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

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Diagnostics library

Memory management library

Metaprogramming library (C++11)

General utilities library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Strings library

Text processing library

Numerics library

Date and time library

Input/output library

Filesystem library (C++17)

Concurrency support library (C++11)

Execution support library (C++26)

Technical specifications

Symbols index

External libraries

 

Strings library

Classes

basic_string

basic_string_view (C++17)

char_traits

 

std::basic_string

Member functions basic_string::basic_string basic_string::~basic_string basic_string::operator= basic_string::assign basic_string::assign_range (C++23)   basic_string::get_allocator Element access basic_string::at basic_string::operator[] basic_string::front (DR*) basic_string::back (DR*) basic_string::data basic_string::c_str basic_string::operator  basic_string_view (C++17) Iterators basic_string::beginbasic_string::cbegin (C++11) basic_string::endbasic_string::cend (C++11) basic_string::rbeginbasic_string::crbegin (C++11) basic_string::rendbasic_string::crend (C++11) Search basic_string::find basic_string::rfind basic_string::find_first_of basic_string::find_first_not_of basic_string::find_last_of basic_string::find_last_not_of

Modifiers basic_string::clear basic_string::insert basic_string::insert_range (C++23) basic_string::erase basic_string::push_back basic_string::pop_back (DR*) basic_string::append basic_string::append_range (C++23) basic_string::operator+= basic_string::replace basic_string::replace_with_range (C++23) basic_string::copy basic_string::resize basic_string::resize_and_overwrite (C++23)  basic_string::swap Capacity basic_string::empty basic_string::sizebasic_string::length basic_string::max_size basic_string::reserve basic_string::capacity basic_string::shrink_to_fit (DR*) Operations basic_string::compare basic_string::starts_with (C++20) basic_string::ends_with (C++20) basic_string::contains (C++23) basic_string::substr

Constants basic_string::npos Non-member functions operator+ swap(std::basic_string) erase(std::basic_string)erase_if(std::basic_string) (C++20)(C++20) I/O operator<<operator>> getline Comparison operator==operator!=operator<operator>operator<=operator>=operator<=> (until C++20)(until C++20)(until C++20)(until C++20)(until C++20)(C++20) Numeric conversions stoistolstoll (C++11)(C++11)(C++11) stoulstoull (C++11)(C++11)  stofstodstold (C++11)(C++11)(C++11) to_string (C++11) to_wstring (C++11) Literals operator""s (C++14) Helper classes hash<std::basic_string> (C++11) Deduction guides (C++17)

 

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)

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

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