std::operator+(std::basic_string)

From cppreference.com
< cpp‎ | string‎ | basic string
 
 
 
std::basic_string
 
template< class CharT, class Traits, class Alloc >

    basic_string<CharT,Traits,Alloc>
        operator+( const basic_string<CharT,Traits,Alloc>& lhs,

                   const basic_string<CharT,Traits,Alloc>& rhs );
(1)
template< class CharT, class Traits, class Alloc >

    basic_string<CharT,Traits,Alloc>
        operator+( const basic_string<CharT,Traits,Alloc>& lhs,

                   const CharT* rhs );
(2)
template<class CharT, class Traits, class Alloc>

    basic_string<CharT,Traits,Alloc>
        operator+( const basic_string<CharT,Traits,Alloc>& lhs,

                   CharT rhs );
(3)
template< class CharT, class Traits, class Alloc >

    basic_string<CharT,Traits,Alloc>
        operator+( const CharT* lhs,

                   const basic_string<CharT,Traits,Alloc>& rhs );
(4)
template< class CharT, class Traits, class Alloc >

    basic_string<CharT,Traits,Alloc>
        operator+( CharT lhs,

                   const basic_string<CharT,Traits,Alloc>& rhs );
(5)
template< class CharT, class Traits, class Alloc >

    basic_string<CharT,Traits,Alloc>
        operator+( basic_string<CharT,Traits,Alloc>&& lhs,

                   basic_string<CharT,Traits,Alloc>&& rhs );
(6) (since C++11)
template< class CharT, class Traits, class Alloc >

    basic_string<CharT,Traits,Alloc>
        operator+( basic_string<CharT,Traits,Alloc>&& lhs,

                   const basic_string<CharT,Traits,Alloc>& rhs );
(7) (since C++11)
template< class CharT, class Traits, class Alloc >

    basic_string<CharT,Traits,Alloc>
        operator+( basic_string<CharT,Traits,Alloc>&& lhs,

                   const CharT* rhs );
(8) (since C++11)
template< class CharT, class Traits, class Alloc >

    basic_string<CharT,Traits,Alloc>
        operator+( basic_string<CharT,Traits,Alloc>&& lhs,

                   CharT rhs );
(9) (since C++11)
template< class CharT, class Traits, class Alloc >

    basic_string<CharT,Traits,Alloc>
        operator+( const basic_string<CharT,Traits,Alloc>& lhs,

                   basic_string<CharT,Traits,Alloc>&& rhs );
(10) (since C++11)
template< class CharT, class Traits, class Alloc >

    basic_string<CharT,Traits,Alloc>
        operator+(const CharT* lhs,

                  basic_string<CharT,Traits,Alloc>&& rhs );
(11) (since C++11)
template< class CharT, class Traits, class Alloc >

    basic_string<CharT,Traits,Alloc>
        operator+( CharT lhs,

                   basic_string<CharT,Traits,Alloc>&& rhs );
(12) (since C++11)

Returns a string containing characters from lhs followed by the characters from rhs.

The allocator used for the result is:

1-3) std::allocator_traits<Alloc>::select_on_container_copy_construction(lhs.get_allocator())
4-5) std::allocator_traits<Alloc>::select_on_container_copy_construction(rhs.get_allocator())
6-9) lhs.get_allocator()
10-12) rhs.get_allocator()

In other words, 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 (6-12), all rvalue basic_string operands are left in valid but unspecified states.

(since C++11)

Parameters

lhs - string, character, or pointer to the first character in a null-terminated array
rhs - string, 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();
 
my_string meow = /* ... */, woof = /* ... */;
meow + cat() + /*...*/; // uses SOCCC on meow's allocator
woof + dog() + /*...*/; // uses allocator of dog()'s return value instead
 
meow + woof + meow; // uses SOCCC 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.

(since C++11)

Example

#include <iostream>
#include <string>
 
int main()
{
    std::string s1 = "Hello";
    std::string s2 = "world";
    std::cout << s1 + ' ' + s2 + "!\n";
}

Output:

Hello world!

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 inconsistent made more consistent

See also

appends characters to the end
(public member function)
appends characters to the end
(public member function)
inserts characters
(public member function)