Question

I would like to do something like:

for(int i=0;i<10;i++)
    addresses[i] = & function(){ callSomeFunction(i) };

Basically, having an array of addresses of functions with behaviours related to a list of numbers.

If it’s possible with external classes like Boost.Lambda is ok.

Edit: after some discussion I’ve come to conclusion that I wasn’t explicit enough. Please read Creating function pointers to functions created at runtime

---

What I really really want to do in the end is:

class X
{
    void action();
}

X* objects;

for(int i=0;i<0xFFFF;i++)
    addresses[i] = & function(){ objects[i]->action() };


void someFunctionUnknownAtCompileTime()
{

}

void anotherFunctionUnknowAtCompileTime()
{

}

patch someFunctionUnknownAtCompileTime() with assembly to jump to function at addresses[0]

patch anotherFunctionUnknownAtCompileTime() with assembly to jump to function at addresses[1]

sth, I don’t think your method will work because of them not being real functions but my bad in not explaining exactly what I want to do.

Answer 1

If I understand you correctly, you’re trying to fill a buffer with machine code generated at runtime and get a function pointer to that code so that you can call it.

It is possible, but challenging. You can use reinterpret_cast<> to turn a data pointer into a function pointer, but you’ll need to make sure that the memory you allocated for your buffer is flagged as executable by the operating system. That will involve a system call (LocalAlloc() on Windows iirc, can’t remember on Unix) rather than a “plain vanilla” malloc/new call.

Assuming you’ve got an executable block of memory, you’ll have to make sure that your machine code respects the calling convention indicated by the function pointer you create. That means pushing/popping the appropriate registers at the beginning of the function, etc.

But, once you’ve done that, you should be able to use your function pointer just like any other function.

It might be worth looking at an open source JVM (or Mono) to see how they do it. This is the essence of JIT compilation.