NAME

std::basic_string::operator+= - std::basic_string::operator+=

Synopsis

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> (4) (since C++11)
ilist ); (constexpr since C++20)
template< class StringViewLike > (5) (since C++17)
basic_string& operator+=( const StringViewLike& t ); (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 added strong exception safety
guarantee guarantee
LWG 2946 C++17 overload (5) caused ambiguity in avoided by making it a
some cases template