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

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

    Replaces the contents of the string.

    1) Equivalent to return *this = str;.
    2) Equivalent to return *this = std::move(str);.
    3) Replaces the contents with count copies of character ch.
    Equivalent to clear(); resize(n, c); return *this;.
    4) Replaces the contents with copies of the characters in the range [s, s + count).
    If [s, s + count) is not a valid range, the behavior is undefined.
    5) Equivalent to return assign(s, Traits::length(s));.
    6,7) Replaces the contents with characters in a string view sv constructed from t.
    • If only t is provided, replaces the contents with all characters in sv.
    • If pos is also provided:
      • If count is npos, replaces the contents with all characters in sv starting from pos.
      • Otherwise, replaces the contents with the std::min(count, sv.size() - pos) characters in sv starting from pos.
    These overloads participate in overload resolution only if all following conditions are satisfied:
    • std::is_convertible_v<const SV&, std::basic_string_view<CharT, Traits>> is true.
    • std::is_convertible_v<const SV&, const CharT*> is false.
    6) Equivalent to std::basic_string_view<CharT, Traits> sv = t;
    return assign(sv.data(), sv.size());    .
    7) Equivalent to std::basic_string_view<CharT, Traits> sv = t;
    return assign(sv.substr(pos, count));    .
    8) Replaces the contents with characters in str.
    • If count is npos, replaces the contents with all characters in str starting from pos.
    • Otherwise, replaces the contents with the std::min(count, str.size() - pos) characters in str starting from pos.
    Equivalent to return assign(std::basic_string_view<CharT, Traits>
    (str).substr(pos, count));    .
    		(since C++20)
    9) Equivalent to return assign(basic_string(first, last, get_allocator()));.

    This overload participates in overload resolution only if InputIt satisfies the requirements of LegacyInputIterator.

    		(since C++11)
    10) Equivalent to return assign(ilist.begin(), ilist.size());.

    Parameters

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

    Return value

    *this

    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 cause size() to exceed max_size(), throws std::length_error.

    7) If pos > sv.size() is true, throws std::out_of_range.
    8) If pos > str.size() is true, throws std::out_of_range.

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

    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(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 overload (2)
    can be implemented by swapping     		corrected to require move assignment
    LWG 2250 C++98 the behavior of overload (8) was
    undefined if pos > str.size() is true     		always throws an exception in this case
    LWG 2579 C++98 overload (1) and the copy assignment
    operator had different effects     		they have the same effect
    LWG 2946 C++17 overload (6) 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) [edit]
    		constructs a basic_string
    (public member function) [edit]
    		assigns values to the string
    (public member function) [edit]