std::operator+(std::basic_string)

 

C++

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Symbols index

External libraries

 

Strings library

Null-terminated strings

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Classes

basic_string

basic_string_view (C++17)

char_traits

 

std::basic_string

Member functions basic_string::basic_string basic_string::~basic_string basic_string::operator= basic_string::assign basic_string::assign_range (C++23) basic_string::get_allocator Element access basic_string::at basic_string::operator[] basic_string::front (DR*) basic_string::back (DR*) basic_string::data basic_string::c_str basic_string::operator  basic_string_view (C++17) Iterators basic_string::beginbasic_string::cbegin (C++11) basic_string::endbasic_string::cend (C++11) basic_string::rbeginbasic_string::crbegin (C++11) basic_string::rendbasic_string::crend (C++11) Capacity basic_string::empty basic_string::sizebasic_string::length basic_string::max_size basic_string::reserve basic_string::capacity basic_string::shrink_to_fit (DR*) Modifiers basic_string::clear basic_string::insert basic_string::insert_range (C++23) basic_string::erase basic_string::push_back basic_string::pop_back (DR*) basic_string::append basic_string::append_range (C++23) basic_string::operator+= basic_string::replace basic_string::replace_with_range (C++23) basic_string::copy basic_string::resize basic_string::resize_and_overwrite (C++23)   basic_string::swap

Search basic_string::find basic_string::rfind basic_string::find_first_of basic_string::find_first_not_of basic_string::find_last_of basic_string::find_last_not_of Operations basic_string::compare basic_string::starts_with (C++20) basic_string::ends_with (C++20) basic_string::contains (C++23) basic_string::substr Constants basic_string::npos Non-member functions operator+ swap(std::basic_string) erase(std::basic_string)erase_if(std::basic_string) (C++20)(C++20) I/O operator<<operator>> getline Comparison operator==operator!=operator<operator>operator<=operator>=operator<=> (until C++20)(until C++20)(until C++20)(until C++20)(until C++20)(C++20) Numeric conversions stoistolstoll (C++11)(C++11)(C++11) stoulstoull (C++11)(C++11) stofstodstold (C++11)(C++11)(C++11) to_string (C++11) to_wstring (C++11) Literals operator""s (C++14) Helper classes hash<std::basic_string> (C++11) Deduction guides (C++17)

 

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

Run this code

#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

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)