NAME¶

std::basic_string::assign - std::basic_string::assign

Synopsis¶

basic_string& assign( (until size_type count, CharT ch ); C++20) constexpr basic_string& (since assign( size_type count, C++20) CharT ch ); basic_string& assign( const (until basic_string& str ); C++20) constexpr basic_string& (since assign( const basic_string& C++20) str ); basic_string& assign( const basic_string& str, (until size_type pos, size_type C++14) count ); basic_string& assign( const (since basic_string& str, C++14) size_type pos, size_type (until count = npos); C++20) constexpr basic_string& assign( const basic_string& (since str, C++20) size_type pos, size_type count = npos);
(since basic_string& assign( C++11) basic_string&& str ); (until
C++17) basic_string& assign( (since basic_string&& str ) C++17) noexcept(/* see below */); (until
C++20) constexpr basic_string& (since assign( basic_string&& str ) C++20) noexcept(/* see below */); basic_string& assign( const (until CharT* s, size_type count ); C++20) constexpr basic_string& (since assign( const CharT* s, C++20) size_type count ); basic_string& assign( const (until CharT* s ); C++20) constexpr basic_string& (since assign( const CharT* s ); (1) C++20) template< class InputIt > basic_string& assign( (2) (until InputIt first, InputIt last C++20) ); template< class InputIt > (3) constexpr basic_string& (since assign( InputIt first, C++20) InputIt last ); basic_string& assign( (since std::initializer_list<CharT> C++11) ilist ); (4) (until
C++20) constexpr basic_string& assign( (since std::initializer_list<CharT> C++20) ilist ); template < class (5) (since StringViewLike > C++17) basic_string& assign( const (6) (until StringViewLike& t ); C++20) template < class (7) StringViewLike > (since constexpr basic_string& C++20) assign( const StringViewLike& t ); (8) template < class StringViewLike >
(since basic_string& assign( const (9) C++17) StringViewLike& t, (until
C++20) size_type pos, size_type count = npos); template < class (10) StringViewLike >

constexpr basic_string& (since assign( const C++20) StringViewLike& t,

size_type pos, size_type count = npos);

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 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.
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 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.

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.

Return value¶

*this

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

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

Example¶

// Run this code

#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¶

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 corrected to require move
a valid implementation of move-assign assignment
LWG 2579 C++11 assign(const basic_string&) doesn't made to propagate
propagate allocators allocators if needed
LWG 2946 C++17 string_view overload causes ambiguity avoided by making it a
in some cases template

See also¶

constructor constructs a basic_string
(public member function)
operator= assigns values to the string
(public member function)