Difference between revisions of "cpp/string/basic string/operator="

Latest revision as of 00:04, 25 May 2024

basic_string& operator=( const basic_string& str );	(1)	(constexpr since C++20)
basic_string& operator=( basic_string&& str ) noexcept(/* see below */);	(2)	(since C++11) (constexpr since C++20)
basic_string& operator=( const CharT* s );	(3)	(constexpr since C++20)
basic_string& operator=( CharT ch );	(4)	(constexpr since C++20)
basic_string& operator=( std::initializer_list<CharT> ilist );	(5)	(since C++11) (constexpr since C++20)
template<class StringViewLike> basic_string& operator=( const StringViewLike& t );	(6)	(since C++17) (constexpr 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 SequenceContainer's move assignment semantics.

Unlike other sequence container move assignments, references, pointers, and iterators to elements of 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.

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

[edit] Return value

*this

[edit] 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.

[edit] 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 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).

[edit] 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"
"#"

[edit] 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	the move assignment operator did not follow SequenceContainer's semantic requirement	follows
LWG 2946	C++17	overload (6) caused ambiguity in some cases	avoided by making it a template

[edit] See also

(constructor)	constructs a `basic_string` (public member function) [edit]
assign	assign characters to a string (public member function) [edit]
operator=	assigns a view (public member function of `std::basic_string_view<CharT,Traits>`) [edit]

