Std::operator+(std::basic_string) (3) Linux Manual Page

std::operator+(std::basic_string) – std::operator+(std::basic_string)

Synopsis

template <class CharT, class Traits, class Alloc>

basic_string<CharT, Traits, Alloc>(1)

operator+(const basic_string<CharT, Traits, Alloc> &lhs,

          const basic_string<CharT, Traits, Alloc> &rhs);

template <class CharT, class Traits, class Alloc>

basic_string<CharT, Traits, Alloc>(2)

operator+(const basic_string<CharT, Traits, Alloc> &lhs,

          const CharT *rhs);

template <class CharT, class Traits, class Alloc>

basic_string<CharT, Traits, Alloc>(3)

operator+(const basic_string<CharT, Traits, Alloc> &lhs,

          CharT rhs);

template <class CharT, class Traits, class Alloc>

basic_string<CharT, Traits, Alloc>(4)

operator+(const CharT *lhs,

          const basic_string<CharT, Traits, Alloc> &rhs);

template <class CharT, class Traits, class Alloc>

basic_string<CharT, Traits, Alloc>(5)

operator+(CharT lhs,

          const basic_string<CharT, Traits, Alloc> &rhs);

template <class CharT, class Traits, class Alloc>

basic_string<CharT, Traits, Alloc>(6)(since C++ 11)

operator+(basic_string<CharT, Traits, Alloc> &&lhs,

          basic_string<CharT, Traits, Alloc> &&rhs);

template <class CharT, class Traits, class Alloc>

basic_string<CharT, Traits, Alloc>(7)(since C++ 11)

operator+(basic_string<CharT, Traits, Alloc> &&lhs,

          const basic_string<CharT, Traits, Alloc> &rhs);

template <class CharT, class Traits, class Alloc>

basic_string<CharT, Traits, Alloc>(8)(since C++ 11)

operator+(basic_string<CharT, Traits, Alloc> &&lhs,

          const CharT *rhs);

template <class CharT, class Traits, class Alloc>

basic_string<CharT, Traits, Alloc>(9)(since C++ 11)

operator+(basic_string<CharT, Traits, Alloc> &&lhs,

          CharT rhs);

template <class CharT, class Traits, class Alloc>

basic_string<CharT, Traits, Alloc>(10)(since C++ 11)

operator+(const basic_string<CharT, Traits, Alloc> &lhs,

          basic_string<CharT, Traits, Alloc> &&rhs);

template <class CharT, class Traits, class Alloc>

basic_string<CharT, Traits, Alloc>(11)(since C++ 11)

operator+(const CharT *lhs,

          basic_string<CharT, Traits, Alloc> &&rhs);

template <class CharT, class Traits, class Alloc>

basic_string<CharT, Traits, Alloc>(12)(since C++ 11)

operator+(CharT lhs,

          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 C++11)
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.

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 = /* ... */;
(since C++ 11)

        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.

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

std::operator+(std::basic_string) (3) Linux Manual Page