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
Standard
Cross-platform

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);

Sub-strings

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

Output
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

std::transform(str.begin(),
               str.end(),
               str.begin(),
               tolower);