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

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< cpp‎ | string‎ | 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.

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

### 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

### 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

### See also

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

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