Why does const_cast not behave as expected?

Question

struct A
{
    A() {}

private:
    A(const A&); // Explicitly disable the copy constructor.
};

int main()
{
    const A a1; // OK.
    A       a2; // OK.
    auto    a3 = const_cast<A&>(a1); // Compiler error C2248! ???       
}

My C++ compiler is the latest VC++ 2013 preview.

The compiler complains for the last line with error C2248: 'A::A' : cannot access private member declared in class 'A'

Why does const_cast not behave as expected?

Answer 1

auto, by itself, is never a reference type. So the last line is equivalent to

A a3 = const_cast<A&>(a1);

which attempts to copy a1 using the private constructor.

If you want a reference, you need to specify a reference:

auto & a3 = const_cast<A&>(a1);

Of course, attempting to use this reference to modify a1 will give undefined behaviour, since the object itself is const.