[Cpp primer] Library string Type

In order to use string type, we need to include the following code

#include<string>
using std::string;

　　

1. Defining and initializing strings

string s1; //Default initialization; s1 is the empty string
string s2(s1); //s2 is a copy of s1.
string s2 = s1; //Equivalent to s2(s1), s2 is a copy of s1.
string s3("value"); //Direct initialization, s3 is a copy of the string literal, not including the null
string s3 = "value"; //Equivalent to s3("value"), s3 is a copy of the string literal.
string s4(n, 'c'); //s4="cccc...ccc", n c

　　

2. Operations on strings

os << s; //Write s onto output stream os. Return os. ex: cout<<s;
is >> s; //Reads whitespace-separeted(if it comes across a whitespace, it stops reading) string from is into s. Rreturn is. ex: cin>>s;
getline(is, s); //Reads a line of input from is to s. Return is. is can be 'cin' and other types. 
//getline(cin, s) This function can read a total line, including whitespace. If it comes across the end-of-line, it stops reading.
s.empty();
s.size(); //Returns the number of characters in s. If s has whitespace, it still counts. Null character is not included in this size.
/*
size() doesn't return a int type value.
However, it returns a string::size_type value.
size_type is a companion type of string.
It's an unsigned type.
We should be aware that we'd better not to mix int with size_type.
*/
s1 == s2;
s1 != s2;
>, <, <=, >=
//We can use the strategy as a(case-sensitive) dictionary to compare twos strings.
s1 + s2;
s1 += s2;
/*
adding two strings together
It won't add a whitespace automatically.
s1 = "666", s2 = "777"
s3 = s1+s2 = "666777"
*/
string s1 = "mdzz";
s1 = s1 + "haha";  //1
s1 = "haha" + s1; //2
/*
String literals are not standard library strings.
Hence, string literals can not be the left-hand operand.
2 is wrong.
1 is ok.
*/

　　

3. Dealing with the Characters in a string

#include<cctype>
//Using this header, we can use many functions to deal with the characters in a string.
isalnum(c); //True: c is a letter or a digit
isalpha(c); //True: c is a letter
iscntrl(c); //True: c is a control character
isdigit(c); //True: c is a digit
isgraph(c); //True: c is not a space but is printable
islower(c); //True: c is a lowercase letter
isupper(c);
isprint(c); //True: c is a printable character
ispunct(c); //True: c is a punctuation character
isspace(c); //True: c is a whitespace
isxdigit(c); //True: c is a hexadecimal digit
tolower(c);
toupper(c);