table of contents
std::operator+(std::basic_string)(3) | C++ Standard Libary | std::operator+(std::basic_string)(3) |
NAME¶
std::operator+(std::basic_string) - std::operator+(std::basic_string)
Synopsis¶
Defined in header <string>
template< class CharT, class Traits, class Alloc >
std::basic_string<CharT,Traits,Alloc> (constexpr since
operator+( const std::basic_string<CharT,Traits,Alloc>& (1)
C++20)
lhs,
const std::basic_string<CharT,Traits,Alloc>& rhs );
template< class CharT, class Traits, class Alloc >
std::basic_string<CharT,Traits,Alloc> (constexpr since
operator+( const std::basic_string<CharT,Traits,Alloc>& (2)
C++20)
lhs,
const CharT* rhs );
template<class CharT, class Traits, class Alloc>
std::basic_string<CharT,Traits,Alloc> (constexpr since
operator+( const std::basic_string<CharT,Traits,Alloc>& (3)
C++20)
lhs,
CharT rhs );
template< class CharT, class Traits, class Alloc >
std::basic_string<CharT,Traits,Alloc> (4) (constexpr since
operator+( const CharT* lhs, C++20)
const std::basic_string<CharT,Traits,Alloc>& rhs );
template< class CharT, class Traits, class Alloc >
std::basic_string<CharT,Traits,Alloc> (5) (constexpr since
operator+( CharT lhs, C++20)
const std::basic_string<CharT,Traits,Alloc>& rhs );
template< class CharT, class Traits, class Alloc >
(since C++11)
std::basic_string<CharT,Traits,Alloc> (6) (constexpr since
operator+( std::basic_string<CharT,Traits,Alloc>&& lhs,
C++20)
std::basic_string<CharT,Traits,Alloc>&& rhs );
template< class CharT, class Traits, class Alloc >
(since C++11)
std::basic_string<CharT,Traits,Alloc> (7) (constexpr since
operator+( std::basic_string<CharT,Traits,Alloc>&& lhs,
C++20)
const std::basic_string<CharT,Traits,Alloc>& rhs );
template< class CharT, class Traits, class Alloc >
(since C++11)
std::basic_string<CharT,Traits,Alloc> (8) (constexpr since
operator+( std::basic_string<CharT,Traits,Alloc>&& lhs,
C++20)
const CharT* rhs );
template< class CharT, class Traits, class Alloc >
(since C++11)
std::basic_string<CharT,Traits,Alloc> (9) (constexpr since
operator+( std::basic_string<CharT,Traits,Alloc>&& lhs,
C++20)
CharT rhs );
template< class CharT, class Traits, class Alloc >
std::basic_string<CharT,Traits,Alloc> (since C++11)
operator+( const std::basic_string<CharT,Traits,Alloc>& (10)
(constexpr since
lhs, C++20)
std::basic_string<CharT,Traits,Alloc>&& rhs );
template< class CharT, class Traits, class Alloc >
(since C++11)
std::basic_string<CharT,Traits,Alloc> (11) (constexpr since
operator+( const CharT* lhs, C++20)
std::basic_string<CharT,Traits,Alloc>&& rhs );
template< class CharT, class Traits, class Alloc >
(since C++11)
std::basic_string<CharT,Traits,Alloc> (12) (constexpr since
operator+( CharT lhs, C++20)
std::basic_string<CharT,Traits,Alloc>&& rhs );
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() (since 10-12) rhs.get_allocator() C++11)
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.
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();
(since my_string meow = /* ... */, woof = /* ... */; C++11) 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.
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 made more consistent
inconsistent
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)
2022.07.31 | http://cppreference.com |