# std::operator+(std::basic_string)

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< cpp‎ | string‎ | 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_string`s 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

Run this code

    #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

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

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