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17.5 — 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:

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:

Output:

cdef

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:

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:

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:

Output:

red blue
blue red
red blue

17.6 -- std::string appending
Index
17.4 -- std::string character access and conversion to c-style strings

13 comments to 17.5 — std::string assignment and swapping

  • ap

    from last code block u forgot to close “blue with …”…

  • abcd

    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.

  • Aryan

    Hey Alex,

    I just want to ask a simple question. We know that the input stream (cin>>) its terminated after it sees a whitespace. But what if I want to include in my sting type object (lets say string name) a whole sentence? Like in my ‘name’ variable I store a string of characters "Hello how are you", but if I did this:

    string name;
    cin>>name; // I wrote ‘Hello how are you’

    So how will win take action on this when it sees a whitespace after ‘Hello’, will it terminate and won’t store the rest of the string?

  • Matt

    Typo ("it’s" should be "its"): "This function is potentially dangerous and it’s use is not recommended."

  • Benjamin

    In the line 2 of the sample code of Swapping section :
    Close the quotation after the word ‘blue’ otherwise the code would not run in the way that which is intended for.
    Good Day.

  • Lokesh

    Last code, line 2: missing <">

  • Sheeplie

    If the string& string::operator= (const string& str) requires a constant string argument, how are you simply able to do

    ?

    • Sheeplie

      After refreshing myself on function parameters, I’ve deduced that this is possible because a function parameter is it’s own new variable inside of a function, simply given the value of whatever argument you pass in. So you could make it constant, sure- it’s its own variable, and that const var would simply receive the value of string( “one”) ( which it would reference. )

    • Alex

      The compiler will convert const literal string “One” into a “string” object via converting constructor std::string(const char*).

      In other words, the compiler is smart enough to know how to convert “One” into a std::string because there’s a std::string constructor that takes C-style string literals.

  • Matt

    Above section Swapping", you wrote:
    "Throws an length_error exception…".

    "an" should be "a".

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