string.md

string

String is a object that manages sequences of characters.
String is a not basic type in C++, but is in Standard Library facility.
So, Appropriate header and namespace std must be stated.

#include <string>
using namespace std;

C++ string is useful. Because we don't need to worry about length of characters, unlikely in C.
Also, it is easy to use with basic operators and member functions.

input and output

operator << and operator >> are overloaded.
or getline(input, str, delim='\n') function can be used.
In getline function,
    input parameter is the stream to get data from.
    str parameter is the string to put data into.
    delim parameter is the character to set the end of input. the function works until it meets the delim character.

cout << "input your name : ";
string name;
cin >> name; 

cout << "input your email : "
string emailHeader; 
geline(cin, emailHeader, '@');

operators

• assign

string a = "hello";
string b = a;

• concatenate and append

string greeting1 = "hello";
string greeting2 = "world";
string greeting = greeting1 + " " + greeting2;
greeting += "!";

• compare
  Compare operations are overloaded and do appropriate comparison.
  == and != return true if two character arrays in string objects are exactly matched.
  <, >, <= and >= compare lexicographical order of string.
  but, !=, <, >, <= and >= are removed in c++20.

string a = 'abc';
string b = 'abc';
if( a == b ) 
	cout << "a and b are same!" << endl; 

string A = 'a';
string B = 'b';
if( a < b ) 
	cout << "a is less than b" << endl; //printed
A = 'ab';
B = 'ac';
if( a < b )
	cout << "a is less than b" << endl; //printed

member function

• size and length
  size() and length() returns the number of characters.
  Since the number of characters is cached in string object, size() and length() both take constant time.

string a = "hello";
cout << a.size() << " " << a.length() << endl; //5 5 

• access strings
  with [] or at(), we can access a character in a string object.

string a = "hello";
cout << a[0] << ", " << a.at(0) << endl; //h, h 

a[0] = 'H';
cout << a << endl; //Hello 

a.at(4) = 'O';
cout << a << endl; //HellO

• c_str
  c_str() returns a pointer to const char of character array that is in string object.
  it means by using c_str(), we can convert string to null terminated character array which is a C-style.

string greeting = "Hello";
const char* hi = greeting.c_str();
int idx = 0;
while(hi[idx] != '\0'){
    cout << hi[idx++];
}

• capacity and resize
  capacity() returns the size of allocated memory for character array in string object.
  if more memory is needed, the new larger memory is newly allocated and whole past memory is copied to new one. reserve() sets the size of allocated memory.
  if we know how many memory is needed, it can be used to prevent inefficient newly allocating and copying process.

string greeting = "hello";
cout << "capacity : " << greeting.capacity() << endl; //15
greeting.reserve(100);
cout << "capacity : " << greeting.capacity() << endl; //100