The allocator used for the result is:

In other words:

• If one operand is abasic_string rvalue, its allocator is used.
• Otherwise,select_on_container_copy_construction is used on the allocator of the lvaluebasic_string operand.

In each case, the left operand is preferred when both arebasic_strings of the same value category.

For(8-16), all rvaluebasic_string operands are left in valid but unspecified states.

• 1Parameters
• 2Return value
• 3Notes
• 4Example
• 5Defect reports
• 6See also

Notes

operator+ should be used with great caution when stateful allocators are involved (such as whenstd::pmr::string is used)(since C++17). Prior toP1165R1, 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 ofoperator+ 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(); my_string meow=/* ... */, woof=/* ... */;meow+ cat()+/* ... */;// uses select_on_container_copy_construction on meow's allocatorwoof+ dog()+/* ... */;// uses allocator of dog()'s return value instead meow+ woof+ meow;// uses select_on_container_copy_construction on meow's allocatormeow+(woof+ meow);// uses SOCCC on woof's allocator instead

For a chain ofoperator+ invocations, the allocator used for the ultimate result may be controlled by prepending an rvaluebasic_string with the desired allocator:

// use my_favorite_allocator for the final resultmy_string(my_favorite_allocator)+ meow+ woof+ cat()+ dog();

For better and portable control over allocators, member functions likeappend,insert, andoperator+= should be used on a result string constructed with the desired allocator.

The usage ofstd::type_identity_t as parameter in overloads(4),(7),(12), and(16) ensures that an object of typestd::basic_string<CharT, Traits, Allocator> can always be concatenated to an object of a typeT with an implicit conversion tostd::basic_string_view<CharT, Traits>, and vice versa, as peroverload resolution rules.