Dynamically creating a C++ function argument list at runtime

Question

I'm trying to generate an argument list for a function call during runtime, but I can't think of a way to accomplish this in c++.

This is for a helper library I'm writing. I'm taking input data from the client over a network and using that data to make a call to a function pointer that the user has set previously. The function takes a string(of tokens, akin to printf), and a varying amount of arguments. What I need is a way to add more arguments depending on what data has been received from the client.

I'm storing the functions in a map of function pointers

typedef void (*varying_args_fp)(string,...);
map<string,varying_args_fp> func_map;

An example usage would be

void printall(string tokens, ...)
{
    va_list a_list;
    va_start(a_list, tokens);

    for each(auto x in tokens)
    {
        if (x == 'i')
        {
            cout << "Int: " << va_arg(a_list, int) << ' ';
        }
        else if(x == 'c')
        {
            cout << "Char: " << va_arg(a_list, char) << ' ';
        }
    }

    va_end(a_list);
}

func_map["printall"] = printall;
func_map["printall"]("iic",5,10,'x');
// prints "Int: 5 Int: 10 Char: x"

This works nicely when hardcoding the function call and it's arguments, but if I've received the data "CreateX 10 20", the program needs to be able to make the argument call itself. eg

// func_name = "CreateX", tokens = 'ii', first_arg = 10, second_arg = 20
func_map[func_name](tokens,first_arg,second_arg);

I can't predict how users are going to lay out the functions and code this beforehand.

If anyone has suggestions on accomplishing this task another way, feel free to suggest. I need the user to be able to "bind" a function to the library, and for the library to call it later after it has received data from a networked client, a callback in essence.

Answer 1

Here is a C++11 solution. It does not support varargs functions like printall or printf, this is impossible with this technique and IMO impossible at all, or at the very least extremely tricky. Such function are difficult to use safely in an environment like yours anyway, since any bad request from any client could crash the server, with absolutely no recourse whatsoever. You probably should move to container-based interface for better safety and stability.

On the other hand, this method supports all (?) other functions uniformly.

#include <vector>
#include <iostream>
#include <functional>
#include <stdexcept>
#include <string>
#include <boost/any.hpp>


template <typename Ret, typename... Args>
Ret callfunc (std::function<Ret(Args...)> func, std::vector<boost::any> anyargs);

template <typename Ret>
Ret callfunc (std::function<Ret()> func, std::vector<boost::any> anyargs)
{
    if (anyargs.size() > 0)
        throw std::runtime_error("oops, argument list too long");
    return func();
}

template <typename Ret, typename Arg0, typename... Args>
Ret callfunc (std::function<Ret(Arg0, Args...)> func, std::vector<boost::any> anyargs)
{
    if (anyargs.size() == 0)
        throw std::runtime_error("oops, argument list too short");
    Arg0 arg0 = boost::any_cast<Arg0>(anyargs[0]);
    anyargs.erase(anyargs.begin());
    std::function<Ret(Args... args)> lambda =
        ([=](Args... args) -> Ret {
         return func(arg0, args...);
    });
    return callfunc (lambda, anyargs);
}

template <typename Ret, typename... Args>
std::function<boost::any(std::vector<boost::any>)> adaptfunc (Ret (*func)(Args...)) {
    std::function<Ret(Args...)> stdfunc = func;
    std::function<boost::any(std::vector<boost::any>)> result =
        ([=](std::vector<boost::any> anyargs) -> boost::any {
         return boost::any(callfunc(stdfunc, anyargs));
         });
    return result;
}

Basically you call adaptfunc(your_function), where your_function is a function of any type (except varargs). In return you get an std::function object that accepts a vector of boost::any and returns a boost::any. You put this object in your func_map, or do whatever else you want with them.

Types of the arguments and their number are checked at the time of actual call.

Functions returning void are not supported out of the box, because boost::any<void> is not supported. This can be dealt with easily by wrapping the return type in a simple template and specializing for void. I've left it out for clarity.

Here's a test driver:

int func1 (int a)
{
    std::cout << "func1(" << a << ") = ";
    return 33;
}

int func2 (double a, std::string b)
{
    std::cout << "func2(" << a << ",\"" << b << "\") = ";
    return 7;
}

int func3 (std::string a, double b)
{
    std::cout << "func3(" << a << ",\"" << b << "\") = ";
    return 7;
}

int func4 (int a, int b)
{
    std::cout << "func4(" << a << "," << b << ") = ";
    return a+b;
}


int main ()
{
    std::vector<std::function<boost::any(std::vector<boost::any>)>> fcs = {
        adaptfunc(func1), adaptfunc(func2), adaptfunc(func3), adaptfunc(func4) };

    std::vector<std::vector<boost::any>> args =
    {{777}, {66.6, std::string("yeah right")}, {std::string("whatever"), 0.123}, {3, 2}};

    // correct calls will succeed
    for (int i = 0; i < fcs.size(); ++i)
        std::cout << boost::any_cast<int>(fcs[i](args[i])) << std::endl;

    // incorrect calls will throw
    for (int i = 0; i < fcs.size(); ++i)
        try {
            std::cout << boost::any_cast<int>(fcs[i](args[fcs.size()-1-i])) << std::endl;
        } catch (std::exception& e) {
            std::cout << "Could not call, got exception: " << e.what() << std::endl;
        }
}