std::basic_string<CharT,Traits,Allocator>::operator+=

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(1)
basic_string& operator+=( const basic_string& str );
(until C++20)
constexpr basic_string& operator+=( const basic_string& str );
(since C++20)
(2)
basic_string& operator+=( CharT ch );
(until C++20)
constexpr basic_string& operator+=( CharT ch );
(since C++20)
(3)
basic_string& operator+=( const CharT* s );
(until C++20)
constexpr basic_string& operator+=( const CharT* s );
(since C++20)
(4)
basic_string& operator+=( std::initializer_list<CharT> ilist );
(since C++11)
(until C++20)
constexpr basic_string& operator+=( std::initializer_list<CharT> ilist );
(since C++20)
(5)
template < class StringViewLike >
basic_string& operator+=( const StringViewLike& t );
(since C++17)
(until C++20)
template < class StringViewLike >
constexpr basic_string& operator+=( const StringViewLike& t );
(since C++20)

Appends additional characters to the string.

1) Appends string str
2) Appends character ch
3) Appends the null-terminated character string pointed to by s.
4) Appends characters in the initializer list ilist.
5) Implicitly converts t to a string view sv as if by std::basic_string_view<CharT, Traits> sv = t;, then appends characters in the string view sv as if by append(sv). 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.

Parameters

str - string to append
ch - character value to append
s - pointer to a null-terminated character string to append
ilist - std::initializer_list with the characters to append
t - object (convertible to std::basic_string_view) with the characters to append

Return value

*this

Complexity

There are no standard complexity guarantees, typical implementations behave similar to std::vector::insert.

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.

Notes

Overload (2) can accept any types that are implicitly convertible to CharT. For std::string, where CharT is char, the set of acceptable types includes all arithmetic types. This may have unintended effects.

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 2946 C++17 string_view overload causes ambiguity in some cases avoided by making it a template

Example

#include <iostream>
#include <iomanip>
#include <string>
 
int main()
{
   std::string str;
   str.reserve(50); //reserves sufficient storage space to avoid memory reallocation
   std::cout << std::quoted(str) << '\n'; //empty string
 
   str += "This";
   std::cout << std::quoted(str) << '\n';
 
   str += std::string(" is ");
   std::cout << std::quoted(str) << '\n';
 
   str += 'a';
   std::cout << std::quoted(str) << '\n';
 
   str += {' ','s','t','r','i','n','g','.'};
   std::cout << std::quoted(str) << '\n';
 
   str += 76.85; // equivalent to str += static_cast<char>(76.85), might not be the intent
   std::cout << std::quoted(str) << '\n';
}

Output:

""
"This"
"This is "
"This is a"
"This is a string."
"This is a string.L"

See also

appends characters to the end
(public member function)