The STL string Class

The STL string class std::string helps you in the following ways:

• Reduces the effort of creation and manipulating strings
• Increases the stability of the application being programmed by internally managing memory allocation details
• Supplies copy constructor and assignment operators that automatically ensure that member strings get correctly copied
• Supplies useful utility functions that help in copying, truncating, finding, and erasing  to name a few
• Provides operators that help in comparisons
• Focuses efforts on your application's primary requirements rather than on string manipulation details

Working with the STL string Class

The most commonly used string functions are

• Copying
• Concatenating
• Finding characters and substrings
• Truncating
• String reversal and case conversion are achieved using algorithms provided by the standard library

Instantiating the STL string and Making Copies

#include <string>
#include <iostream>

int main ()
{
    using namespace std;
    const char* pszConstString = "Hello String!";
    cout << "Constant string is: " << pszConstString << endl;

    std::string strFromConst (pszConstString);
    cout << "strFromConst is: " << strFromConst << endl;

    std::string str2 ("Hello String!");
    std::string str2Copy (str2);
    cout << "str2Copy is: " << str2Copy << endl;

    // Initialize a string to the first 5 characters of another
    std::string strPartialCopy (pszConstString, 5);
    cout << "strPartialCopy is: " << strPartialCopy << endl;

    // Initialize a string object to contain 10 'a's
    std::string strRepeatChars (10, 'a');
    cout << "strRepeatChars is: " << strRepeatChars << endl;

    return 0;
}

Output:　　

　　Constant string is: Hello String!

　　strFromConst is: Hello String!

　　str2Copy is: Hello String!

　　strPartialCopy is: Hello

　　strRepeatChars is: aaaaaaaaaa

Accessing a string and Its Contents

#include <string>
#include <iostream>

int main ()
{
    using namespace std;

    // The sample string
    string strSTLString ("Hello String");

    // Access the contents of the string using array syntax
    cout << "Displaying characters using array-syntax: " << endl;
    for ( size_t nCharCounter = 0
        ; nCharCounter < strSTLString.length ()
        ; ++ nCharCounter )
    {
        cout << "Character [" << nCharCounter << "] is: ";
        cout << strSTLString [nCharCounter] << endl;
    }
    cout << endl;

    // Access the contents of a string using iterators
    cout << "Displaying characters using iterators: " << endl;
    int nCharOffset = 0;
    string::const_iterator iCharacterLocator;
    for ( iCharacterLocator = strSTLString.begin ()
        ; iCharacterLocator != strSTLString.end ()
        ; ++ iCharacterLocator )
    {
        cout << "Character [" << nCharOffset ++ << "] is: ";
        cout << *iCharacterLocator << endl;
    }
    cout << endl;

    // Access the contents of a string as a C-style string
    cout << "The char* representation of the string is: ";
    cout << strSTLString.c_str () << endl;

    return 0;
}

Output:

　　Displaying the elements in the string using array-syntax:

　　Character [0] is: H

　　Character [1] is: e

　　Character [2] is: l

　　Character [3] is: l

　　Character [4] is: o

　　Character [5] is:

　　Character [6] is: S

　　Character [7] is: t

　　Character [8] is: r

　　Character [9] is: i

　　Character [10] is: n

　　Character [11] is: g

　　Displaying the contents of the string using iterators:

　　Character [0] is: H

　　Character [1] is: e

　　Character [2] is: l

　　Character [3] is: l

　　Character [4] is: o

　　Character [5] is:

　　Character [6] is: S

　　Character [7] is: t

　　Character [8] is: r

　　Character [9] is: i

　　Character [10] is: n

　　Character [11] is: g

String Concatenation

#include <string>
#include <iostream>

int main ()
{
    using namespace std;

    string strSample1 ("Hello");
    string strSample2 (" String!");

    // Concatenate
    strSample1 += strSample2;
    cout << strSample1 << endl << endl;

    string strSample3 (" Fun is not needing to use pointers!");
    strSample1.append (strSample3);
    cout << strSample1 << endl << endl;

    const char* pszConstString = "You however still can!";
    strSample1.append (pszConstString);
    cout << strSample1 << endl;

    return 0;
}

Output:

　　Hello String!

　　Hello String! Fun is not needing to use pointers!

　　Hello String! Fun is not needing to use pointers when working with strings. You

　　however still can!

