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std::basic_string::assign

basic_string& assign( size_type count, CharT ch );	(1)
basic_string& assign( const basic_string& str );	(2)
	(3)
basic_string& assign( const basic_string& str, size_type pos, size_type count );		(until C++14)
basic_string& assign( const basic_string& str, size_type pos, size_type count = npos);		(since C++14)
basic_string& assign( basic_string&& str );		(4)	(since C++11)
basic_string& assign( const CharT* s, size_type count );		(5)
basic_string& assign( const CharT* s );		(6)
template< class InputIt > basic_string& assign( InputIt first, InputIt last );		(7)
basic_string& assign( std::initializer_list<CharT> ilist );		(8)	(since C++11)
basic_string& assign( std::basic_string_view<CharT, Traits> sv);		(9)	(since C++17)
template < class T > basic_string& assign( const T& t, size_type pos, size_type count = npos);		(10)	(since C++17)

Replaces the contents of the string.

1) Replaces the contents with count copies of character ch.

2) Replaces the contents with a copy of str.

3) Replaces the contents with a substring [pos, pos+count) of str. If the requested substring lasts past the end of the string, or if count == npos, the resulting substring is [pos, str.size()). If pos > str.size(), std::out_of_range is thrown.

4) Replaces the contents with those of str using move semantics. Equivalent to *this = std::move(str).

5) Replaces the contents with the first count characters of character string pointed to by s. s can contain null characters.

6) Replaces the contents with those of null-terminated character string pointed to by s. The length of the string is determined by the first null character.

7) Replaces the contents with copies of the characters in the range [first, last). This overload does not participate in overload resolution if InputIt does not satisfy InputIterator. (since C++11)

8) Replaces the contents with those of the initializer list ilist.

9) Replaces the contents with those of the string view sv, as if by assign(sv.data(), sv.size())

10) Converts t to a string view sv as if by std::basic_string_view<CharT, Traits> sv = t;, then replaces the contents with the characters from the subview [pos, pos+count) of sv. If the requested subview lasts past the end of sv, or if count == npos, the resulting subview is [pos, sv.size()). If pos > sv.size(), std::out_of_range is thrown. This overload only participates in overload resolution if std::is_convertible_v<const T&, std::basic_string_view<CharT, Traits>> is true and std::is_convertible_v<const T&, const CharT*> is false.

Parameters

count	-	size of the resulting string
pos	-	index of the first character to take
ch	-	value to initialize characters of the string with
first, last	-	range to copy the characters from
str	-	string to be used as source to initialize the characters with
s	-	pointer to a character string to use as source to initialize the string with
init	-	`std::initializer_list` to initialize the characters of the string with
sv	-	`std::basic_string_view` to initialize the characters of the string with
t	-	object (convertible to `std::basic_string_view`) to initialize the characters of the string with
Type requirements
-`InputIt` must meet the requirements of `InputIterator`.

Return value

*this.

Complexity

1) linear in count

2) linear in size of str

3) linear in count

4) constant. If alloc is given and alloc != other.get_allocator(), then linear.

5) linear in count

6) linear in size of s

7) linear in distance between first and last

8) linear in size of init

Exceptions

If an exception is thrown for any reason, this function has no effect (strong exception guarantee). (since C++11).

If the operation would result in size() > max_size(), throws std::length_error.

4) `noexcept` specification: `noexcept`	(since C++11) (until C++17)
4) `noexcept` specification: `noexcept(std::allocator_traits<Allocator>::propagate_on_container_move_assignment::value \|\| std::allocator_traits<Allocator>::is_always_equal::value)`	(since C++17)

Example

#include <iostream>
#include <iterator>
#include <string>
 
int main()
{
  std::string s;
  // assign(size_type count, CharT ch)
  s.assign(4, '=');
  std::cout << s << '\n'; // "===="
 
  std::string const c("Exemplary");
  // assign(basic_string const& str)
  s.assign(c);
  std::cout << c << "==" << s <<'\n'; // "Exemplary == Exemplary"
 
  // assign(basic_string const& str, size_type pos, size_type count)
  s.assign(c, 0, c.length()-1);
  std::cout << s << '\n'; // "Exemplar";
 
  // assign(basic_string&& str)
  s.assign(std::string("C++ by ") + "example");
  std::cout << s << '\n'; // "C++ by example"
 
  // assign(charT const* s, size_type count)
  s.assign("C-style string", 7);
  std::cout << s << '\n'; // "C-style"
 
  // assign(charT const* s)
  s.assign("C-style\0string");
  std::cout << s << '\n'; // "C-style"
 
  char mutable_c_str[] = "C-style string";
  // assign(InputIt first, InputIt last)
  s.assign(std::begin(mutable_c_str), std::end(mutable_c_str)-1);
  std::cout << s << '\n'; // "C-style string"
 
  // assign(std::initializer_list<charT> ilist)
  s.assign({ 'C', '-', 's', 't', 'y', 'l', 'e' });
  std::cout << s << '\n'; // "C-style"
}

Output:

====
Exemplary==Exemplary
Exemplar
C++ by example
C-style
C-style
C-style string
C-style

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 2063	C++11	non-normative note stated that swap is a valid implementation of move-assign	corrected to require move assignment

(constructor)	constructs a `basic_string` (public member function)
operator=	assigns values to the string (public member function)