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 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

#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

See also

appends characters to the end
(public member function)