Finding a Character or Substring in a string

#include <string>
#include <iostream>

int main ()
{
    using namespace std;

    string strSample ("Good day String! Today is beautiful!");
    cout << "The sample string is: " << endl;
    cout << strSample << endl << endl;

    // Find substring "day" in it...
    size_t nOffset = strSample.find ("day", 0);

    // Check if the substring was found...
    if (nOffset != string::npos)
        cout << "First instance of \"day\" was found at offset " << nOffset;
    else
        cout << "Substring not found." << endl;

    cout << endl << endl;

    cout << "Locating all instances of substring \"day\"" << endl;
    size_t nSubstringOffset = strSample.find ("day", 0);

    while (nSubstringOffset != string::npos)
    {
        cout << "\"day\" found at offset " << nSubstringOffset << endl;

        // Make the 'find' function search the next character onwards
        size_t nSearchOffset = nSubstringOffset + 1;

        nSubstringOffset = strSample.find ("day", nSearchOffset);
    }

    cout << endl;

    cout << "Locating all instances of character 'a'" << endl;
    const char chCharToSearch = 'a';
    size_t nCharacterOffset = strSample.find (chCharToSearch, 0);

    while (nCharacterOffset != string::npos)
    {
        cout << "'" << chCharToSearch << "' found";
        cout << " at position: " << nCharacterOffset << endl;

        // Make the 'find' function search forward from the next character
        size_t nCharSearchOffset = nCharacterOffset + 1;

        nCharacterOffset = strSample.find(chCharToSearch,nCharSearchOffset);
    }

    return 0;
}

Output:

　　The sample string is:

　　Good day String! Today is beautiful!

　　First instance of “day” was found at offset 5

　　Locating all instances of substring “day”

　　“day” found at offset 5

　　“day” found at offset 19

　　Locating all instances of character ‘a’

　　‘a’ found at position: 6

　　‘a’ found at position: 20

　　‘a’ found at position: 28

Truncate a STL string

#include <string>
#include <algorithm>
#include <iostream>

int main ()
{
    using namespace std;

    string strSample ("Hello String! Wake up to a beautiful day!");
    cout << "The original sample string is: " << endl;
    cout << strSample << endl << endl;

    // Delete characters from the string given position and count
    cout << "Truncating the second sentence: " << endl;
    strSample.erase (13, 28);
    cout << strSample << endl << endl;

    // Find a character 'S' in the string using STL find algorithm
    string::iterator iCharS = find ( strSample.begin ()
        , strSample.end (), 'S');

    // If character found, 'erase' to deletes a character
    cout << "Erasing character 'S' from the sample string:" << endl;
    if (iCharS != strSample.end ())
        strSample.erase (iCharS);

    cout << strSample << endl << endl;

    // Erase a range of characters using an overloaded version of erase()
    cout << "Erasing a range between begin() and end(): " << endl;
    strSample.erase (strSample.begin (), strSample.end ());

    // Verify the length after the erase() operation above
    if (strSample.length () == 0)
        cout << "The string is empty" << endl;

    return 0;
}

Output:

　　The original sample string is:

　　Hello String! Wake up to a beautiful day!

　　Truncating the second sentence:

　　Hello String!

　　Erasing character ‘S’ from the sample string:

　　Hello tring!

　　Erasing a range between begin() and end():

　　The string is empty

String Reversal

// Listing 17.6 - Reversing an STL String
#include <string>
#include <iostream>
#include <algorithm>

int main ()
{
    using namespace std;

    string strSample ("Hello String! We will reverse you!");
    cout << "The original sample string is: " << endl;
    cout << strSample << endl << endl;

    reverse (strSample.begin (), strSample.end ());

    cout << "After applying the std::reverse algorithm: " << endl;
    cout << strSample;

    return 0;
}

Output:

　　The original sample string is:

　　Hello String! We will reverse you!

　　After applying the std::reverse algorithm:

　　!uoy esrever lliw eW !gnirtS olleH

String Case Conversion

// Listing 17.7 - Performing Case Conversions on an STL String Using std::transform
#include <string>
#include <iostream>
#include <algorithm>

int main ()
{
    using namespace std;

    cout << "Please enter a string for case-convertion:" << endl;
    cout << "> ";

    string strInput;
    getline (cin, strInput);
    cout << endl;

    transform (strInput.begin(),strInput.end(),strInput.begin(),toupper);
    cout << "The string converted to upper case is: " << endl;
    cout << strInput << endl << endl;

    transform (strInput.begin(),strInput.end(),strInput.begin(),tolower);
    cout << "The string converted to lower case is: " << endl;
    cout << strInput << endl << endl;

    return 0;
}

Output:

　　Please enter a string for case-conversion:

　　> ConverT thIS StrINg!

　　The string converted to upper case is:

　　CONVERT THIS STRING!

　　The string converted to lower case is:

　　convert this string!