rule_of_zero_example.cpp

/***********************************************************
*
* Example based on an idea from the article "Rule of Zero"
* URL: https://blog.rmf.io/cxx11/rule-of-zero
*
* Must be compiled as C++11
************************************************************/

#define _SCL_SECURE_NO_WARNINGS
#define UNICODE
#include <iostream>
#include <memory>
#include <type_traits>
#include <Windows.h>

using namespace std;

// declare a function pointer to MessageBox API from User32.dll
using FunctionPtr = std::add_pointer<int __stdcall (HWND, LPCWSTR, LPCWSTR, UINT)>::type; 
// alternative declaration
// using FunctionPtr = int (__stdcall *)(HWND, LPCWSTR, LPCWSTR, UINT);

// this class loads/unloads a DLL automatically
// no need for explicit constructors, destructor etc.
class module{
public:
	explicit module(wstring const& name)
	: handle { ::LoadLibraryW(name.c_str()), &FreeLibraryInternal } {}
   
        // define a wrapper for Windows' FreeLibrary
	static bool FreeLibraryInternal(void* dll) {
	   return (true == ::FreeLibrary(static_cast<HMODULE>(dll)));
	}

private:
        // declare a module_handle as unique_ptr to void + custom deleter function FreeLibraryInternal
	using module_handle = unique_ptr<void, decltype(&module::FreeLibraryInternal)>;
	module_handle handle;

public: 
	bool isValid() {
	   return handle != nullptr;
	}

	HMODULE const getHandle() {
	   return static_cast<HMODULE>(this->handle.get());
	}

};

// function that calls the MessageBoxW API
void showMsgBox(wstring const message, module& const mod) {
   if (mod.isValid()) {
      FunctionPtr ptr = reinterpret_cast<FunctionPtr>(::GetProcAddress(mod.getHandle(), "MessageBoxW"));
      ptr(nullptr, message.c_str(), L"Message", MB_OK);
   }
}

int main(int argc, char **argv) {
  // load DLL
  module mod{ L"User32.dll" };
  showMsgBox(L"hello", mod);

  return 0;
}