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

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< cpp‎ | string‎ | basic string
(reorder)
m (→‎Synopsis: "notes=" ⇒ "constexpr=")
 
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−
{{cpp/string/basic_string/title | operator{{=}}}}
+
{{cpp/string/basic_string/title|operator{{=}}}}
 
{{cpp/string/basic_string/navbar}}
 
{{cpp/string/basic_string/navbar}}
 
{{dcl begin}}
 
{{dcl begin}}
−
{{dcl | num=1 | 1=
+
{{dcla|anchor=no|num=1|constexpr=c++20|1=
 
basic_string& operator=( const basic_string& str );
 
basic_string& operator=( const basic_string& str );
 
}}
 
}}
−
{{dcl | num=2 | since=c++11 | 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 */);
 
}}
 
}}
−
{{dcl | num=3 | 1=
+
{{dcla|anchor=no|num=3|constexpr=c++20|1=
 
basic_string& operator=( const CharT* s );
 
basic_string& operator=( const CharT* s );
 
}}
 
}}
−
{{dcl | num=4 | 1=
+
{{dcla|anchor=no|num=4|constexpr=c++20|1=
 
basic_string& operator=( CharT ch );
 
basic_string& operator=( CharT ch );
 
}}
 
}}
−
{{dcl | num=5 | since=c++11 | 1=
+
{{dcla|anchor=no|num=5|since=c++11|constexpr=c++20|1=
 
basic_string& operator=( std::initializer_list<CharT> ilist );
 
basic_string& operator=( std::initializer_list<CharT> ilist );
 
}}
 
}}
−
{{dcl | num=6 | since=c++17 | 1=
+
{{dcla|num=6|since=c++17|constexpr=c++20|1=
−
template<class T>
+
template<class StringViewLike>
−
basic_string& operator=( const T& t );
+
basic_string& operator=( const StringViewLike& t );
 +
}}
 +
{{dcl|num=7|since=c++23|1=
 +
basic_string& operator=( std::nullptr_t ) = delete;
 
}}
 
}}
 
{{dcl end}}
 
{{dcl end}}
Line 25: Line 29:
 
Replaces the contents of the string.
 
Replaces the contents of the string.
  
−
@1@ Replaces the contents with a copy of {{tt|str}}. If {{tt|*this}} and {{tt|str}} are the same object, this function has no effect.
+
@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.
  
−
{{rev begin}}
+
@2@ Replaces the contents with those of {{c|str}} using {{named req|SequenceContainer}}'s move assignment semantics.
−
{{rev|until=c++17|
+
@@ Unlike other sequence container move assignments, references, pointers, and iterators to elements of {{c|str}} may be invalidated.
−
@2@ Replaces the contents with those of {{tt|str}} using move semantics. Leaves {{tt|str}} in valid, but unspecified state. If {{tt|*this}} and {{tt|str}} are the same object, the function has no effect.
+
@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)}}.
−
{{rev|since=c++17|
+
@5@ Replaces the contents with those of the initializer list {{c|ilist}} as if by {{c|assign(ilist.begin(), ilist.size())}}.
−
@2@ Replaces the contents with those of {{tt|str}} using move semantics. {{tt|str}} is in a valid but unspecified state afterwards. If {{lc|std::allocator_traits<Allocator>::propagate_on_container_move_assignment()}} is {{c|true}}, the target allocator is replaced by a copy of the source allocator. If it is {{c|false}} and the source and the target allocators do not compare equal, the target cannot take ownership of the source memory 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 {{tt|str}} may be invalidated.}}
+
@6@ {{cpp/string/sv hack|replaces the contents with those of the {{c|sv}} as if by {{c|assign(sv)}}}}
−
{{rev end}}
+
@7@ {{tt|std::basic_string}} cannot be assigned from {{c|nullptr}}.
−
@3@ Replaces the contents with those of null-terminated character string pointed to by {{tt|s}} as if by {{c|*this {{=}} basic_string(s)}}, which involves a call to {{c|Traits::length(s)}}.
+
−
@4@ Replaces the contents with character {{tt|ch}} as if by {{c|*this {{=}} basic_string(1,c)}}
+
−
@5@ Replaces the contents with those of the initializer list {{tt|ilist}} as if by {{c|*this {{=}} basic_string(ilist)}}
+
−
@6@ {{cpp/string/sv hack|replaces the contents with those of the {{tt|sv}} as if by {{c|assign(sv)}}}}
+
  
 
===Parameters===
 
===Parameters===
 
{{par begin}}
 
{{par begin}}
−
{{par | ch | value to initialize characters of the string with}}
+
{{par|ch|value to initialize characters of the string with}}
−
{{par | 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}}
−
{{par | s | pointer to a null-terminated 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}}
−
{{par | init | {{lc|std::initializer_list}} to initialize the string with}}
+
{{par|ilist|{{lc|std::initializer_list}} to initialize the string with}}
−
{{par | t | object convertible to {{lc|std::basic_string_view}} to initialize the string with}}
+
{{par|t|object convertible to {{lc|std::basic_string_view}} to initialize the string with}}
 
{{par end}}  
 
{{par end}}  
  
Line 52: Line 52:
  
 
===Complexity===
 
===Complexity===
 +
@1@ Linear in size of {{c|str}}.
  
