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std::basic_string::operator+=

basic_string& operator+=( const basic_string& str );	(1)
basic_string& operator+=( CharT ch );	(2)
basic_string& operator+=( const CharT* s );	(3)
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.

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

1) linear in size of str

2) constant

3) linear in size of s

4) linear in size of ilist

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"