NAME¶

std::basic_string::operator= - std::basic_string::operator=

Synopsis¶

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

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) 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.

Exceptions¶

2) noexcept specification: (since noexcept(std::allocator_traits<Allocator>::propagate_on_container_move_assignment::value C++17) || 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 (since C++11)
(strong exception guarantee).

Example¶

// Run this code

#include <string>
#include <iostream>
#include <iomanip>

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 2063 C++11 non-normative note stated that swap is a corrected to support
valid implementation of move assignment allocators
LWG 2946 C++17 string_view overload causes ambiguity in avoided by making it a
some cases template

See also¶

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