By using auto&& var = <initializer> you are saying: I will accept any initializer regardless of whether it is an lvalue or rvalue expression and I will preserve its constness. This is typically used for forwarding (usually with T&&). The reason this works is because a “universal reference”, auto&& or T&&, will bind to anything.
You might say, well why not just use a const auto& because that will also bind to anything? The problem with using a const reference is that it’s const! You won’t be able to later bind it to any non-const references or invoke any member functions that are not marked const.
As an example, imagine that you want to get a std::vector, take an iterator to its first element and modify the value pointed to by that iterator in some way:
auto&& vec = some_expression_that_may_be_rvalue_or_lvalue;
auto i = std::begin(vec);
(*i)++;
This code will compile just fine regardless of the initializer expression. The alternatives to auto&& fail in the following ways:
auto => will copy the vector, but we wanted a reference
auto& => will only bind to modifiable lvalues
const auto& => will bind to anything but make it const, giving us const_iterator
const auto&& => will bind only to rvalues
So for this, auto&& works perfectly! An example of using auto&& like this is in a range-based for loop. See my other question for more details.
If you then use std::forward on your auto&& reference to preserve the fact that it was originally either an lvalue or an rvalue, your code says: Now that I’ve got your object from either an lvalue or rvalue expression, I want to preserve whichever valueness it originally had so I can use it most efficiently – this might invalidate it. As in:
auto&& var = some_expression_that_may_be_rvalue_or_lvalue;
// var was initialized with either an lvalue or rvalue, but var itself
// is an lvalue because named rvalues are lvalues
use_it_elsewhere(std::forward<decltype(var)>(var));
This allows use_it_elsewhere to rip its guts out for the sake of performance (avoiding copies) when the original initializer was a modifiable rvalue.
What does this mean as to whether we can or when we can steal resources from var? Well since the auto&& will bind to anything, we cannot possibly try to rip out vars guts ourselves – it may very well be an lvalue or even const. We can however std::forward it to other functions that may totally ravage its insides. As soon as we do this, we should consider var to be in an invalid state.
Now let’s apply this to the case of auto&& var = foo();, as given in your question, where foo returns a T by value. In this case we know for sure that the type of var will be deduced as T&&. Since we know for certain that it’s an rvalue, we don’t need std::forward‘s permission to steal its resources. In this specific case, knowing that foo returns by value, the reader should just read it as: I’m taking an rvalue reference to the temporary returned from foo, so I can happily move from it.
As an addendum, I think it’s worth mentioning when an expression like some_expression_that_may_be_rvalue_or_lvalue might turn up, other than a “well your code might change” situation. So here’s a contrived example:
std::vector<int> global_vec{1, 2, 3, 4};
template <typename T>
T get_vector()
{
return global_vec;
}
template <typename T>
void foo()
{
auto&& vec = get_vector<T>();
auto i = std::begin(vec);
(*i)++;
std::cout << vec[0] << std::endl;
}
Here, get_vector<T>() is that lovely expression that could be either an lvalue or rvalue depending on the generic type T. We essentially change the return type of get_vector through the template parameter of foo.
When we call foo<std::vector<int>>, get_vector will return global_vec by value, which gives an rvalue expression. Alternatively, when we call foo<std::vector<int>&>, get_vector will return global_vec by reference, resulting in an lvalue expression.
If we do:
foo<std::vector<int>>();
std::cout << global_vec[0] << std::endl;
foo<std::vector<int>&>();
std::cout << global_vec[0] << std::endl;
We get the following output, as expected:
2
1
2
2
If you were to change the auto&& in the code to any of auto, auto&, const auto&, or const auto&& then we won’t get the result we want.
An alternative way to change program logic based on whether your auto&& reference is initialised with an lvalue or rvalue expression is to use type traits:
if (std::is_lvalue_reference<decltype(var)>::value) {
// var was initialised with an lvalue expression
} else if (std::is_rvalue_reference<decltype(var)>::value) {
// var was initialised with an rvalue expression
}