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std::basic_string<CharT,Traits,Allocator>::assign

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

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 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)

If an exception is thrown for any reason, this function has no effect (strong exception safety guarantee).

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(const basic_string& str)
    s.assign(c);
    std::cout << c << " == " << s << '\n'; // "Exemplary == Exemplary"
 
    // assign(const basic_string& 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(const CharT* s, size_type count)
    s.assign("C-style string", 7);
    std::cout << s << '\n'; // "C-style"
 
    // assign(const CharT* 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 847 C++98 there was no exception safety guarantee added strong exception safety guarantee
LWG 2063 C++11 non-normative note stated that swap is
a valid implementation of move-assign 		corrected to require move assignment
LWG 2579 C++11 assign(const basic_string&)
did not propagate allocators 		made to propagate allocators if needed
LWG 2946 C++17 overload (9) caused ambiguity in some cases avoided by making it a template

See also

(C++23)
 assign a range of characters to a string
(public member function)
constructs a basic_string
(public member function)
assigns values to the string
(public member function)

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