&&

[Popping mad]

What is the purpose of the double ampersand in this use of auto

http://en.cppreference.com/w/cpp/container/vector/push_back

#include <vector>
#include <iostream>
#include <iomanip>

int main()
{
std::vector<std::string> numbers;

numbers.push_back("abc");
std::string s = "def";
numbers.push_back(std::move(s));

std::cout << "vector holds: ";
for (auto &&i : numbers) std::cout << std::quoted(i) << ' ';
std::cout << "\nMoved-from string holds " << std::quoted(s) << '\n';
}

[Alf P. Steinbach]

In this context, where the type is deduced, the `&&` makes that type a
“universal reference” , also known as a “forwarding reference”.

In effect, if `i` is bound to an rvalue expression (a temporary), then
type deduction plus the rules for collapsing of `&`-sequences make `i`
of type `T&&`, an rvalue reference. And if `i` is bound to an lvalue
expression, then those same rules make `i` of type `T&`, an ordinary
C++03 reference. This is the sort of shape shifting nature of a
universal reference.

For this particular case `i` is bound to an lvalue in each iteration of
the loop, and thus its type is `T&`, namely `std::string&`.

If the programmer was well aware of things and wrote that intentionally,
not just blindly copying what he/she had seen elsewhere, then the only
purpose I can think of would be to support a later modification of the
code where `i` might instead be bound to an rvalue.

In that case the reference makes the code compile and avoids copying the
value (in each iteration).


Cheers & hth.,

- Alf