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

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< cpp‎ | string‎ | basic string
 
 
 
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Helper classes
 
(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.

Contents

[edit] 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.

[edit] Return value

*this

[edit] 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

[edit] 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)

[edit] 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(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

[edit] 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 2579 C++11 assign(const basic_string&) doesn't propagate allocators made to propagate allocators if needed
LWG 2946 C++17 string_view overload causes ambiguity in some cases avoided by making it a template

[edit] See also

constructs a basic_string
(public member function) [edit]
assigns values to the string
(public member function) [edit]