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

basic_string& operator+=( const basic_string& str );	(1)	(constexpr since C++20)
basic_string& operator+=( CharT ch );	(2)	(constexpr since C++20)
basic_string& operator+=( const CharT* s );	(3)	(constexpr since C++20)
basic_string& operator+=( std::initializer_list<CharT> ilist );	(4)	(since C++11) (constexpr since C++20)
template< class StringViewLike > basic_string& operator+=( const StringViewLike& t );	(5)	(since C++17) (constexpr 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
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Language
Standard library
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Feature test macros (C++20)
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Ranges library (C++20)
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Null-terminated strings
Byte strings
Multibyte strings
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Classes
basic_string
basic_string_view (C++17)
char_traits