Constexpr find implementation - c++

Question

After answering this question and reading this talk and looking at this code, I want to implement constexpr find with just simple array class.
Consider following example:
#include <cstddef>
template <class It, class T>
constexpr auto constexpr_find(const It& b, const It& e, T value) {
auto begin = b;
while (begin != e) {
if (*begin == value) break;
++begin;
}
return *begin;
}
template<typename T, size_t N>
class array
{
public:
typedef T* iterator;
typedef const T* const_iterator;
constexpr auto begin() const { return const_iterator(array_); }
constexpr auto end() const { return const_iterator(array_ + N); }
T array_[N];
static constexpr size_t size = N;
};
int main()
{
constexpr array<int, 3> array{{0,2,3}};
static_assert(constexpr_find(array.begin(), array.end(), 0) == 0, "");
}
compiles as expected
And with custom constexpr iterator:
template<class T>
class array_iterator
{
public:
constexpr array_iterator(const T* v) : iterator(v)
{
}
constexpr const T& operator * () const { return *iterator; }
constexpr array_iterator& operator ++()
{
++iterator;
return *this;
}
constexpr bool operator != (const array_iterator& other) const { return iterator != other.iterator; }
private:
const T* iterator;
};
In array class:
typedef const array_iterator<const T> const_iterator;
that's the only difference, compiler give me error:
in constexpr expansion of
constexpr_find<array_iterator<const int>, int>(array.array<T,
N>::begin<int, 3u>(), array.array<T, N>::end<int, 3u>(), 0)
error: (((const int*)(& array.array<int, 3u>::array_)) != (((const
int*)(& array.array<int, 3u>::array_)) + 12u)) is not a constant
expression
Live example
Is this gcc bug, since clang compile this fine, or there is difference in two snippets?

Answer 1

I cannot say for sure, but you store pointers for array's member into external iterator class, it may be the reason for that error.
--------- update start ---------
Here is the minimal snippet that demonstrates the problem:
constexpr const struct A { int i[2]; } a {{0,0}};
int main ()
{
static_assert (nullptr != a.i , ""); // ok
static_assert (nullptr != a.i+0, ""); // ok
static_assert (nullptr != a.i+1, ""); // error
}
It seems to be forbidden to have pointers to array elements (with non-zero offset) in constant expressions.
--------- update end ---------
The workaround is trivial - store the pointer to array object and offset.
Live
#include <cstddef>
template <class It, class T>
constexpr auto constexpr_find(const It& b, const It& e, T value) {
auto begin = b, end = e;
while (begin != end) {
if (*begin == value) break;
++begin;
}
return *begin;
}
template<class Array>
class array_iterator
{
public:
constexpr array_iterator(const Array& a, size_t pos=0u) : array_(&a), pos_ (pos)
{
}
constexpr const typename Array::value_type&
operator * () const { return (*array_)[pos_]; }
constexpr array_iterator& operator ++()
{
++pos_;
return *this;
}
constexpr bool operator != (const array_iterator& other) const
{ return array_ != other.array_ || pos_ != other.pos_; }
private:
const Array* array_;
size_t pos_;
};
template<typename T, size_t N>
class array
{
public:
typedef T value_type;
typedef const array_iterator<array> const_iterator;
constexpr T const& operator[] (size_t idx) const { return array_[idx]; }
constexpr auto begin() const { return const_iterator(*this); }
constexpr auto end() const { return const_iterator(*this, N); }
T array_[N];
static constexpr size_t size = N;
};
int main()
{
constexpr array<int, 3> array{{0,2,3}};
static_assert(constexpr_find(array.begin(), array.end(), 0) == 0, "");
}
By the way, it is possible to implement C++11 version of constexpr enabled find:
Live
#include <cstddef>
#include <cassert>
#if !defined(__clang__) && __GNUC__ < 5
// TODO: constexpr asserts does not work in gcc4, but we may use
// "thow" workaround from
// http://ericniebler.com/2014/09/27/assert-and-constexpr-in-cxx11/
# define ce_assert(x) ((void)0)
#else
# define ce_assert(x) assert(x)
#endif
namespace my {
template <class It, class T>
inline constexpr It
find (It begin, It end, T const& value) noexcept
{
return ! (begin != end && *begin != value)
? begin
: find (begin+1, end, value);
}
template<class Array>
class array_iterator
{
public:
using value_type = typename Array::value_type;
constexpr array_iterator(const Array& array, size_t size = 0u) noexcept
: array_ (&array)
, pos_ (size)
{}
constexpr const value_type operator* () const noexcept
{
return ce_assert (pos_ < Array::size), (*array_) [pos_];
}
#if __cplusplus >= 201402L // C++14
constexpr
#endif
array_iterator& operator ++() noexcept
{
return ce_assert (pos_ < Array::size), ++pos_, *this;
}
constexpr array_iterator operator+ (size_t n) const noexcept
{
return ce_assert (pos_+n <= Array::size), array_iterator (*array_, pos_+n);
}
friend constexpr bool
operator != (const array_iterator& i1, const array_iterator& i2) noexcept
{
return i1.array_ != i2.array_ || i1.pos_ != i2.pos_;
}
friend constexpr size_t
operator- (array_iterator const& i1, array_iterator const& i2) noexcept
{
return ce_assert (i1.array_ == i2.array_), i1.pos_ - i2.pos_;
}
private:
const Array* array_;
size_t pos_;
};
template<typename T, size_t N>
class array
{
public:
using value_type = T;
using const_iterator = const array_iterator<array>;
constexpr value_type const&
operator[] (size_t idx) const noexcept
{ return array_[idx]; }
constexpr const_iterator begin() const noexcept
{ return const_iterator(*this); }
constexpr const_iterator end() const noexcept
{ return const_iterator(*this, N); }
T array_[N];
static constexpr size_t size = N;
};
}
int main()
{
static constexpr my::array<int, 3> array{{0,2,3}};
static_assert (
find (array.begin(), array.end(), 2) - array.begin () == 1,
"error");
}
You may also be interested to check Sprout library, it contains a lot of constexpr data structures and algorithms.