std::basic_string<CharT,Traits,Allocator>::assign

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Execution support library (C++26)

Technical specifications

Symbols index

External libraries

 

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::~basic_string basic_string::operator= basic_string::assign basic_string::assign_range (C++23) basic_string::get_allocator Element access basic_string::at basic_string::operator[] basic_string::front (DR*) basic_string::back (DR*) 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 (DR*) Modifiers basic_string::clear basic_string::insert basic_string::insert_range (C++23) basic_string::erase basic_string::push_back basic_string::pop_back (DR*) basic_string::append basic_string::append_range (C++23) basic_string::operator+= basic_string::replace basic_string::replace_with_range (C++23) basic_string::copy basic_string::resize basic_string::resize_and_overwrite (C++23)   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 Operations basic_string::compare basic_string::starts_with (C++20) basic_string::ends_with (C++20) basic_string::contains (C++23) basic_string::substr Constants basic_string::npos Non-member functions operator+ swap(std::basic_string) erase(std::basic_string)erase_if(std::basic_string) (C++20)(C++20) I/O operator<<operator>> getline Comparison operator==operator!=operator<operator>operator<=operator>=operator<=> (until C++20)(until C++20)(until C++20)(until C++20)(until C++20)(C++20) Numeric conversions 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) Literals operator""s (C++14) Helper classes hash<std::basic_string> (C++11) Deduction guides (C++17)

 

basic_string& assign( size_type count, CharT ch );	(1)	(constexpr since C++20)
basic_string& assign( const basic_string& str );	(2)	(constexpr since C++20)
	(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) (constexpr since C++20)
basic_string& assign( basic_string&& str ) noexcept(/* see below */);		(4)	(since C++11) (constexpr since C++20)
basic_string& assign( const CharT* s, size_type count );		(5)	(constexpr since C++20)
basic_string& assign( const CharT* s );		(6)	(constexpr since C++20)
template< class InputIt > basic_string& assign( InputIt first, InputIt last );		(7)	(constexpr since C++20)
basic_string& assign( std::initializer_list<CharT> ilist );		(8)	(since C++11) (constexpr since C++20)
template< class StringViewLike > basic_string& assign( const StringViewLike& t );		(9)	(since C++17) (constexpr since C++20)
template< class StringViewLike > basic_string& assign( const StringViewLike& t,                       size_type pos, size_type count = npos);		(10)	(since C++17) (constexpr since C++20)

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. Equivalent to *this = str;. In particular, allocator propagation may take place.(since C++11)

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). In particular, allocator propagation may take place.

5) Replaces the contents with copies of the characters in the range [s, s + count). This range 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 using Traits::length(s).

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 LegacyInputIterator.(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 participates in overload resolution only if std::is_convertible_v<const StringViewLike&,
                      std::basic_string_view<CharT, Traits>> is true and std::is_convertible_v<const StringViewLike&, 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 participates in overload resolution only if std::is_convertible_v<const StringViewLike&,
                      std::basic_string_view<CharT, Traits>> is true and std::is_convertible_v<const StringViewLike&, 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 LegacyInputIterator.

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.

9) Linear in size of t.

Exceptions

4)

noexcept specification:  

noexcept(std::allocator_traits<Allocator>::

             propagate_on_container_move_assignment::value ||

         std::allocator_traits<Allocator>::is_always_equal::value)

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

If an exception is thrown for any reason, this function has no effect (strong exception safety guarantee).

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(const basic_string& str)
    s.assign(c);
    std::cout << c << " == " << s << '\n'; // "Exemplary == Exemplary"
 
    // assign(const basic_string& 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(const CharT* s, size_type count)
    s.assign("C-style string", 7);
    std::cout << s << '\n'; // "C-style"
 
    // assign(const CharT* 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 847	C++98	there was no exception safety guarantee	added strong exception safety guarantee
LWG 2063	C++11	non-normative note stated that swap is a valid implementation of move-assign	corrected to require move assignment
LWG 2579	C++11	assign(const basic_string&) did not propagate allocators	made to propagate allocators if needed
LWG 2946	C++17	overload (9) caused ambiguity in some cases	avoided by making it a template

See also

assign_range (C++23)	assign a range of characters to a string (public member function)
(constructor)	constructs a basic_string (public member function)
operator=	assigns values to the string (public member function)