Storing function pointer in std::function

问题

I'm trying to write a C++0x wrapper around dlopen()/dlsym() to dynamically load functions from shared objects:

class DynamicLoader
{
  public:
    DynamicLoader(std::string const& filename);

    template<class Signature>
      std::function<Signature> load(std::string const& functionName);

  private:
    void *itsLibraryHandle;
};


DynamicLoader::DynamicLoader(std::string const& filename)
{
  itsLibraryHandle = dlopen(filename.c_str(), RTLD_LAZY);

  if(!itsLibraryHandle) 
  { /* Throw Some Error */ }
}

  template<class Signature>
std::function<Signature> DynamicLoader::load(std::string const& functionName)
{
  return <insert magic here> dlsym(itsHandle, functionName.c_str());
}

Is there a way to convert the void* function pointer returned by dlsym into a std::function?

回答1:

try this:

static_cast<Signature*>()

seems works in VC10

complete test:

#include <functional>

void test()
{}

template <typename Signature>
std::function<Signature> cast(void* f)
{
    return static_cast<Signature*>(f);
}

int main()
{
    std::function<void()> f = cast<void()>(&test);
    return 0;
}

回答2:

Based on what I see here: http://pubs.opengroup.org/onlinepubs/009695399/functions/dlsym.html

#include <boost/function_types/components.hpp>
#include <boost/function_types/function_pointer.hpp>

template< typename Signature >
std::function<Signature> DynamicLoader::load(std::string const& name)
{
  namespace ft = boost::function_types;
  typedef typename ft::function_pointer< typename ft::components<Signature>::type >::type fp_t;
  fp_t fun_ptr;

  *reinterpret_cast<void**>(&fun_ptr) = dlsym(itsHandle, name.c_str());

  return fun_ptr;
}

I've never used dlsym so I don't understand why the cast is being done that way and not simply casting dlsym's return like so:

fun_ptr = reinterpret_cast<fp_t>(dlsym(itsHandle, name.c_str());

回答3:

You just need to cast result of dlsym() call to a proper type. Here's a complete working example:

#include <functional>
#include <iostream>
#include <stdexcept>
#include <string>
#include <dlfcn.h>

class DynamicLoader
{
public:
    DynamicLoader(std::string const& filename) :
        m_handle(dlopen(filename.c_str(), RTLD_LAZY))
    {
        if (!m_handle)
        {
            throw std::logic_error("can't load library named \"" + filename + "\"");
        }
    }

    template<class T>
    std::function<T> load(std::string const& functionName) const
    {
        dlerror();
        void* const result = dlsym(m_handle, functionName.c_str());
        if (!result)
        {
            char* const error = dlerror();
            if (error)
            {
                throw std::logic_error("can't find symbol named \"" + functionName + "\": " + error);
            }
        }

        return reinterpret_cast<T*>(result);
    }

private:
    void* const m_handle;
};

int main()
{
    DynamicLoader const loader("/lib64/libz.so.1");
    auto const zlibVersion = loader.load<char const* (void)>("zlibVersion");
    std::cout << "zlib version: " << zlibVersion() << std::endl;
    return 0;
}

来源：https://stackoverflow.com/questions/4770968/storing-function-pointer-in-stdfunction