@@ Line 1: / Line 1: @@
-{{cpp/string/basic_string/title | operator{{=}}}}
+{{cpp/string/basic_string/title|operator{{=}}}}
-{{cpp/string/basic_string/sidebar}}
+{{cpp/string/basic_string/navbar}}
-{{ddcl list begin}}
+{{dcl begin}}
-{{ddcl list item | num=1 | 1=
+{{dcla|anchor=no|num=1|constexpr=c++20|1=
 basic_string& operator=( const basic_string& str );
 }}
-{{ddcl list item | num=2 | notes={{mark c++11 version}} | 1=
+{{dcla|anchor=no|num=2|since=c++11|constexpr=c++20|1=
-basic_string& operator=( basic_string&& str );
+basic_string& operator=( basic_string&& str )
+    noexcept(/* see below */);
 }}
-{{ddcl list item | num=3 | 1=
+{{dcla|anchor=no|num=3|constexpr=c++20|1=
 basic_string& operator=( const CharT* s );
 }}
-{{ddcl list item | num=4 | 1=
+{{dcla|anchor=no|num=4|constexpr=c++20|1=
 basic_string& operator=( CharT ch );
 }}
-{{ddcl list item | num=5 | notes={{mark c++11 feature}} | 1=
+{{dcla|anchor=no|num=5|since=c++11|constexpr=c++20|1=
 basic_string& operator=( std::initializer_list<CharT> ilist );
 }}
-{{ddcl list end}}
+{{dcla|num=6|since=c++17|constexpr=c++20|1=
+template<class StringViewLike>
+basic_string& operator=( const StringViewLike& t );
+}}
+{{dcl|num=7|since=c++23|1=
+basic_string& operator=( std::nullptr_t ) = delete;
+}}
+{{dcl end}}
 Replaces the contents of the string.
-) Replaces the contents with a copy of {{tt|str}}
+@1@ Replaces the contents with a copy of {{c|str}}. If {{c|*this}} and {{c|str}} are the same object, this function has no effect.
-) Replaces the contents with those of {{tt|str}} using move semantics. {{tt|str}} is in undefined state after the operation.
+@2@ Replaces the contents with those of {{c|str}} using {{named req|SequenceContainer}}'s move assignment semantics.
+@@ Unlike other sequence container move assignments, references, pointers, and iterators to elements of {{c|str}} may be invalidated.
-) Replaces the contents with those of null-terminated character string pointed to by {{tt|s}}.
+@3@ Replaces the contents with those of null-terminated character string pointed to by {{c|s}} as if by {{c|assign(s, Traits::length(s))}}.
+@4@ Replaces the contents with character {{c|ch}} as if by {{c|assign(std::addressof(ch), 1)}}.
-) Replaces the contents with character {{tt|ch}}
+@5@ Replaces the contents with those of the initializer list {{c|ilist}} as if by {{c|assign(ilist.begin(), ilist.size())}}.
+@6@ {{cpp/string/sv hack|replaces the contents with those of the {{c|sv}} as if by {{c|assign(sv)}}}}
-) Replaces the contents with those of the initializer list {{tt|ilist}}.
+@7@ {{tt|std::basic_string}} cannot be assigned from {{c|nullptr}}.
 ===Parameters===
-{{param list begin}}
+{{par begin}}
-{{param list item | ch | value to initialize characters of the string with}}
+{{par|ch|value to initialize characters of the string with}}
-{{param list item | str | string to be used as source to initialize the string with}}
+{{par|str|string to be used as source to initialize the string with}}
-{{param list item | s | pointer to a null-termanated character string <br>to use as source to initialize the string with}}
+{{par|s|pointer to a null-terminated character string to use as source to initialize the string with}}
-{{param list item | init | initializer list to initialize the string with}}
+{{par|ilist|{{lc|std::initializer_list}} to initialize the string with}}
-{{param list end}}
+{{par|t|object convertible to {{lc|std::basic_string_view}} to initialize the string with}}
+{{par end}}
 ===Return value===
-{{cpp|*this}}
+{{c|*this}}
 ===Complexity===
+@1@ Linear in size of {{c|str}}.
-) linear in size of {{tt|str}}
+@2@ Linear in the size of {{c|*this}} (formally, each {{tt|CharT}} has to be destroyed). If allocators do not compare equal and do not propagate, then also linear in the size of {{c|str}} (copy must be made).
-) constant. If {{tt|alloc}} is given and {{cpp|alloc !{{=}} other.get_allocator()}}, then linear.
+@3@ Linear in size of {{c|s}}.
-) linear in size of {{tt|s}}
+@4@ Constant.
-) constant.
+@5@ Linear in size of {{c|ilist}}.
-) linear in size of {{tt|init}}
+@6@ Linear in size of {{c|t}}.
+===Exceptions===
+@2@ {{noexcept|std::allocator_traits<Allocator>::
+             propagate_on_container_move_assignment::value {{!!}}
+         std::allocator_traits<Allocator>::is_always_equal::value}}
+{{cpp/string/basic_string/length error note}}
+{{cpp/strong exception safety guarantee}}
 ===Example===
-{{example cpp
+{{example|
- | code=
+|code=
- | output=
+#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)
+}
+|p=true
+|output=
+"alpha" "alpha"
+"alpha" ""
+"beta"
+"!"
+"gamma"
+"#"
 }}
+===Defect reports===
+{{dr list begin}}
+{{dr list item|wg=lwg|dr=847|std=C++98|before=there was no exception safety guarantee|after=added strong exception safety guarantee}}
+{{dr list item|wg=lwg|dr=2063|std=C++11|before=the move assignment operator did not follow<br>{{named req|SequenceContainer}}'s semantic requirement|after=follows}}
+{{dr list item|wg=lwg|dr=2946|std=C++17|before=overload {{vl|6}} caused ambiguity in some cases|after=avoided by making it a template}}
+{{dr list end}}
 ===See also===
+{{dsc begin}}
+{{dsc inc|cpp/string/basic_string/dsc constructor}}
+{{dsc inc|cpp/string/basic_string/dsc assign}}
+{{dsc inc|cpp/string/basic_string_view/dsc operator{{=}}}}
+{{dsc end}}
-{{dcl list begin}}
+{{langlinks|de|es|fr|it|ja|pt|ru|zh}}
-{{dcl list template | cpp/string/basic_string/dcl list constructor}}
-{{dcl list template | cpp/string/basic_string/dcl list assign}}
-{{dcl list end}}

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Difference between revisions of "cpp/string/basic string/operator="

Latest revision as of 00:04, 25 May 2024

Contents

[edit] Parameters