−
1) 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).
  
−
2) linear in the size of {{tt|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 {{tt|str}} (copy must be made)
+
@3@ Linear in size of {{c|s}}.
  
−
3) linear in size of {{tt|s}}
+
@4@ Constant.
  
−
4) constant.
+
@5@ Linear in size of {{c|ilist}}.
  
−
5) linear in size of {{tt|init}}
+
@6@ Linear in size of {{c|t}}.
  
 
===Exceptions===
 
===Exceptions===
−
{{rev begin}}
+
@2@ {{noexcept|std::allocator_traits<Allocator>::
−
{{rev|since=c++11|until=c++17|
+
            propagate_on_container_move_assignment::value {{!!}}
−
@2@ {{unreviewed noexcept}}
+
        std::allocator_traits<Allocator>::is_always_equal::value}}
 +
 
 +
{{cpp/string/basic_string/length error note}}
 +
 
 +
{{cpp/strong exception safety guarantee}}
 +
 
 +
===Example===
 +
{{example|
 +
|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)
 +
}
 +
|p=true
 +
|output=
 +
"alpha" "alpha"
 +
"alpha" ""
 +
"beta"
 +
"!"
 +
"gamma"
 +
"#"
 
}}
 
}}
−
{{rev|since=c++17|
 
−
@2@ {{unreviewed noexcept|std::allocator_traits<Allocator>::propagate_on_container_move_assignment::value
 
−
{{!!}} std::allocator_traits<Allocator>::is_always_equal::value}}
 
−
}}
 
−
{{rev end}}
 
−
 
−
If the operation would result in {{tt|size() > max_size()}}, throws {{lc|std::length_error}}.
 
−
{{rev begin}}
 
−
{{rev|since=c++11|
 
−
If an exception is thrown for any reason, this function has no effect (strong exception guarantee).}}
 
−
{{rev end}}
 
  
 
===Defect reports===
 
===Defect reports===
 
{{dr list begin}}
 
{{dr list begin}}
−
{{dr list item|wg=lwg|dr=2063|std=C++11|before=non-normative note stated that swap is a valid implementation of move assignment|after=corrected to support allocators}}
+
{{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=2946|std=C++17|before={{tt|string_view}} overload causes ambiguity in some cases|after=avoided by making it a template}}
+
{{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}}
 
{{dr list end}}
−
 
−
===Example===
 
−
{{example
 
−
| code=
 
−
| output=
 
−
}}
 
  
 
===See also===
 
===See also===
−
 
 
{{dsc begin}}
 
{{dsc begin}}
−
{{dsc inc | cpp/string/basic_string/dsc constructor}}
+
{{dsc inc|cpp/string/basic_string/dsc constructor}}
−
{{dsc inc | cpp/string/basic_string/dsc assign}}
+
{{dsc inc|cpp/string/basic_string/dsc assign}}
 +
{{dsc inc|cpp/string/basic_string_view/dsc operator{{=}}}}
 
{{dsc end}}
 
{{dsc end}}
  
 
{{langlinks|de|es|fr|it|ja|pt|ru|zh}}
 
{{langlinks|de|es|fr|it|ja|pt|ru|zh}}

Latest revision as of 00:04, 25 May 2024

 
C++
 
Strings library
 
std::basic_string
Member functions
basic_string::operator=
(C++23)
Element access
(DR*)
(DR*)
(C++17)
Iterators
(C++11)
(C++11)
(C++11)
(C++11)
Capacity
(DR*)
Modifiers
(C++23)
(DR*)
(C++23)
(C++23)
(C++23)  
Search
Operations
(C++20)
(C++20)
(C++23)
Constants
Non-member functions
(C++20)(C++20)
I/O
Comparison
(until C++20)(until C++20)(until C++20)(until C++20)(until C++20)(C++20)
Numeric conversions
(C++11)(C++11)(C++11)
(C++11)(C++11)
(C++11)(C++11)(C++11)
(C++11)
(C++11)
Literals
(C++14)
Helper classes
(C++11)
Deduction guides (C++17)

 
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.

Contents

[edit] 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

[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

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