< cpp‎ | string‎ | basic string
basic_string& operator+=( const basic_string& str );
basic_string& operator+=( CharT ch );
basic_string& operator+=( const CharT* s );
basic_string& operator+=( std::initializer_list<CharT> ilist );
(4) (since C++11)
template < class T >
basic_string& operator+=( const T& t );
(5) (since C++17)

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 only participates in overload resolution if std::is_convertible_v<const T&, std::basic_string_view<CharT, Traits>> is true and std::is_convertible_v<const T&, const CharT*> is false.


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



1) linear in size of str
2) constant
3) linear in size of s
4) linear in size of ilist


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.


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


#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';


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

See also

appends characters to the end
(public member function)