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

From cppreference.net
std::basic_string
basic_string & assign ( const basic_string & str ) ;
(1) (constexpr since C++20)
basic_string & assign ( basic_string && str ) noexcept ( /* see below */ ) ;
(2) (since C++11)
(constexpr since C++20)
basic_string & assign ( size_type count, CharT ch ) ;
(3) (constexpr since C++20)
basic_string & assign ( const CharT * s, size_type count ) ;
(4) (constexpr since C++20)
basic_string & assign ( const CharT * s ) ;
(5) (constexpr since C++20)
template < class SV >
basic_string & assign ( const SV & t ) ;
(6) (since C++17)
(constexpr since C++20)
template < class SV >

basic_string & assign ( const SV & t,

size_type pos, size_type count = npos ) ;
(7) (since C++17)
(constexpr since C++20)
(8)
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)
template < class InputIt >
basic_string & assign ( InputIt first, InputIt last ) ;
(9) (constexpr since C++20)
basic_string & assign ( std:: initializer_list < CharT > ilist ) ;
(10) (since C++11)
(constexpr since C++20)

Replaces the contents of the string.

1) Equivalent to return * this = str ; .
2) Equivalent to return * this = std :: move ( str ) ; .
3) Replaces the contents with count copies of character ch .
Equivalent to clear ( ) ; resize ( n, c ) ; return * this ; .
4) Replaces the contents with copies of the characters in the range [ s , s + count ) .
If [ s , s + count ) is not a valid range , the behavior is undefined.
5) Equivalent to return assign ( s, Traits :: length ( s ) ) ; .
6,7) Replaces the contents with characters in a string view sv constructed from t .
  • If only t is provided, replaces the contents with all characters in sv .
  • If pos is also provided:
    • If count is npos , replaces the contents with all characters in sv starting from pos .
    • Otherwise, replaces the contents with the std:: min ( count, sv. size ( ) - pos ) characters in sv starting from pos .
These overloads participate in overload resolution only if all following conditions are satisfied:
6) Equivalent to std:: basic_string_view < CharT, Traits > sv = t ;
return assign ( sv. data ( ) , sv. size ( ) ) ;
.
7) Equivalent to std:: basic_string_view < CharT, Traits > sv = t ;
return assign ( sv. substr ( pos, count ) ) ;
.
8) Replaces the contents with characters in str .
  • If count is npos , replaces the contents with all characters in str starting from pos .
  • Otherwise, replaces the contents with the std:: min ( count, str. size ( ) - pos ) characters in str starting from pos .
Equivalent to return assign ( std:: basic_string_view < CharT, Traits >
( str ) . substr ( pos, count ) ) ;
.
(since C++20)
9) Equivalent to return assign ( basic_string ( first, last, get_allocator ( ) ) ) ; .

This overload participates in overload resolution only if InputIt satisfies the requirements of LegacyInputIterator .

(since C++11)
10) Equivalent to return assign ( ilist. begin ( ) , ilist. size ( ) ) ; .

Contents

Parameters

str - string to be used as source to initialize the characters with
count - size of the resulting string
ch - value to initialize characters of the string with
s - pointer to a character string to use as source to initialize the string with
t - object (convertible to std::basic_string_view ) to initialize the characters of the string with
pos - index of the first character to take
first, last - range to copy the characters from
ilist - std::initializer_list to initialize the characters of the string with

Return value

* this

Exceptions

2)
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 cause size() to exceed max_size() , throws std::length_error .

7) If pos > sv. size ( ) is true , throws std::out_of_range .
8) If pos > str. size ( ) is true , throws std::out_of_range .

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 overload ( 2 )
can be implemented by swapping
corrected to require move assignment
LWG 2250 C++98 the behavior of overload ( 8 ) was
undefined if pos > str. size ( ) is true
always throws an exception in this case
LWG 2579 C++98 overload ( 1 ) and the copy assignment
operator had different effects
they have the same effect
LWG 2946 C++17 overload ( 6 ) 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)