std::operator+(std::basic_string)(3)

NAME
std::operator+(std::basic_string) - std::operator+(std::basic_string)

Synopsis
   Defined in header <string>
   template< class CharT, class Traits, class Alloc >
   std::basic_string<CharT,Traits,Alloc> (1) (constexpr
       operator+( const std::basic_string<CharT,Traits,Alloc>& lhs, since C++20)
                  const std::basic_string<CharT,Traits,Alloc>& rhs );
   template< class CharT, class Traits, class Alloc >
   std::basic_string<CharT,Traits,Alloc> (2) (constexpr
       operator+( const std::basic_string<CharT,Traits,Alloc>& lhs, since C++20)
                  const CharT* rhs );
   template< class CharT, class Traits, class Alloc >
   std::basic_string<CharT,Traits,Alloc> (3) (constexpr
       operator+( const std::basic_string<CharT,Traits,Alloc>& lhs, since C++20)
                  CharT rhs );
   template< class CharT, class Traits, class Alloc >
   constexpr std::basic_string<CharT,Traits,Alloc> (since
       operator+( const std::basic_string<CharT,Traits,Alloc>& lhs, (4) C++26)
    std::type_identity_t<std::basic_string_view<CharT,Traits>> rhs );
   template< class CharT, class Traits, class Alloc >
   std::basic_string<CharT,Traits,Alloc> (5) (constexpr
       operator+( const CharT* lhs, since C++20)
                  const std::basic_string<CharT,Traits,Alloc>& rhs );
   template< class CharT, class Traits, class Alloc >
   std::basic_string<CharT,Traits,Alloc> (6) (constexpr
       operator+( CharT lhs, since C++20)
                  const std::basic_string<CharT,Traits,Alloc>& rhs );
   template< class CharT, class Traits, class Alloc >
   constexpr std::basic_string<CharT,Traits,Alloc> (since
       operator+( (7) C++26)
   std::type_identity_t<std::basic_string_view<CharT,Traits>> lhs,
                  const std::basic_string<CharT,Traits,Alloc>& rhs );
   template< class CharT, class Traits, class Alloc >
                        (since
   std::basic_string<CharT,Traits,Alloc> (8) C++11)
       operator+( std::basic_string<CharT,Traits,Alloc>&& lhs, (constexpr
                        since C++20)
                  std::basic_string<CharT,Traits,Alloc>&& rhs );
   template< class CharT, class Traits, class Alloc >
                        (since
   std::basic_string<CharT,Traits,Alloc> (9) C++11)
       operator+( std::basic_string<CharT,Traits,Alloc>&& lhs, (constexpr
                        since C++20)
                  const std::basic_string<CharT,Traits,Alloc>& rhs );
   template< class CharT, class Traits, class Alloc >
                        (since
   std::basic_string<CharT,Traits,Alloc> (10) C++11)
       operator+( std::basic_string<CharT,Traits,Alloc>&& lhs, (constexpr
                        since C++20)
                  const CharT* rhs );
   template< class CharT, class Traits, class Alloc >
                        (since
   std::basic_string<CharT,Traits,Alloc> (11) C++11)
       operator+( std::basic_string<CharT,Traits,Alloc>&& lhs, (constexpr
                        since C++20)
                  CharT rhs );
   template< class CharT, class Traits, class Alloc >
   constexpr std::basic_string<CharT,Traits,Alloc> (since
       operator+( std::basic_string<CharT,Traits,Alloc>&& lhs, (12) C++26)
    std::type_identity_t<std::basic_string_view<CharT,Traits>> rhs );
   template< class CharT, class Traits, class Alloc >
                        (since
   std::basic_string<CharT,Traits,Alloc> (13) C++11)
       operator+( const std::basic_string<CharT,Traits,Alloc>& lhs, (constexpr
                        since C++20)
                  std::basic_string<CharT,Traits,Alloc>&& rhs );
   template< class CharT, class Traits, class Alloc >
                        (since
   std::basic_string<CharT,Traits,Alloc> (14) C++11)
       operator+( const CharT* lhs, (constexpr
                        since C++20)
                  std::basic_string<CharT,Traits,Alloc>&& rhs );
   template< class CharT, class Traits, class Alloc >
                        (since
   std::basic_string<CharT,Traits,Alloc> (15) C++11)
       operator+( CharT lhs, (constexpr
                        since C++20)
                  std::basic_string<CharT,Traits,Alloc>&& rhs );
   template< class CharT, class Traits, class Alloc >
   constexpr std::basic_string<CharT,Traits,Alloc> (since
       operator+( (16) C++26)
   std::type_identity_t<std::basic_string_view<CharT,Traits>> lhs,
                  std::basic_string<CharT,Traits,Alloc>&& rhs );
   Returns a string containing characters from lhs followed by the characters from rhs.
   Equivalent to:
   1,2) std::basic_string<CharT, Traits, Allocator> r = lhs; r.append(rhs); return r;
   3) std::basic_string<CharT, Traits, Allocator> r = lhs; r.push_back(rhs); return r;
   4) std::basic_string<CharT, Traits, Allocator> r = lhs; r.append(rhs); return r;
   5) std::basic_string<CharT, Traits, Allocator> r = rhs; r.insert(0, lhs); return r;
   6) std::basic_string<CharT, Traits, Allocator> r = rhs; r.insert(r.begin(), lhs);
   return r;
   7) std::basic_string<CharT, Traits, Allocator> r = rhs; r.insert(0, lhs); return r;
   8) lhs.append(rhs); return std::move(lhs); except that both lhs and rhs are left in
   valid but unspecified states. If lhs and rhs have equal allocators, the
   implementation can move from either.
   9,10) lhs.append(rhs); return std::move(lhs);
   11) lhs.push_back(rhs); return std::move(lhs);
   12) lhs.append(rhs); return std::move(lhs);
   13,14) rhs.insert(0, lhs); return std::move(rhs);
   15) rhs.insert(rhs.begin(), lhs); return std::move(rhs);
   16) rhs.insert(0, lhs); return std::move(rhs); The allocator used for the result is: 1-4) std::allocator_traits<Alloc>::select_on_container_copy_construction(lhs.get_allocator()) 5-7) std::allocator_traits<Alloc>::select_on_container_copy_construction(rhs.get_allocator()) 8-12) lhs.get_allocator() 13-16) rhs.get_allocator()
                        (since In other words: C++11)
  * If one operand is a basic_string rvalue, its allocator is used.
  * Otherwise, select_on_container_copy_construction is used on the allocator of the
    lvalue basic_string operand. In each case, the left operand is preferred when both are basic_strings of the same value category. For (8-16), all rvalue basic_string operands are left in valid but unspecified states.

