std::basic_string<CharT,Traits,Allocator>::operator=

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(1)
basic_string& operator=( const basic_string& str );
 (until C++20)
constexpr basic_string& operator=( const basic_string& str );
 (since C++20)
(2)
basic_string& operator=( basic_string&& str );
 (since C++11)
(until C++17)
basic_string& operator=( basic_string&& str ) noexcept(/* see below */);
 (since C++17)
(until C++20)
constexpr basic_string&
    operator=( basic_string&& str ) noexcept(/* see below */);
 (since C++20)
(3)
basic_string& operator=( const CharT* s );
 (until C++20)
constexpr basic_string& operator=( const CharT* s );
 (since C++20)
(4)
basic_string& operator=( CharT ch );
 (until C++20)
constexpr basic_string& operator=( CharT ch );
 (since C++20)
(5)
basic_string& operator=( std::initializer_list<CharT> ilist );
 (since C++11)
(until C++20)
constexpr basic_string& operator=( std::initializer_list<CharT> ilist );
 (since C++20)
(6)
template<class StringViewLike>
basic_string& operator=( const StringViewLike& t );
 (since C++17)
(until C++20)
template<class StringViewLike>
constexpr basic_string& operator=( const StringViewLike& t );
 (since C++20)
basic_string& operator=( std::nullptr_t ) = delete;
 (7) (since C++23)

Replaces the contents of the string.

1) Replaces the contents with a copy of str. If *this and str are the same object, this function has no effect.
2) Replaces the contents with those of str using move semantics. str is in a valid but unspecified state afterwards. If std::allocator_traits<Allocator>::
    propagate_on_container_move_assignment::value is true, the allocator of *this is replaced by a copy of that of str. If it is false and the allocators of *this and str do not compare equal, *this cannot take ownership of the memory owned by str and must assign each character individually, allocating additional memory using its own allocator as needed. Unlike other container move assignments, references, pointers, and iterators to str may be invalidated.
3) Replaces the contents with those of null-terminated character string pointed to by s as if by assign(s, Traits::length(s)).
4) Replaces the contents with character ch as if by assign(std::addressof(ch), 1).
5) Replaces the contents with those of the initializer list ilist as if by assign(ilist.begin(), ilist.size()).
6) 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 the sv as if by assign(sv).
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.
7) std::basic_string cannot be assigned from nullptr.

Parameters

ch - value to initialize characters of the string with
str - string to be used as source to initialize the string with
s - pointer to a null-terminated character string to use as source to initialize the string with
ilist - std::initializer_list to initialize the string with
t - object convertible to std::basic_string_view to initialize the string with

Return value

*this

Complexity

1) Linear in size of str.
2) Linear in the size of *this (formally, each CharT has to be destroyed). If allocators do not compare equal and do not propagate, then also linear in the size of str (copy must be made).
3) Linear in size of s.
4) Constant.
5) Linear in size of ilist.
6) Linear in size of t.

Exceptions

2)
noexcept specification:  
noexcept(std::allocator_traits<Allocator>::

             propagate_on_container_move_assignment::value ||

         std::allocator_traits<Allocator>::is_always_equal::value)
 (since C++17)

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 <iomanip>
#include <iostream>
#include <string>
 
int main()
{
    std::string str1;
    std::string str2{"alpha"};
 
    // (1) operator=(const basic_string&);
    str1 = str2;
    std::cout << std::quoted(str1) << ' '   // "alpha"
              << std::quoted(str2) << '\n'; // "alpha"
 
    // (2) operator=(basic_string&&);
    str1 = std::move(str2);
    std::cout << std::quoted(str1) << ' '   // "alpha"
              << std::quoted(str2) << '\n'; // "" or "alpha" (unspecified)
 
    // (3) operator=(const CharT*);
    str1 = "beta";
    std::cout << std::quoted(str1) << '\n'; // "beta"
 
    // (4) operator=(CharT);
    str1 = '!'; 
    std::cout << std::quoted(str1) << '\n'; // "!"
 
    // (5) operator=(std::initializer_list<CharT>);
    str1 = {'g', 'a', 'm', 'm', 'a'};
    std::cout << std::quoted(str1) << '\n'; // "gamma"
 
    // (6) operator=(const T&);
    str1 = 35U; // equivalent to str1 = static_cast<char>(35U);
    std::cout << std::quoted(str1) << '\n'; // "#" (ASCII = 35)
}

Possible output: 

"alpha" "alpha"
"alpha" ""
"beta"
"!"
"gamma"
"#"

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 a non-normative note stated that swap is a
valid implementation of move assignment 		corrected to support allocators
LWG 2946 C++17 overload (6) caused ambiguity in some cases avoided by making it a template

See also

 constructs a basic_string
(public member function)
assign characters to a string
(public member function)
(C++17)
 assigns a view
(public member function of std::basic_string_view<CharT,Traits>)
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