Clash Royale CLAN TAG#URR8PPP

Using `hana::is_valid` fails with const references

I am using hana to determine if an object has a Length member like so:

Length

using boost::hana::is_valid;


static const auto has_length
= is_valid((const auto& obj)
-> decltype(obj.Length(),void()) );


This works fine....I can do static asserts with it all day to my hearts content. So the next step is logically to enable_if a function:

enable_if

template<typename T>
auto foo(T t) -> std::enable_if_t<has_length(t), void>


struct mine

int Length() const return 0;
;

int main()

foo(mine);



This works just fine....but as soon as I change the T to const T&, we get errors that there is no suitable overload: godbolt

T

const T&

So my question is: why does this happen?

std::enable_if_t<has_length(T), void>

constexpr mine()

constexpr mine(const mine&)

constexpr

auto foo(T)

1 Answer
1

The problem is that calling a function with a reference that is not constexpr is not constexpr. This is where hana::is_valid is useful because it returns an integral_constant-like value that contains a static constexpr boolean so we can just look at the return type. See bool_.

hana::is_valid

integral_constant

bool_

Here is an example:

#include <boost/hana.hpp>
#include <type_traits>

namespace hana = boost::hana;

static auto has_length = hana::is_valid((auto&& t)
-> decltype(t.Length()) );

template <typename T>
auto foo(T const& t)
-> std::enable_if_t<decltype(has_length(t)), void>
// ^-- notice the return type
// is a boolean integral constant


struct mine

int Length() const return 0;
;

int main()

foo(mine);



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