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Shifting from C to C++ strings

Shifting from C to C++ strings

Why change?

Avoid memory corruption errors
Easier to maintain
Simple to use
Should be fast (is reference counted)
Supports wide, narrow, anything-else characters

C++ std::basic_string

Available in two flavours
  • std::string for storing narrow characters
  • std::wstring for storing wide characters
  • Not normally needed, but if wanted you can instantiate basic_string for your own character type
    E.g. basic_string<MyCharacterClass>

    Comparing C and C++

    #include <string.h>
    #include <assert.h>
    int main()
       char name[15]; // !!!
       strcpy(name, "David");
       int len = strlen(name);
       strcat(name, " Moore");
       name[5] = 'J';
       assert(strcmp(name, "Oliver"));
       return 0;
    #include <string>
    #include <assert.h>
    int main()
       std::string name;
       name = "David";
       int len = name.length();
       name += " Moore";
       name[5] = 'J';
       assert(name != "Oliver");
       return 0;

    Interoperability between C and C++

    A C++ string can be constructed from a C character pointer
    char* p = "David";
    std::string str = p;
    A C++ string can return a C character pointer to its contents. But...
    Not possible to change the string with this pointer
    Don't use the pointer after the string has been destroyed
    std::string str = "Cocacola";
    char* p = str.c_str();

    Searching within a string

    Many different search functions

    Find substring
  • find
  • rfind
  • Find character
  • find_first_of
  • find_last_of
  • find_first_not_of
  • find_last_not_of
  • Special value string::npos means "not in the string"

    Searching example

    std::string line = "abcd    Apple butter chicken delicious    $0.62"; 
    // abcd
    string::size_type codeEnd = line.find_first_of(" \t");
    string code = line.substr(0, codeEnd);
    // 0.62
    string::size_type priceBegin = line.find_last_of('$');
    Double price = atof(line.substr(priceBegin + 1).c_str());
    // Apple butter chicken delicous
    string::size_type descBegin = line.find_first_not_of(" \t", codeEnd);
    string::size_type descEnd   = line.find_last_not_of(" \t", priceBegin - 1);
    string desc = line.substr(descBegin, descEnd - descBegin + 1);


    substr(i, n)
    returns a new string object of n characters starting from i
    n defaults to being to the end of the string
    std::wstring str = L"Cocacola";
    std::wcout << str.substr(4) << std::endl;

    Replace / Insert / Erase

    Easy to change the contents of a string
    std::string name("David Moore");
    name.replace(3, 2, "e");          // Dave Moore
    name.insert(0, "Mr. ");           // Mr. Dave Moore
    name.erase(4, 5);                 // Mr. Moore

    String I/O

    C++ strings support streaming

    std::cout << name << std::endl;

    Input a word
    std::cin >> name;

    Input a whole line
    getline(cin, line);

    Some other operations

    std::string a = "Coca";
    std::string b = "cola";
    std::string c = a + b;
    c += "(R)";
    int v = c.compare("Pepsi");

    Strings as collections

    A C++ string object looks a lot like a C++ collection (of characters)
    Has iterators, begin(), end(), push_back() etc.
    Can be used with generic algorithms