Read this article: Appearing and Disappearing consts in C++
Type deduction for auto variables in C++0x is essentially the same as
for template parameters. (As far as I know, the only difference
between the two is that the type of auto variables may be deduced from
initializer lists, while the types of template parameters may not be.)
Each of the following declarations therefore declare variables of type
int (never const int):
auto a1 = i;
auto a2 = ci;
auto a3 = *pci;
auto a4 = pcs->i;
During type deduction for template parameters and auto variables, only
top-level consts are removed. Given a function template taking a
pointer or reference parameter, the constness of whatever is pointed
or referred to is retained:
template<typename T>
void f(T& p);
int i;
const int ci = 0;
const int *pci = &i;
f(i); // as before, calls f<int>, i.e., T is int
f(ci); // now calls f<const int>, i.e., T is const int
f(*pci); // also calls f<const int>, i.e., T is const int
This behavior is old news, applying as it does to both C++98 and
C++03. The corresponding behavior for auto variables is, of course,
new to C++0x:
auto& a1 = i; // a1 is of type int&
auto& a2 = ci; // a2 is of type const int&
auto& a3 = *pci; // a3 is also of type const int&
auto& a4 = pcs->i; // a4 is of type const int&, too
Since you can retain the cv-qualifier if the type is a reference or pointer, you can do:
auto& my_foo2 = GetFoo();
Instead of having to specify it as const (same goes for volatile).
Edit: As for why auto deduces the return type of GetFoo() as a value instead of a reference (which was your main question, sorry), consider this:
const Foo my_foo = GetFoo();
The above will create a copy, since my_foo is a value. If auto were to return an lvalue reference, the above wouldn’t be possible.