CODEWITHSUNDEEP
c++function-pointers
nwatson
nwatson

Reputation: 33

Getting function pointers from specific files

I have a lot (perhaps hundreds) of different c++ files. Each one contains 10 functions, all of them taking in an int and a double and returning an int.

So the pointer to one of these functions in one of these files would look like this:

int (*foo)(int, double);

And then I have a class, which contains 10 of these function pointers.

Is it possible to have the constructor of this class take in a file name of one of these c++ files, put that file's functions into its pointers, and be able to use the functions later?

Preferably it would work so that even if two functions from different files had the same name it would still work (the idea is that multiple programmers could submit different files into the list, and they might use the same names for their 10 functions), but if that's not possible I could figure out something to avoid that.

From what I've searched, I can't seem to find anything that lets you differentiate between files when choosing functions, and even if I were to concatenate the functions into one file, there's still the problem of trying to designate which 10 functions to pick (as they all have the same arguments).

Is there any way to do this? Is there any better solution that I'm just not thinking of?

Upvotes: 0

Views: 245

Answers (3)

bobah
bobah

Reputation: 18864

To achieve what you ask literally, you can turn each C++ file into a dynamic library and in the library constructor register the set of functions in some global map using FILE string as a key or declare them static, and make sure each file has some static variable whose initializer triggers functions' registration in the same global map.

To implement the same using plugin approach you can do it the way similar to what I have done it here (just download plugin.tgz, article is not yet ready). Contents:

  • app.cc - application, loads all plugins' libraries
  • module.cc - a plugin class implementing a business interface module_ifc.h and a "loadable" interface bootstrap_ifc.h
  • client.cc - a plugin implementing bootstrap_ifc.h and using method from module.cc resolved at runtime

Each of your C++ files having the same set of function would have a class implementing abstract business interface (useful part, all your functions) and bootstrap interface (unified initialization part, will be used by plugin loader). Each such class would be put into a separate shared library that declares class instance constructor and destructor methods.

A simple class to work with shared libraries on Linux:

#include <dlfcn.h>

class library {
  void* _handle;
public:
  library(char const* path);
  ~library();

  template <typename F> F func(char const* name);
};

library::library(char const* path) {
  _handle = dlopen(path, RTLD_NOW);
  if (!_handle) throw std::runtime_error(dlerror());
  std::clog << "opened library " << path << ", handle=" << std::hex << _handle << std::dec << "\n";
}

library::~library() {
  if (_handle) dlclose(_handle);
  std::clog << "closed library, handle=" << std::hex << _handle << std::dec << "\n";
}

template <typename F> F library::func(char const* name) {
  dlerror();
  F func = reinterpret_cast<F>(dlsym(_handle, name));
  const char *dlsym_error = dlerror();
  if (dlsym_error) throw std::runtime_error(dlsym_error);
  std::clog << "loaded symbol " << name << ", ptr=" << std::hex << ((void*)func) << std::dec << "\n";
  return func;
}

Upvotes: 0

cnicutar
cnicutar

Reputation: 182724

Is there any way to do this? Is there any better solution that I'm just not thinking of?

You could just use different namespaces for them I think ? I mean each group of 10 functions in their own namespace; that way they won't conflict any more.

Other than that, you could try some dlsym + dlopen weirdness (or their win32 counterparts). It's not something I would do though.

Upvotes: 1

Crashworks
Crashworks

Reputation: 41472

I believe you are describing a dynamic linking library (aka shared object in Linux-land).

Upvotes: 0

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