In template: constexpr variable 'kFirst' must be initialized by a constant expression

[engun]

This is a great open-source project. I encountered a little problem while using it.

The compilation tool is CLion, and I'm using C++17.

#include <shared_mutex>
#include <vector>
#include <iostream>

#include "parallel_hashmap/phmap.h"

template <class K, class V>
using HashT = phmap::parallel_flat_hash_map<K, V, phmap::priv::hash_default_hash<K>,
        phmap::priv::hash_default_eq<K>,
        std::allocator<std::pair<const K, V>>, 4, std::shared_mutex>;

int main() {
    HashT<uint64_t, uint32_t> hash;
    hash[1] = 1;
    std::cout << hash.at(1) << std::endl;
    return 0;
}

The compiler is showing an annoying error message:

"In template: constexpr variable 'kFirst' must be initialized by a constant expression."

[greg7mdp]

Hi @engun , thanks for reporting this.
I'm not using CLion, but I just made a change that I think may fix this. Can you try the latest version of the repo?

[engun]

I just tried it. It's still the same. maybe more detailed information would help.

begin----------------------------------------------------------------------------------------------------------

In template: constexpr variable 'kFirst' must be initialized by a constant expression

error occurred here 
in instantiation of static data member 'phmap::priv::memory_internal::OffsetOf<phmap::priv::memory_internal::IsLayoutCompatible<unsigned long long, unsigned int>::Pair>::kFirst' requested here 
in instantiation of static data member 'phmap::priv::memory_internal::IsLayoutCompatible<unsigned long long, unsigned int>::value' requested here 
in instantiation of function template specialization 'phmap::priv::map_slot_policy<unsigned long long, unsigned int>::transfer<std::allocator<std::pair<const unsigned long long, unsigned int>>>' requested here 
in instantiation of function template specialization 'phmap::priv::FlatHashMapPolicy<unsigned long long, unsigned int>::transfer<std::allocator<std::pair<const unsigned long long, unsigned int>>>' requested here 
in instantiation of function template specialization 'phmap::priv::hash_policy_traits<phmap::priv::FlatHashMapPolicy<unsigned long long, unsigned int>>::transfer_impl<std::allocator<std::pair<const unsigned long long, unsigned int>>, ph... (skipping 4 contexts in backtrace; use -ftemplate-backtrace-limit=0 to see all) 
in instantiation of function template specialization 'phmap::priv::parallel_hash_set<4, phmap::priv::raw_hash_set, std::shared_mutex, phmap::priv::FlatHashMapPolicy<unsigned long long, unsigned int>, phmap::Hash<uint64_t>, phmap::EqualT... 
in instantiation of function template specialization 'phmap::priv::parallel_hash_map<4, phmap::priv::raw_hash_set, std::shared_mutex, phmap::priv::FlatHashMapPolicy<unsigned long long, unsigned int>, phmap::Hash<uint64_t>, phmap::EqualT... 
in instantiation of function template specialization 'phmap::priv::parallel_hash_map<4, phmap::priv::raw_hash_set, std::shared_mutex, phmap::priv::FlatHashMapPolicy<unsigned long long, unsigned int>, phmap::Hash<uint64_t>, phmap::EqualT... 
in instantiation of function template specialization 'phmap::priv::parallel_hash_map<4, phmap::priv::raw_hash_set, std::shared_mutex, phmap::priv::FlatHashMapPolicy<unsigned long long, unsigned int>, phmap::Hash<uint64_t>, phmap::EqualT... 
in instantiation of function template specialization 'phmap::priv::parallel_hash_map<4, phmap::priv::raw_hash_set, std::shared_mutex, phmap::priv::FlatHashMapPolicy<unsigned long long, unsigned int>, phmap::Hash<uint64_t>, phmap::EqualT...

:4387:39:
note: cast that performs the conversions of a reinterpret_cast is not allowed in a constant expression

Declaration location: phmap.h

namespace phmap::priv  
public: 
template<K = key_type, P = Policy, K *  = nullptr> 
MappedReference<P> parallel_hash_map::operator[](key_arg<K> &&key)

end----------------------------------------------------------------------------------------------------------

[greg7mdp]

What compiler are you using within CLion, on what operating system?

[engun]

• Compiler: MSVC
• System: win11 22h2

[greg7mdp]

Hum, weird, I cannot reproduce the issue with Visual Studio 2022.
Do you have a custom version of the macro offsetof in your code? Either that or you use an older compiler which provides a non constexpr version of offsetof.

[engun]

Yes, VS doesn't show this error and, in fact, doesn't affect CLION compilation, but my OCD tells me to raise an issue.
after a brief study, this error is from the Clang prompt.