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

	(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 the operation would result in size() > max_size(), throws std::length_error.

If an exception is thrown for any reason, this function has no effect (strong exception safety guarantee).

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.

Example

Run this code

#include <iomanip>
#include <iostream>
#include <string>
 
int main()
{
    std::string str;
 
    // reserve sufficient storage space to avoid memory reallocation
    str.reserve(50);
 
    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 += 69.96; // Equivalent to str += static_cast<char>(69.96);
                  // 'E' (ASCII code 69) is appended by overload (2),
                  // which might not be the intent.
 
    // To add a numeric value, consider std::to_string():
    str += std::to_string(1729);
 
    std::cout << std::quoted(str) << '\n';
}

Output:

""
"This"
"This is "
"This is a"
"This is a string."
"This is a string.E1729"

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 2946	C++17	overload (5) caused ambiguity in some cases	avoided by making it a template

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)
Metaprogramming library (C++11)
Diagnostics library
General utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Localizations library
Input/output library
Filesystem library (C++17)
Regular expressions library (C++11)
Concurrency support library (C++11)
Technical specifications
Symbols index
External libraries

Null-terminated strings
Byte strings
Multibyte strings
Wide strings
Classes
basic_string
basic_string_view (C++17)
char_traits