For implementing a conditional type I highly enjoy std::conditional_t
as it keeps the code short and very readable:
template<std::size_t N>
using bit_type =
std::conditional_t<N == std::size_t{ 8 }, std::uint8_t,
std::conditional_t<N == std::size_t{ 16 }, std::uint16_t,
std::conditional_t<N == std::size_t{ 32 }, std::uint32_t,
std::conditional_t<N == std::size_t{ 64 }, std::uint64_t, void>>>>;
using it works quite intuitively:
bit_type<8u> a; // == std::uint8_t
bit_type<16u> b; // == std::uint16_t
bit_type<32u> c; // == std::uint32_t
bit_type<64u> d; // == std::uint64_t
But since this is a pure conditional type there must be a default type - void
, in this case. Therefore if N
is any other value said type yields:
bit_type<500u> f; // == void
Now this doesn't compile, but the yielding type is still valid.
Meaning you could say bit_type<500u>* f;
and would have a valid program!
So is there a nice way to let compilation fail when the fail case of an conditional type is reached?
One idea immediately would be to replace the last std::conditional_t
with std::enable_if_t
:
template<std::size_t N>
using bit_type =
std::conditional_t<N == std::size_t{ 8 }, std::uint8_t,
std::conditional_t<N == std::size_t{ 16 }, std::uint16_t,
std::conditional_t<N == std::size_t{ 32 }, std::uint32_t,
std::enable_if_t< N == std::size_t{ 64 }, std::uint64_t>>>>;
The problem with that is that templates are always fully evaluated, meaning that the std::enable_if_t
is always fully evaluated - and that will fail if N != std::size_t{ 64 }
. Urgh.
My current go-to workaround to this is rather clumsy introducing a struct and 3 using
declarations:
template<std::size_t N>
struct bit_type {
private:
using vtype =
std::conditional_t<N == std::size_t{ 8 }, std::uint8_t,
std::conditional_t<N == std::size_t{ 16 }, std::uint16_t,
std::conditional_t<N == std::size_t{ 32 }, std::uint32_t,
std::conditional_t<N == std::size_t{ 64 }, std::uint64_t, void>>>>;
public:
using type = std::enable_if_t<!std::is_same_v<vtype, void>, vtype>;
};
template<std::size_t N>
using bit_type_t = bit_type<N>::type;
static_assert(std::is_same_v<bit_type_t<64u>, std::uint64_t>, "");
Which generally works, but I dislike it as it adds so much stuff, I might as well just use template specialization. It also reserves void
as a special type - so it won't work where void
is actually a yield from a branch. Is there a readable, short solution?
You can solve this by adding a level of indirection, so that the result of the outermost conditional_t
is not a type but a metafunction that needs ::type
to be applied to it. Then use enable_if
instead of enable_if_t
so you don't access the ::type
unless it's actually needed:
template<typename T> struct identity { using type = T; };
template<std::size_t N>
using bit_type = typename
std::conditional_t<N == std::size_t{ 8 }, identity<std::uint8_t>,
std::conditional_t<N == std::size_t{ 16 }, identity<std::uint16_t>,
std::conditional_t<N == std::size_t{ 32 }, identity<std::uint32_t>,
std::enable_if<N == std::size_t{ 64 }, std::uint64_t>>>>::type;
In this version the type in the final branch is enable_if<
condition
, uint64_t>
which is always a valid type, and you only get an error if that branch is actually taken and enable_if<false, uint64_t>::type
is needed. When one of the earlier branches is taken you end up using identity<uintNN_t>::type
for one of the smaller integer types, and it doesn't matter that enable_if<false, uint64_t>
has no nested type (because you don't use it).
Just for fun... what about using std::tuple
and std::tuple_element
avoiding at all std::conditional
?
If you can use C++14 (so template variables and specialization of template variables) you can write a template variable for conversion size/index-in-the-tuple
template <std::size_t>
constexpr std::size_t bt_index = 100u; // bad value
template <> constexpr std::size_t bt_index<8u> = 0u;
template <> constexpr std::size_t bt_index<16u> = 1u;
template <> constexpr std::size_t bt_index<32u> = 2u;
template <> constexpr std::size_t bt_index<64u> = 3u;
so bit_type
become
template <std::size_t N>
using bit_type = std::tuple_element_t<bt_index<N>,
std::tuple<std::uint8_t, std::uint16_t, std::uint32_t, std::uint64_t>>;
If you can use only C++11, you can develop a bt_index()
constexpr
function that return the correct (or incorrect) value.
You can verify that are satisfied
static_assert( std::is_same_v<bit_type<8u>, std::uint8_t>, "!" );
static_assert( std::is_same_v<bit_type<16u>, std::uint16_t>, "!" );
static_assert( std::is_same_v<bit_type<32u>, std::uint32_t>, "!" );
static_assert( std::is_same_v<bit_type<64u>, std::uint64_t>, "!" );
and that using bit_type
with an unsupported dimension
bit_type<42u> * pbt42;
cause a compilation error.
-- EDIT -- As suggested by Jonathan Wakely, if you can use C++20, so std::ispow2()
and std::log2p1()
, you can simplify a lot: you can avoid bt_index
at all and simply write
template <std::size_t N>
using bit_type = std::tuple_element_t<std::ispow2(N) ? std::log2p1(N)-4u : -1,
std::tuple<std::uint8_t, std::uint16_t, std::uint32_t, std::uint64_t>>;
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