Is it possible to detect namespace membership in C++?

Question

For C++ types, the header gives us many useful compile-time reflection capabilities. E.g. std::is_base_of::value de

Answer 1

You can test whether the namespace is accessible (looked up by the compiler), via ADL, from the type.

Suppose that we want to check if type A comes from namespace foo, we can try to use a type that appears only in foo (e.g. a generic function foo::foo_inner_func(T&&)) via the use of A to see if we reach the namespace. If we do it in a SFINAE context then this can result with the answer we are looking for: whether namespace foo is accessible via A.

In many cases that would be the answer of whether the type belongs to this namespace, but in some cases it may identify a namespace as accessible by ADL even though the type doesn't come from this namespace. For example if A is from namespace foo and B which derives from A is from another namespace, B still "sees" foo via ADL. Also std::vector<A> "sees" foo via ADL (and also "sees" std via ADL).

The idea of using ADL was already presented here: Check if a type is from a particular namespace.

Here is the macro version that allows querying any type (almost) for any namespace (almost):

#define create_ns_checker(ns) \
namespace ns { \
    template <typename T> \
    constexpr std::true_type ns##FindmeNsADLHelper(T&&); \
} \
namespace ns##_type_traits { \
    class ns##SecondBestMatchType {}; \
    class ns##BestExactMatchType : public ns##SecondBestMatchType {}; \
    namespace helpers { \
        template <typename T> \
        auto TestNs(ns##_type_traits::ns##BestExactMatchType) \
               -> decltype(ns##FindmeNsADLHelper(std::declval<T>())); \
        template <typename T> \
        auto TestNs(ns##_type_traits::ns##SecondBestMatchType) \
               -> std::false_type; \
    } \
    template <typename T> \
    constexpr bool ns##IsFindmeNs() { \
        return decltype(helpers::TestNs<std::decay_t<T>> \
                           (ns##BestExactMatchType{}))::value; \
    } \
}

#define is_in_ns(Type, ns) \
(ns##_type_traits::ns##IsFindmeNs<Type>())

A small printing utility:

#define print_is_in_ns(Type, ns) \
[]() { \
    std::cout << #Type << " in " << #ns << ": "  \
              << is_in_ns(Type, ns) << std::endl; \
}()

Creating the checkers with the macro:

create_ns_checker(findme)
create_ns_checker(other)
create_ns_checker(std)

Checking it for the following types:

namespace other {
    struct B {};
}

struct C {};

namespace findme {
    struct A {};

    namespace inner {
        struct A {};
    }
    create_ns_checker(inner)
}

Testing in findme context:

namespace findme {
    void test() {
        using namespace other;
        // add the below in and the results change, as it should!
          // using inner::A;
        using std::string;
        std::cout << std::boolalpha;
        print_is_in_ns(int, std);          // false
        print_is_in_ns(string, std);       // true
        print_is_in_ns(A, findme);         // true
        print_is_in_ns(A, inner);          // false
        print_is_in_ns(inner::A, findme);  // false
        print_is_in_ns(inner::A, inner);   // true
        print_is_in_ns(B, findme);         // false
        print_is_in_ns(B, other);          // true
        print_is_in_ns(C, findme);         // false
    }
}

Testing in main:

int main() {
    using std::string;
    using findme::A;
    std::cout << std::boolalpha;
    print_is_in_ns(int, std);                 // false
    print_is_in_ns(string, std);              // true
    print_is_in_ns(string, findme);           // false
    print_is_in_ns(findme::A, findme);        // true
    print_is_in_ns(findme::inner::A, findme); // false
    print_is_in_ns(other::B, findme);         // false
    print_is_in_ns(other::B, other);          // true
    print_is_in_ns(C, findme);                // false
    print_is_in_ns(std::vector<A>, findme); // falsely says true :-(
    print_is_in_ns(std::vector<A>, std);      // true
    std::cout << "-----------------" << std::endl;
    findme::test();
}

Code: https://godbolt.org/z/8Ed89v

Answer 2

A namespace is not a valid template parameter, so it could never be a class trait. Perhaps you can do something obscure with macros though. You could maybe inject functions in the test namespace and use ADL together with a sizeof/decltype trick to see which overload gets picked.