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

    From cppreference.com
     
    C++
     
    Strings library
     
    std::basic_string
     
    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 cause size() to exceed 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

    See also

    		 appends characters to the end
    (public member function) [edit]