22.5 — std::string assignment and swapping

String assignment

The easiest way to assign a value to a string is to 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:

std::string sString;

// Assign a string value
sString = std::string("One");
std::cout << sString << '\n';

const std::string sTwo("Two");
sString.assign(sTwo);
std::cout << sString << '\n';

// Assign a C-style string
sString = "Three";
std::cout << sString << '\n';

sString.assign("Four");
std::cout << sString << '\n';

// Assign a char
sString = '5';
std::cout << sString << '\n';

// Chain assignment
std::string sOther;
sString = sOther = "Six";
std::cout << sString << ' ' << sOther << '\n';

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:

const std::string sSource("abcdefg");
std::string sDest;

sDest.assign(sSource, 2, 4); // assign a substring of source from index 2 of length 4
std::cout << sDest << '\n';

Output:

cdef

string& string::assign (const char* chars, size_type len)
  • Assigns len characters from the C-style array chars
  • Throws an length_error exception if the result exceeds the maximum number of characters
  • Returns *this so it can be “chained”.

Sample code:

std::string sDest;

sDest.assign("abcdefg", 4);
std::cout << sDest << '\n';

Output:

abcd

This function is potentially dangerous and its use is not recommended.

string& string::assign (size_type len, char c)
  • Assigns len occurrences of the character c
  • Throws a length_error exception if the result exceeds the maximum number of characters
  • Returns *this so it can be “chained”.

Sample code:

std::string sDest;

sDest.assign(4, 'g');
std::cout << sDest << '\n';

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:

std::string sStr1("red");
std::string sStr2("blue");

std::cout << sStr1 << ' ' << sStr2 << '\n';
swap(sStr1, sStr2);
std::cout << sStr1 << ' ' << sStr2 << '\n';
sStr1.swap(sStr2);
std::cout << sStr1 << ' ' << sStr2 << '\n';

Output:

red blue
blue red
red blue
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