std::basic_string::assign

C++

Language

Headers

Freestanding and hosted implementations

Named requirements

Language support library

Concepts library (C++20)

Diagnostics library

Utilities library

Strings library

Containers library

Algorithms library

Iterators library

Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

Filesystem library (C++17)

Technical Specifications

Strings library

Null-terminated strings

Byte strings

Multibyte strings

Wide strings

Classes

basic_string

basic_string_view (C++17)

char_traits

std::basic_string

Member functions basic_string::basic_string basic_string::operator= basic_string::assign basic_string::get_allocator Element access basic_string::at basic_string::operator[] basic_string::front (C++11) basic_string::back (C++11) basic_string::data basic_string::c_str basic_string::operator basic_string_view (C++17) Iterators basic_string::beginbasic_string::cbegin (C++11) basic_string::endbasic_string::cend (C++11) basic_string::rbeginbasic_string::crbegin (C++11) basic_string::rendbasic_string::crend (C++11) Capacity basic_string::empty basic_string::sizebasic_string::length basic_string::max_size basic_string::reserve basic_string::capacity basic_string::shrink_to_fit (C++11)

Operations basic_string::clear basic_string::insert basic_string::erase basic_string::push_back basic_string::pop_back (C++11) basic_string::append basic_string::operator+= basic_string::compare basic_string::starts_with (C++20) basic_string::ends_with (C++20) basic_string::replace basic_string::substr basic_string::copy basic_string::resize basic_string::swap Search basic_string::find basic_string::rfind basic_string::find_first_of basic_string::find_first_not_of basic_string::find_last_of basic_string::find_last_not_of Constants basic_string::npos

Non-member functions operator+ operator==operator!=operator<operator>operator<=operator>= swap(std::basic_string) operator<<operator>> getline stoistolstoll (C++11)(C++11)(C++11) stoulstoull (C++11)(C++11) stofstodstold (C++11)(C++11)(C++11) to_string (C++11) to_wstring (C++11) operator""s (C++14) Helper classes hash<std::string>hash<std::wstring>hash<std::u32string>hash<std::u16string> (C++11)(C++11)(C++11)(C++11) hash<std::pmr::string>hash<std::pmr::wstring>hash<std::pmr::u32string>hash<std::pmr::u16string> (C++20)(C++20)(C++20)(C++20) Deduction guides(C++17)

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 ) noexcept(/* see below */);		(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)
template < class T > basic_string& assign( const T& t );		(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) Implicitly converts t to a string view sv as if by std::basic_string_view<CharT, Traits> sv = t;, then replaces the contents with those of sv, as if by assign(sv.data(), sv.size()). 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.

10) Implicitly 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
ilist	-	std::initializer_list 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 ilist

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)

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
LWG 2946	C++17	string_view overload causes ambiguity in some cases	avoided by making it a template

Example

Run this code

#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

See also

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