Could I create a function based on another function at runtime?

I'm playing with Microsoft's Detours to hook api, for example, I can change what happens when MessageBoxA is called in this way:

  int (WINAPI* pMessageBoxA)(HWND, LPCTSTR, LPCTSTR, UINT) = MessageBoxA;

  int WINAPI MyMessageBoxA(HWND hWnd, LPCTSTR lpText, LPCTSTR lpCaption, UINT uType)
  {
      printf("A function is called here!\n");
      return pMessageBoxA(hWnd, lpText, lpCaption, uType);  // call the regular MessageBoxA
  }

  DetourTransactionBegin();
  DetourUpdateThread(GetCurrentThread());
  DetourAttach(&(PVOID&)pMessageBoxA, MyMessageBoxA); 

So when you call MessageBoxA, you are actually calling MyMessageBoxA.

Now I want to write a function Hook(), which would do what codes above do at runtime. For example, if I pass function pointer MessageBoxA to the function, it will do exactly what the above开发者_Python百科 code did.

Of course, I can pass other function pointer to it too.

Then there is a question, when I get a function pointer in Hook, how could I define a function with the same return value and parameter as the given function(in this case, MessageBoxA to int WINAPI MyMessageBoxA(HWND hWnd, LPCTSTR lpText, LPCTSTR lpCaption, UINT uType)) and then fill the function's function body?

In C++, functions aren't first-class object, that means they cannot be created at runtime.

However, you can use an array of function-pointers, each pointer pointing to an already defined function, and choosing the appropriate function-pointer at runtime based on some conditions, and call it. And it looks like you're already using function-pointer in the code-snippet.

Which isn't entirely true. You can easily store a (member) function reference, so you can have a function call another function (decidable at runtime).

You can also use a functor which is a struct/class olverloading the () operator. This can then use the class's state to remember which actual function to call. An example of a functor:

STL has a <functional> header that contains a load of helpful utilities to make handling (member) function references 'easier' (slightly). Random example from cplusplus.com:

// mem_fun example
#include <iostream>
#include <functional>
#include <vector>
#include <algorithm>
#include <string>
using namespace std;

int main () 
{
  vector <string*> numbers;

  // populate vector of pointers:
  numbers.push_back ( new string ("one") );
  numbers.push_back ( new string ("two") );
  numbers.push_back ( new string ("three") );
  numbers.push_back ( new string ("four") );
  numbers.push_back ( new string ("five") );

  vector <int> lengths ( numbers.size() );

  transform (numbers.begin(), numbers.end(), lengths.begin(), mem_fun(&string::length));

  for (int i=0; i<5; i++) {
      cout << *numbers[i] << " has " << lengths[i] << " letters.\n";
  }
  return 0;
}

c++0x has a lot of nifty new features (including 'auto' type inference and lambda expressions) that will make a lot of this easier