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 > constexpr std::basic_string<CharT,Traits,Alloc> operator+( const std::basic_string<CharT,Traits,Alloc>& lhs, std::type_identity_t<std::basic_string_view<CharT,Traits>> rhs );	(4)	(since C++26)
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 );	(5)	(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 );	(6)	(constexpr since C++20)
template< class CharT, class Traits, class Alloc > constexpr std::basic_string<CharT,Traits,Alloc> operator+( std::type_identity_t<std::basic_string_view<CharT,Traits>> lhs, const std::basic_string<CharT,Traits,Alloc>& rhs );	(7)	(since C++26)
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 );	(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, const std::basic_string<CharT,Traits,Alloc>& rhs );	(9)	(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 );	(10)	(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 );	(11)	(since C++11) (constexpr since C++20)
template< class CharT, class Traits, class Alloc > constexpr std::basic_string<CharT,Traits,Alloc> operator+( std::basic_string<CharT,Traits,Alloc>&& lhs, std::type_identity_t<std::basic_string_view<CharT,Traits>> rhs );	(12)	(since C++26)
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 );	(13)	(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 );	(14)	(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 );	(15)	(since C++11) (constexpr since C++20)
template< class CharT, class Traits, class Alloc > constexpr std::basic_string<CharT,Traits,Alloc> operator+( std::type_identity_t<std::basic_string_view<CharT,Traits>> lhs, std::basic_string<CharT,Traits,Alloc>&& rhs );	(16)	(since C++26)

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

1,2) std::basic_string<CharT, Traits, Allocator> r = lhs; r.append(rhs); return r;

3) std::basic_string<CharT, Traits, Allocator> r = lhs; r.push_back(rhs); return r;

4) std::basic_string<CharT, Traits, Allocator> r = lhs; r.append(rhs); return r;

5) std::basic_string<CharT, Traits, Allocator> r = rhs; r.insert(0, lhs); return r;

6) std::basic_string<CharT, Traits, Allocator> r = rhs; r.insert(r.begin(), lhs); return r;

7) std::basic_string<CharT, Traits, Allocator> r = rhs; r.insert(0, lhs); return r;

8) lhs.append(rhs); return std::move(lhs); except that both lhs and rhs are left in valid but unspecified states. If lhs and rhs have equal allocators, the implementation can move from either.

9,10) lhs.append(rhs); return std::move(lhs);

11) lhs.push_back(rhs); return std::move(lhs);

12) lhs.append(rhs); return std::move(lhs);

13,14) rhs.insert(0, lhs); return std::move(rhs);

15) rhs.insert(rhs.begin(), lhs); return std::move(rhs);

16) rhs.insert(0, lhs); return std::move(rhs);

The allocator used for the result is:

1-4) std::allocator_traits<Alloc>::select_on_container_copy_construction(lhs.get_allocator()) 5-7) std::allocator_traits<Alloc>::select_on_container_copy_construction(rhs.get_allocator()) 8-12) lhs.get_allocator() 13-16) 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 (8-16), all rvalue basic_string operands are left in valid but unspecified states.

(since C++11)

Parameters

lhs	-	string, string view(since C++26), character, or pointer to the first character in a null-terminated array
rhs	-	string, string view(since C++26), 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)

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

Feature-test macro	Value	Std	Feature
`__cpp_lib_string_view`	`202403`	(C++26)	Concatenation of strings and string views, overloads (4), (7), (12), (16)

(since C++26)

Example

#include <iostream>
#include <string>
#include <string_view>

int main()
{
    std::string s1 = "Hello";
    std::string s2 = "world";
    const char* end = "!\n";
    std::cout << s1 + ' ' + s2 + end;

    std::string_view water{" Water"};
    #if __cpp_lib_string_view >= 202403
    std::cout << s1 + water + s2 << end; // overload (4), then (1)
    #else
    std::cout << s1 + std::string(water) + s2 << end; // OK, but less efficient
    #endif
}

Output:

Hello world!
Hello Waterworld!

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