyz ijk");
   string c;

   cout << a << " " << b << endl;                        // Output: abcd efg xyz ijk

   cout << "String empty: "    << c.empty()    << endl;  // String empty: 1 
                                                         // Is string empty? Yes it is empty. (TRUE)
   c = a + b;                                            // concatenation
   cout << c << endl;                                    // abcd efgxyz ijk
   cout << "String length: "   << c.length()   << endl;  // String length: 15
   cout << "String size: "     << c.size()     << endl;  // String size: 15
   cout << "String capacity: " << c.capacity() << endl;  // String capacity: 15
   cout << "String empty: "    << c.empty()    << endl;  // String empty: 0 
                                                         // Is string empty? No it is NOT empty. (FALSE)
   string d = c;
   cout << d << endl;                                    // abcd efgxyz ijk

                                                         // First character: a
   cout << "First character: " << c[0] << endl;          // Strings start with index 0 just like C.

   string f("    Leading and trailing blanks      ");
   cout << "String f:" << f << endl;
   cout << "String length: " << f.length() << endl;      // String length: 37
   cout << "String f:" << f.append("ZZZ") << endl;       // String f:    Leading and trailing blanks      ZZZ
   cout << "String length: " << f.length() << endl;      // String length: 40

   string g("abc abc abd abc");
   cout << "String g: " << g << endl;                    // String g: abc abc abd abc
   cout << "Replace 12,1,\"xyz\",3: " << g.replace(12,1,"xyz",3) << endl;  // Replace 12,1,"xyz",3: abc abc abd xyzbc
   cout << g.replace(0,3,"xyz",3) << endl;               // xyz abc abd xyzbc
   cout << g.replace(4,3,"xyz",3) << endl;               // xyz xyz abd xyzbc
   cout << g.replace(4,3,"ijk",1) << endl;               // xyz i abd xyzbc
   cout << "Find: " << g.find("abd",1) << endl;          // Find: 6
   cout << g.find("qrs",1) << endl;

   string h("abc abc abd abc");
   cout << "String h: " << h << endl;
   cout << "Find \"abc\",0: " << h.find("abc",0) << endl; // Find "abc",0: 0
   cout << "Find \"abc\",1: " << h.find("abc",1) << endl; // Find "abc",1: 4
   cout << "Find_first_of \"abc\",0: " << h.find_first_of("abc",0) << endl; // Find_first_of "abc",0: 0
   cout << "Find_last_of \"abc\",0: " << h.find_last_of("abc",0) << endl;   // Find_last_of "abc",0: 0
   cout << "Find_first_not_of \"abc\",0: " << h.find_first_not_of("abc",0) << endl;  // Find_first_not_of "abc",0: 3
   cout << "Find_first_not_of \" \": " << h.find_first_not_of(" ") << endl;  // Find_first_not_of " ": 0
   cout << "Substr 5,9: " << h.substr(5,9) << endl;       // Substr 5,9: bc abd ab
   cout << "Compare 0,3,\"abc\": " << h.compare(0,3,"abc") << endl;  // Compare 0,3,"abc": 0
   cout << "Compare 0,3,\"abd\": " << h.compare(0,3,"abd") << endl;  // Compare 0,3,"abd": -1
   cout << h.assign("xyz",0,3) << endl;                   // xyz
   cout << "First character: " << h[0] << endl; // Strings start with 0 // First character: x


}

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

String class functions:

    * Constructors:

    string sVar1("abc");
    string sVar1(C-string);
    string sVar2(10," ");  // Generate string initialized to 10 blanks.
    string sVar3(Var1,string-index);  // Initialize with characters from string starting with index string-index.
    string sVar4(iterator-index-begin, iterator-index-end)
    

    * Destructor:

    Var.~string();         // Destructor
    

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

      Code samples:

       string g("abc abc abd abc");
       cout << g.replace(4,1,"ijk",3) << endl;

       string h("abc abc abd abc");
       cout << h.replace(4,6,"ijk",3) << endl;

       string k("abc abc abd abc");
       cout << k.replace(4,3,"ijk",3) << endl;
 
       string l("abc abc abd abc");
       cout << k.replace(12,1,"xyz",3) << endl;
    

      Output:

    abc ijkbc abd abc          - Beginning with the fourth index (character number 5) replace one character with three characters from the 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:
          o Val.find(const string& argument)
            Find first occurence of argument within string Val
          o find(const string& argument, size_type index)
            Find first occurence of argument within string Val starting search from position index.
          o find(const char* argument)
          o find(const char* argument, size_type index)
          o 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. 