Parameters
         string
   lhs - , string view
         (since C++26), character, or pointer to the first character in a
         null-terminated array
         string
   rhs - , string view
         (since C++26), character, or pointer to the first character in a
         null-terminated array

Return value
   A string containing characters from lhs followed by the characters from rhs
   , using the allocator determined as above
   (since C++11).

Notes
operator+ should be used with great caution when stateful allocators are involved (such as when std::pmr::string is used) (since C++17). Prior to P1165R1, the allocator used for the result was determined by historical accident and can vary from overload to overload for no apparent reason. Moreover, for (1-5), the allocator propagation behavior varies across major standard library implementations and differs from the behavior depicted in the standard. Because the allocator used by the result of operator+ is sensitive to value category, operator+ is not associative with respect to allocator propagation: using my_string = std::basic_string<char, std::char_traits<char>, my_allocator<char>>; my_string cat(); const my_string& dog(); (since
                        C++11) my_string meow = /* ... */, woof = /* ... */; meow + cat() + /* ... */; // uses select_on_container_copy_construction on meow's allocator woof + dog() + /* ... */; // uses allocator of dog()'s return value instead meow + woof + meow; // uses select_on_container_copy_construction on meow's allocator meow + (woof + meow); // uses SOCCC on woof's allocator instead For a chain of operator+ invocations, the allocator used for the ultimate result may be controlled by prepending an rvalue basic_string with the desired allocator: // use my_favorite_allocator for the final result my_string(my_favorite_allocator) + meow + woof + cat() + dog(); For better and portable control over allocators, member functions like append, insert, and operator+= should be used on a result string constructed with the desired allocator.
   The usage of std::type_identity_t as parameter in overloads (4), (7), (12),
   and (16) ensures that an object of type std::basic_string<CharT, Traits,
   Allocator> can always be concatenated to an object of a type T with an
   implicit conversion to std::basic_string_view<CharT, Traits>, and vice versa, (since
   as per overload resolution rules. C++26)
    Feature-test macro Value Std Feature
   __cpp_lib_string_view 202403 (C++26) Concatenation of strings and string
                        views, overloads (4), (7), (12), (16)

Example
// Run this code
 #include <iostream>
 #include <string>
 #include <string_view>
 int main()
 {
     std::string s1 = "Hello";
     std::string s2 = "world";
     const char* end = "!\n";
     std::cout << s1 + ' ' + s2 + end;
     std::string_view water{" Water"};
     #if __cpp_lib_string_view >= 202403
     std::cout << s1 + water + s2 << end; // overload (4), then (1)
     #else
     std::cout << s1 + std::string(water) + s2 << end; // OK, but less efficient
     #endif
 }

Output:
 Hello world!
 Hello Waterworld!
   Defect reports
   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.
     DR Applied to Behavior as published Correct behavior
   P1165R1 C++11 allocator propagation is haphazard and made more consistent
                      inconsistent

See also
   operator+= appends characters to the end
              (public member function)
   append appends characters to the end
              (public member function)
   insert inserts characters
              (public member function)