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

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)
basic_string& assign( const basic_string& str, size_type pos, size_type count );	(8)	(until C++14)
basic_string& assign( const basic_string& str, size_type pos, size_type count = npos);	(8)		(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)

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

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

#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