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

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std::basic_string
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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

#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 a range of characters to a string
(public member function)
constructs a basic_string
(public member function)
assigns values to the string
(public member function)