std::operator+(std::basic_string)

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)

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 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)

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

Docs

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