Factory function returning unique_ptr with custom deleter

Question

I'm making a helper function for loading shared libs and putting the result into a std::unique_ptr with a custom deleter (which is the module destruction function).

This works fine when I'm not setting custom deleter but as soon as I add them I get an error saying that the type of the custom deleter cannot be deduced (which is fair).

The thing is, if I where to specify the deleter type when I call the function, it will end up looking very ugly.

The question is, how can my function deduce automatically the type of the custom deleter ? And how should I declare a std::vector of the resulting std::unique_ptr s ?

My code (I'm also open for any suggestion on the code):

template <typename T, typename D>
std::unique_ptr<T, D> openLib(const std::string &lib_path,
                       const std::string &csym = "create",
                       const std::string &dsym = "destroy")
{
  void *handle;
  if (!(handle = dlopen(lib_path.c_str(), RTLD_LAZY)))
  {
    std::cerr << "dlopen " << dlerror() << std::endl;
    return nullptr;
  }
  T *(*create)();
  if (!(create = (T * (*)()) dlsym(handle, csym.c_str())))
  {
    std::cerr << "dlsym " << csym << dlerror() << std::endl;
    return nullptr;
  }
  void (*destroy)();
  if (!(destroy = (void (*)()) dlsym(handle, dsym.c_str())))
  {
    std::cerr << "dlsym " << dsym << dlerror() << std::endl;
    return nullptr;
  }
  auto cDel = [destroy](T *lib) { destroy(); };
  std::unique_ptr<T, decltype(cDel)> lib_ptr((T *)create(), cDel);
  return lib_ptr;
}

Answer 1

Write a deleter type and factory.

using deleter = void();
using ptr_deleter = deleter*;
struct lib_unloader {
  deleter* del = 0;
  void operator()(void*)const { if (del) del(); }
  explicit operator bool() const { return del; }
};
lib_unloader get_lib_unloader( void* handle, char const* destroy_name ) {
  auto del = ptr_deleter( dlsym( handle, destroy_name ) );
  return {del};
}
template<class T>
using lib_up = std::unique_ptr< T, lib_unloader >;

template<class T>>
lib_up<T> openLib(const std::string &lib_path,
                       const std::string &csym = "create",
                       const std::string &dsym = "destroy")
{
  void *handle;
  if (!(handle = dlopen(lib_path.c_str(), RTLD_LAZY)))
  {
    std::cerr << "dlopen " << dlerror() << std::endl;
    return nullptr;
  }
  T *(*create)();
  if (!(create = (T * (*)()) dlsym(handle, csym.c_str())))
  {
    std::cerr << "dlsym " << csym << dlerror() << std::endl;
    return nullptr;
  }
  auto destroy = get_lib_unloader( handle, dsym.c_str() );
  if (!destroy) {
    std::cerr << "dlsym " << dsym << dlerror() << std::endl;
    return nullptr;
  }
  return {create(), destroy};
}

myself, I'd write a better handle. Note that your code leaks dynamic library handles like crazy.

using libhandle_ptr = std::unique_ptr<void, std::integral_constant<int(*)(void*), dlclose>>;

libhandle_ptr make_libhandle(const char* filename, int flags) {
  return libhandle_ptr( dlopen(filename, flags) );
}
template<class T>
T* get_sym( libhandle_ptr const& handle, const char* symbol ) {
  if (!handle || !symbol) return nullptr;
  return static_cast<T*>( dlsym( handle.get(), symbol ) );
}

now we get:

lib_unloader get_lib_unloader( libhandle const& handle, char const* destroy_name ) {
  auto del = get_sym<deleter>( handle, destroy_name );
  return {del};
}
template<class T>>
lib_up<T> openLib( libhandle_ptr const& handle,
                       const char* csym = "create",
                       const char* dsym = "destroy")
{
  if (!handle)
    return {};
  auto create = get_sym<T*()>( handle, csym );
  if (!create)
  {
    std::cerr << "dlsym " << csym << dlerror() << std::endl;
    return {};
  }
  auto destroy = get_lib_unloader( handle, dsym.c_str() );
  if (!destroy) {
    std::cerr << "dlsym " << dsym << dlerror() << std::endl;
    return {};
  }
  return {create(), destroy};
}

where users are expected to provide a libhandle_ptr when they wantna symbol.

Note that boost has a dll symbol loading library that probably does a better job.