using hash_value() for in different namespace

[ghost]

Hi. I'm trying to implement a hash_value() from this example, but I'm getting a compilation error

full gist

#include <iostream>

#include <boost/describe.hpp>
#include <boost/mp11.hpp>
#include <boost/container_hash/hash.hpp>

#include <gtest/gtest.h>

#if BOOST_VERSION < 108100

template <class T, class Bd = boost::describe::describe_bases<T, boost::describe::mod_any_access>,
          class Md = boost::describe::describe_members<T, boost::describe::mod_any_access>>
std::size_t hash_value(const T& t) {
    std::size_t r = 0;
    boost::mp11::mp_for_each<Bd>([&](auto D) {
        using B = typename decltype(D)::type;
        boost::hash_combine(r, (const B&)t);
    });
    boost::mp11::mp_for_each<Md>([&](auto D) { boost::hash_combine(r, t.*D.pointer); });
    return r;
}

#endif

namespace namespace_a {
struct PODTypeN1 {
    int a;
    int b;
    int c;
};
BOOST_DESCRIBE_STRUCT(PODTypeN1, (), (a, b, c))
} // namespace namespace_a

namespace namespace_b {
struct PODTypeN2 {
    float d;
    double e;
    bool f;
};
BOOST_DESCRIBE_STRUCT(PODTypeN2, (), (d, e, f))
} // namespace namespace_b

namespace namespace_c {
struct ComplexPodTypeN {
    namespace_a::PODTypeN1 podType1;
    namespace_b::PODTypeN2 podType2;
};
BOOST_DESCRIBE_STRUCT(ComplexPodTypeN, (), (podType1, podType2))
} // namespace namespace_c

namespace test {

TEST(test_hash, ComplexPodType_with_namespace) {
    namespace_c::ComplexPodTypeN a = {
        {1, 2, 3},
        {0.3, 0.4, true}
    };
    namespace_c::ComplexPodTypeN b = a;

    auto hash_a = boost::hash<namespace_c::ComplexPodTypeN>()(a);
    auto hash_b = boost::hash<namespace_c::ComplexPodTypeN>()(b);
    std::cout << "hash_a = " << hash_a << std::endl;
    std::cout << "hash_b = " << hash_b << std::endl;
    ASSERT_TRUE((hash_a == hash_b));

    b.podType2.e = 42.42;
    hash_b = boost::hash<namespace_c::ComplexPodTypeN>()(b);
    std::cout << "hash_a = " << hash_a << std::endl;
    std::cout << "hash_b = " << hash_b << std::endl;
    ASSERT_TRUE((hash_a != hash_b));
}

} // namespace test

Scanning dependencies of target boost_describe
[ 50%] Building CXX object CMakeFiles/boost_describe.dir/test_hash.cpp.o
In file included from /home/pftest/.conan/data/boost/1.78.0/_/_/package/4c0d21de97108c6489e888fd2e50a95c38b9f3aa/include/boost/container_hash/hash.hpp:766:0,
                 from /home/pftest/Documents/boost_describe3/test_hash.cpp:5:
/home/pftest/.conan/data/boost/1.78.0/_/_/package/4c0d21de97108c6489e888fd2e50a95c38b9f3aa/include/boost/container_hash/extensions.hpp: In instantiation of ‘std::size_t boost::hash<T>::operator()(const T&) const [with T = namespace_c::ComplexPodTypeN; std::size_t = long unsigned int]’:
/home/pftest/Documents/boost_describe3/test_hash.cpp:62:64:   required from here
/home/pftest/.conan/data/boost/1.78.0/_/_/package/4c0d21de97108c6489e888fd2e50a95c38b9f3aa/include/boost/container_hash/extensions.hpp:305:30: error: no matching function for call to ‘hash_value(const namespace_c::ComplexPodTypeN&)’
             return hash_value(val);

[pdimov]

This will work as-is under Boost 1.81, but if you need to support earlier versions, you have to define the overloads of hash_value in the corresponding namespace of the type. The easiest way to do it is to add a using declaration for your existing hash_value:

#if BOOST_VERSION < 108100
using ::hash_value;
#endif

after each BOOST_DESCRIBE_STRUCT, as in https://godbolt.org/z/M1n3Me7eY.

You can also put the common definition of hash_value into a namespace of your own, it doesn't need to be global.

(Unrelated, unless you're on C++20, you can also add using boost::describe::operator== and using boost::describe::operator!= to generate the equality operators, as these are generally required to use the hash containers.)

[ghost]

ths