bond::is_writer does not work for derived types if they don't implement WriteStructBegin

[biocomp]

Repro (I copied check_method and is_writer from bond's headers): https://godbolt.org/g/2F76N9

The way is_writer works - it tries to assign member function pointer from T::WriteStructBegin. When derived class relies on base's WriteStructBegin, this fails because it tries to assign base's pointer to member function to derived one.

I'm also guessing is_writer is not the only problem.

Copy of repro above:

#include <type_traits>
#include <boost/type_traits.hpp>

struct Metadata
{};

struct Base
{
    void WriteStructBegin(const Metadata&, bool) {}
};

struct Derived : Base {};

template <typename T, T> struct 
check_method
    : std::true_type {};

template <typename T, typename Enable = void> struct
is_writer
    : std::false_type {};

template <typename T> struct
is_writer<T, typename boost::enable_if<check_method<void (T::*)(const Metadata&, bool), &T::WriteStructBegin>>::type>
     : std::true_type {};

int main()
{
    static_assert(is_writer<Base>::value, "!Base is writer");
    static_assert(is_writer<Derived>::value, "!Derived is writer"); // Fails
}

[chwarr]

Is there a deeper problem that this is causing? We need to know that to make sure that the fix addresses the core problem too.

Are you, for example, hitting the static assert in the Serialization transform?

As a work around, try adding an explicit specialization of is_writer for your derived type:

template <> struct
is_writer<Derived> : std::true_type {};

[biocomp]

Yes, this was exaclty the issue - I hit that assert. And yes, I did figure out this workaround, or I can just implement the method in derived class too. I just filed the issue that future users won't have to suffer :)

[chwarr]

Thanks for confirming it was the static assert. We'll make sure the fix actually fixes that case.