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std::operator+(std::basic_string)

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Defined in header <string>
template< class CharT, class Traits, class Alloc >

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

                   const std::basic_string<CharT,Traits,Alloc>& rhs );
(1) (constexpr since C++20)
template< class CharT, class Traits, class Alloc >

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

                   const CharT* rhs );
(2) (constexpr since C++20)
template<class CharT, class Traits, class Alloc>

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

                   CharT rhs );
(3) (constexpr since C++20)
template< class CharT, class Traits, class Alloc >

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

                   const std::basic_string<CharT,Traits,Alloc>& rhs );
(4) (constexpr since C++20)
template< class CharT, class Traits, class Alloc >

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

                   const std::basic_string<CharT,Traits,Alloc>& rhs );
(5) (constexpr since C++20)
template< class CharT, class Traits, class Alloc >

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

                   std::basic_string<CharT,Traits,Alloc>&& rhs );
(6) (since C++11)
(constexpr since C++20)
template< class CharT, class Traits, class Alloc >

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

                   const std::basic_string<CharT,Traits,Alloc>& rhs );
(7) (since C++11)
(constexpr since C++20)
template< class CharT, class Traits, class Alloc >

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

                   const CharT* rhs );
(8) (since C++11)
(constexpr since C++20)
template< class CharT, class Traits, class Alloc >

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

                   CharT rhs );
(9) (since C++11)
(constexpr since C++20)
template< class CharT, class Traits, class Alloc >

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

                   std::basic_string<CharT,Traits,Alloc>&& rhs );
(10) (since C++11)
(constexpr since C++20)
template< class CharT, class Traits, class Alloc >

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

                   std::basic_string<CharT,Traits,Alloc>&& rhs );
(11) (since C++11)
(constexpr since C++20)
template< class CharT, class Traits, class Alloc >

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

                   std::basic_string<CharT,Traits,Alloc>&& rhs );
(12) (since C++11)
(constexpr since C++20)

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)

Contents

[edit] 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

[edit] 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 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.

(since C++11)

[edit] Example

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

Output:

Hello world!

[edit] 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

[edit] See also

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