std::string assignment and swapping

String assignment

The easiest way to assign a value to a string is to the use the overloaded operator= function. There is also an assign() member function that duplicates some of this functionality.

string& string::operator= (const string& str) string& string::assign (const string& str) string& string::operator= (const char* str) string& string::assign (const char* str) string& string::operator= (char c) These functions assign values of various types to the string. These functions return *this so they can be “chained”. Note that there is no assign() function that takes a single char. Sample code: 01 string sString; 02   03 // Assign a string value 04 sString = string("One"); 05 cout << sString << endl; 06   07 const string sTwo("Two"); 08 sString.assign(sTwo); 09 cout << sString << endl; 10   11 // Assign a C-style string 12 sString = "Three"; 13 cout << sString << endl; 14   15 sString.assign("Four"); 16 cout << sString << endl; 17   18 // Assign a char 19 sString = '5'; 20 cout << sString << endl; 21   22 // Chain assignment 23 string sOther; 24 sString = sOther = "Six"; 25 cout << sString << " " << sOther << endl; Output: One Two Three Four 5 Six Six

The assign() member function also comes in a few other flavors:

string& string::assign (const string& str, size_type index, size_type len) Assigns a substring of str, starting from index, and of length len Throws an out_of_range exception if the index is out of bounds Returns *this so it can be “chained”. Sample code: 1 const string sSource("abcdefg"); 2 string sDest; 3   4 sDest.assign(sSource, 2, 4); // assign a substring of source from index 2 of length 4 5 cout << sDest << endl; Output: cdef

string& string::assign (const char* chars, size_type len) Assigns len characters from the C-style array chars Ignores special characters (including ‘\0′) Throws an length_error exception if the result exceeds the maximum number of characters Returns *this so it can be “chained”. Sample code: 1 string sDest; 2   3 sDest.assign("abcdefg", 4); 4 cout << sDest << endl; Output: abcd This function is potentially dangerous and it’s use is not recommended.

string& string::assign (size_type len, char c) Assigns len occurrences of the character c Throws an length_error exception if the result exceeds the maximum number of characters Returns *this so it can be “chained”. Sample code: 1 string sDest; 2   3 sDest.assign(4, 'g'); 4 cout << sDest << endl; Output: gggg

Swapping
If you have two strings and want to swap their values, there are two functions both named swap() that you can use.
void string::swap (string &str)
void swap (string &str1, string &str2)

• Both functions swap the value of the two strings. The member function swaps *this and str, the global function swaps str1 and str2.
• These functions are efficient and should be used instead of assignments to perform a string swap.

Sample code:

1	string sStr1("red");

2	string sStr2("blue);

3

4	cout << sStr1 << " " << sStr2 << endl;

5	swap(sStr1, sStr2);

6	cout << sStr1 << " " << sStr2 << endl;

7	sStr1.swap(sStr2);

8	cout << sStr1 << " " << sStr2 << endl;

Output:

red blue
blue red
red blue