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std::basic_string::assign

From cppreference.com
< cpp‎ | string‎ | basic string
 
 
 
std::basic_string
 
basic_string& assign( size_type count, CharT ch );
(1)
basic_string& assign( const basic_string& str );
(2)
(3)
basic_string& assign( const basic_string& str,

                      size_type pos,

                      size_type count );
(until C++14)
basic_string& assign( const basic_string& str,

                      size_type pos,

                      size_type count = npos);
(since C++14)
(4)
basic_string& assign( basic_string&& str );
(since C++11)
(until C++17)
basic_string& assign( basic_string&& str ) noexcept(/* see below */);
(since C++17)
basic_string& assign( const CharT* s,
                      size_type count );
(5)
basic_string& assign( const CharT* s );
(6)
template< class InputIt >
basic_string& assign( InputIt first, InputIt last );
(7)
basic_string& assign( std::initializer_list<CharT> ilist );
(8) (since C++11)
template < class T >
basic_string& assign( const T& t );
(9) (since C++17)
template < class T >

basic_string& assign( const T& t,
                      size_type pos,

                      size_type count = npos);
(10) (since C++17)

Replaces the contents of the string.

1) Replaces the contents with count copies of character ch.
2) Replaces the contents with a copy of str. Equivalent to *this = str;. In particular, allocator propagation may take place. (since C++11)
3) Replaces the contents with a substring [pos, pos+count) of str. If the requested substring lasts past the end of the string, or if count == npos, the resulting substring is [pos, str.size()). If pos > str.size(), std::out_of_range is thrown.
4) Replaces the contents with those of str using move semantics. Equivalent to *this = std::move(str). In particular, allocator propagation may take place.
5) Replaces the contents with copies of the characters in the range [s, s+count). This range can contain null characters.
6) Replaces the contents with those of null-terminated character string pointed to by s. The length of the string is determined by the first null character using Traits::length(s).
7) Replaces the contents with copies of the characters in the range [first, last). This overload does not participate in overload resolution if InputIt does not satisfy LegacyInputIterator. (since C++11)
8) Replaces the contents with those of the initializer list ilist.
9) 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 sv, as if by assign(sv.data(), sv.size()). This overload only participates in overload resolution if std::is_convertible_v<const T&, std::basic_string_view<CharT, Traits>> is true and std::is_convertible_v<const T&, const CharT*> is false.
10) Implicitly converts t to a string view sv as if by std::basic_string_view<CharT, Traits> sv = t;, then replaces the contents with the characters from the subview [pos, pos+count) of sv. If the requested subview lasts past the end of sv, or if count == npos, the resulting subview is [pos, sv.size()). If pos > sv.size(), std::out_of_range is thrown. This overload only participates in overload resolution if std::is_convertible_v<const T&, std::basic_string_view<CharT, Traits>> is true and std::is_convertible_v<const T&, const CharT*> is false.

Contents

[edit] Parameters

count - size of the resulting string
pos - index of the first character to take
ch - value to initialize characters of the string with
first, last - range to copy the characters from
str - string to be used as source to initialize the characters with
s - pointer to a character string to use as source to initialize the string with
ilist - std::initializer_list to initialize the characters of the string with
t - object (convertible to std::basic_string_view) to initialize the characters of the string with
Type requirements
-
InputIt must meet the requirements of LegacyInputIterator.

[edit] Return value

*this

[edit] Complexity

1) linear in count
2) linear in size of str
3) linear in count
4) constant. If alloc is given and alloc != other.get_allocator(), then linear.
5) linear in count
6) linear in size of s
7) linear in distance between first and last
8) linear in size of ilist

[edit] Exceptions

If an exception is thrown for any reason, this function has no effect (strong exception guarantee). (since C++11)

If the operation would result in size() > max_size(), throws std::length_error.

4)
noexcept specification:  
noexcept(std::allocator_traits<Allocator>::propagate_on_container_move_assignment::value
 || std::allocator_traits<Allocator>::is_always_equal::value)
(since C++17)

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

[edit] Example

#include <iostream>
#include <iterator>
#include <string>
 
int main()
{
  std::string s;
  // assign(size_type count, CharT ch)
  s.assign(4, '=');
  std::cout << s << '\n'; // "===="
 
  std::string const c("Exemplary");
  // assign(basic_string const& str)
  s.assign(c);
  std::cout << c << "==" << s <<'\n'; // "Exemplary == Exemplary"
 
  // assign(basic_string const& str, size_type pos, size_type count)
  s.assign(c, 0, c.length()-1);
  std::cout << s << '\n'; // "Exemplar";
 
  // assign(basic_string&& str)
  s.assign(std::string("C++ by ") + "example");
  std::cout << s << '\n'; // "C++ by example"
 
  // assign(charT const* s, size_type count)
  s.assign("C-style string", 7);
  std::cout << s << '\n'; // "C-style"
 
  // assign(charT const* s)
  s.assign("C-style\0string");
  std::cout << s << '\n'; // "C-style"
 
  char mutable_c_str[] = "C-style string";
  // assign(InputIt first, InputIt last)
  s.assign(std::begin(mutable_c_str), std::end(mutable_c_str)-1);
  std::cout << s << '\n'; // "C-style string"
 
  // assign(std::initializer_list<charT> ilist)
  s.assign({ 'C', '-', 's', 't', 'y', 'l', 'e' });
  std::cout << s << '\n'; // "C-style"
}

Output:

====
Exemplary==Exemplary
Exemplar
C++ by example
C-style
C-style
C-style string
C-style

[edit] See also

constructs a basic_string
(public member function) [edit]
assigns values to the string
(public member function) [edit]