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17.5 — std::string assignment and swapping
By Alex on July 18th, 2010 | last modified by Alex on July 18th, 2010 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:
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string sString; // Assign a string value sString = string("One"); cout << sString << endl; const string sTwo("Two"); sString.assign(sTwo); cout << sString << endl; // Assign a C-style string sString = "Three"; cout << sString << endl; sString.assign("Four"); cout << sString << endl; // Assign a char sString = '5'; cout << sString << endl; // Chain assignment string sOther; sString = sOther = "Six"; cout << sString << " " << sOther << endl; |
Output:
One
Two
Three
Four
5
Six Six
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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:
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const string sSource("abcdefg"); string sDest; sDest.assign(sSource, 2, 4); // assign a substring of source from index 2 of length 4 cout << sDest << endl; |
Output:
cdef
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string& string::assign (const char* chars, size_type len)
- Assigns len characters from the C-style array chars
- Ignores special characters (including ‘ ’)
- Throws an length_error exception if the result exceeds the maximum number of characters
- Returns *this so it can be “chained”.
Sample code:
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string sDest; sDest.assign("abcdefg", 4); cout << sDest << endl; |
Output:
abcd
This function is potentially dangerous and it’s use is not recommended.
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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:
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string sDest; sDest.assign(4, 'g'); cout << sDest << endl; |
Output:
gggg
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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:
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string sStr1("red"); string sStr2("blue); cout << sStr1 << " " << sStr2 << endl; swap(sStr1, sStr2); cout << sStr1 << " " << sStr2 << endl; sStr1.swap(sStr2); cout << sStr1 << " " << sStr2 << endl; |
Output:
red blue
blue red
red blue
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17.6 -- std::string appending
Index
17.4 -- std::string character access and conversion to c-style strings
Hi Alex,
Thanks a lot for these wonderful tutorials. Can you please add some tutorials on Vector, other Container classes, Shared_ptr, and other advanced C++ topics?
Thanks a lot again.
string sStr1("red"); string sStr2("blue); cout << sStr1 << " " << sStr2 << endl; swap(sStr1, sStr2); cout << sStr1 << " " << sStr2 << endl; sStr1.swap(sStr2); cout << sStr1 << " " << sStr2 << endl;from last code block u forgot to close "blue with ..."...