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Compilation error with std::hash<gsl::not_null<std::shared_ptr<T>>> in GSL 4.2.0 #1209
Description
Describe the bug
The specialization of std::hash for gsl::not_null<std::shared_ptr<T>> does not compile in GSL version 4.2.0.
To Reproduce
#include <gsl/pointers>
void test()
{
std::hash<gsl::not_null<std::shared_ptr<int>>> hasher{};
}Compilation Error
error C2280: 'gsl::not_null_hash<gsl::not_null<std::shared_ptr<int>>,const T &,false>::not_null_hash(void)': attempting to reference a deleted function
error C2280: with
error C2280: [
error C2280: T=std::shared_ptr<int>
error C2280: ]
GSL-4.2.0\include\gsl\pointers(244,5):
see declaration of 'gsl::not_null_hash<gsl::not_null<std::shared_ptr<int>>,const T &,false>::not_null_hash'
with
[
T=std::shared_ptr<int>
]
GSL-4.2.0\include\gsl\pointers(244,5):
'gsl::not_null_hash<gsl::not_null<std::shared_ptr<int>>,const T &,false>::not_null_hash(void)': function was explicitly deleted
with
[
T=std::shared_ptr<int>
]
Expected behavior
The specialization of std::hash for gsl::not_null<std::shared_ptr<T>> should compile successfully.
Spec:
- OS: Windows
- Compiler: MSVC 17.13.6
- C++ Version: C++20
Additional context
In the following template from <gsl/pointers>:
template <class T, class U = decltype(std::declval<const T&>().get()), bool = std::is_default_constructible<std::hash<U>>::value>
struct not_null_hash
{
std::size_t operator()(const T& value) const { return std::hash<U>{}(value.get()); }
};it might be necessary to apply std::remove_cv_t<std::remove_reference_t<...>> to the type U to properly deduce the underlying type and avoid issues with const/reference qualifiers.
Reacted by Copilot
Activity
carsonRadtke commentedon May 29, 2025
Thanks for reporting this issue! I'll investigate and post an update when I know more.
carsonRadtke commentedon Jun 2, 2025
Yes,
Uwas resolving toconst std::shared_ptr<int> &instead ofstd::shared_ptr<int>which was causing further failures to the template instantiations forstd::hash<T>. Your solution of applying the nestedremove_*would have worked, but I think we can just usegsl::not_null<T>::element_typehere instead.carsonRadtke commentedon Jun 2, 2025
Let's update the template for
not_null_hashto grabT::element_typeand use that to construct the hasher.