List of operations:
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
    ==, !=, <, <=, >, >=
    Var.compare(string)	Compare strings.
    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.
    Var.find()
    Var.rfind()	Find first occurance of string or substring.
    Find last occurance of string or substring.
    Var.find_first_of()
    Var.find_last_of()	Find strings and substrings.
    Var.find_first_not_of()
    Var.find_last_not_of()	Find strings and substrings.
    Var.replace()	Replace section of string with new characters.
    Var.substr()	Return substring of text
    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.

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 

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.

#include <strings.h>
#include <string>
#include <stdio.h>
using namespace std;
   
int main()
{
   char *phrase1="phrase";
   string phrase2("Second phrase");
   char  phraseA[128];
   char  *phraseB;
   
   strcpy(phraseA,phrase2.c_str()); 
   phraseB = strstr(phrase2.c_str(),phrase1);

   printf("phraseA: %s\n",phraseA);
   printf("phraseB: %s\n",phraseB);
   printf("phrase2: %s\n",phrase2.c_str());
}   

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

#include <iostream>
#include <sstream>
#include <string>
using namespace std;
 
string int2string(const int& number)
{
   ostringstream oss;
   oss << number;
   return oss.str();
}
 
main()
{
   int number=7878;
   string test="SSSSS";
   test += int2string(number);
   cout << test << endl;
}
    

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

#include <iostream>
#include <sstream>
#include <string>
using namespace std;
 
main()
{
   string test="AAA 123 SSSSS 3.141592654";
   istringstream totalSString( test );
   string string1, string2;
   int    integer1;
   double PI;

   totalSString >> string1 >> integer1 >> string2 >> PI;
   
   cout << "Individual parsed variables:" << endl;
   cout << "First string:  " << string1   << endl;
   cout << "First integer: " << integer1  << endl;
   cout << "Value of PI:   " << PI        << endl;
}
    

Compile and run:

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

Code snipets:

    * Read lines from standard input:

    while( getline(std::cin, sLine) )
    {
       if( sLine.empty() );                     // Ignore empty lines
       else
       {
          cout << sLine[0] << sLine[1] << endl;
          ....
          ...
       }
    }
    

    * Read lines from input file:

    #define SYS_CONFIG_FILE "/etc/file.conf"
    #include <string>
    #include <algorithm>
    #include <vector>
    #include <cctype>
    #include <iostream>
    #include <fstream>

    using namespace std;

    string::size_type posBeginIdx, posEndIdx;
    string::size_type ipos=0;
    string            sLine, sValue;
    string            sKeyWord;
    const string      sDelim( ":" );

    ifstream myInputFile(SYS_CONFIG_FILE, ios::in);
    if( !myInputFile )
    {
       sError = "File SYS_CONFIG_FILE could not be opened";
       return sError; // ERROR
    }

    while( getline(myInputFile,sLine) )
    {
       if( sLine.empty() );                     // Ignore empty lines
       else
       {
          posEndIdx = sLine.find_first_of( sDelim );
          sKeyWord  = sLine.substr( ipos, posEndIdx ); // Extract word
          posBeginIdx = posEndIdx + 1;  // Beginning of next word (after ':')
       ....
       ...
       }
    }
    

The String Class and Debugging in GDB:

The first thing you will notice when using the GNU 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 or use string class funtions to print out the value of the string variable.

#include <string>
#include <iostream>

using namespace std;

// Helper routine ps to print a string class variable.

void ps(string& s){ cout << s << endl; }

int main()
{
   string a("String A");
   string b;

   b = "String B";

   cout << "Hello!" << endl;
}
    

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
    

One may also 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