std::basic_string<CharT,Traits,Allocator>::operator=
(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) | |
constexpr 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))
.
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:
noexcept(std::allocator_traits<Allocator>::propagate_on_container_move_assignment::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 guarantee). |
(since C++11) |
Example
#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 valid implementation of move assignment | corrected to support allocators |
LWG 2946 | C++17 | string_view overload causes 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> ) |
© cppreference.com
Licensed under the Creative Commons Attribution-ShareAlike Unported License v3.0.
https://en.cppreference.com/w/cpp/string/basic_string/operator%3D