Onay Urfalioglu Onay Urfalioglu - 1 month ago 12
C++ Question

C++11 trailing return member function using decltype and constness

I am trying to understand the trailing return based new function declaration syntax in C++11, using decltype.

In the following code, I am trying to define a member function returning a const & to allow for read-only access to

i


#include <iostream>
#include <type_traits>

struct X {
int &i;

X(int &ii) : i(ii) {}

// auto acc() const -> std::add_const<decltype((i))>::type { return i; } // fails the constness test
auto acc() const -> decltype(i) { return i; } // fails the constness test
// const int &acc() const { return i; } // works as expected
};

void modify_const(const X &v) {
v.acc() = 1;
}

int main() {
int i = 0;
X x(i);

modify_const(x);
std::cout << i << std::endl;

return 0;
}


As mentioned in the comments, only the last commented version of
acc()
works, whereas using the others, the code just compiles and prints value
1
.

Question: How do we have to define the
acc()
function using the new function declaration syntax based on
decltype
, such that the compilation here fails due to returning a
const &int
in
modify_const
, or in other words, such that
acc()
has a proper
const &int
return type.

Remark: using
int i;
instead of
int &i;
as the member variable in
X
produces a compile error, as expected.

Edited to better distinguish between constness of
v
and
X::i
, respectively. It is the latter I am trying to impose in
acc()
.

Answer

The problem is that decltype((i)) return int& and apply const to that type has no effect. You want something like

template <typename T> struct add_ref_const { typedef T const type; };
template <typename T> struct add_ref_const<T&> { typedef T const& type; };

... and then use

auto acc() const -> typename add_ref_const<decltype((i))>::type { return i; }

That is, the const needs to go between the type T and the &. The solution would have been obvious if you had put the const into the correct location: const should go to the right.