Calling a function using a string containing the function's name

Question

I have a class defined as

class modify_field 
{
    public:
        std::string modify(std::string str)
        {
            return str;
        }

};

Is there any way to store this function name inside a string in main function and then call it. I tried this but it's not working.

int main()
{
modify_field  mf;
std::string str,str1,str2;
str = fetch_function_name(); //fetch_function_name() returns string modify
str2 = "check";
cout << str; //prints modify
str1 = str + "(" +str2 + ")";
mf.str1(); 
}

I know this is wrong. But I just want to know if there is any way to call a function name using variable.

Answer 1

This is not directly possible in C++. C++ is a compiled language, so the names of functions and variables are not present in the executable file - so there is no way for the code to associate your string with the name of a function.

You can get a similar effect by using function pointers, but in your case you are trying to use a member function as well, which complicates matters a little bit.

I will make a little example, but wanted to get an answer in before I spend 10 minutes to write code.

Edit: Here's some code to show what I mean:

#include <algorithm>
#include <string>
#include <iostream>
#include <functional>

class modify_field 
{
public:
    std::string modify(std::string str)
        {
            return str;
        }

    std::string reverse(std::string str)
        {
            std::reverse(str.begin(), str.end());
            return str;
        }
};


typedef std::function<std::string(modify_field&, std::string)> funcptr;


funcptr fetch_function(std::string select)
{
    if (select == "forward")
        return &modify_field::modify;
    if (select == "reverse")
        return &modify_field::reverse;
    return 0;
}



int main()
{
    modify_field mf;

    std::string example = "CAT";

    funcptr fptr = fetch_function("forward");
    std::cout << "Normal: " << fptr(mf, example) << std::endl;

    fptr = fetch_function("reverse");
    std::cout << "Reverse: " << fptr(mf, example) << std::endl;
}

Of course, if you want to store the functions in a map<std::string, funcptr>, then that is entirely possible.

Answer 2

You have two choices:

1 - manually create a map with pointers to the functions you need and their identifiers as a string key so you can perform lookups, or..

2 - create a dynamic link library/shared object and use name lookup to get the pointer. Use extern "C" to prohibit identifier mangling. You may not be able to use certain C++ features and performance will be slightly worse, depending on your actual usage scenario.

Answer 3

It surely possible with C functions, but will be really tricky and unportable for C++ - because different OSes (and even different compilers on the same OS) uses different ABIs for C++, so real function names differ quite significantly from what you named them in your code.

If C functions is ok (e.g. you can declare them as extern "C"), you can use dlsym for POSIX OSes and GetProcAddress for windows. Of course, you'll need to add this functions into dynamic symbols table - something like '-rdynamic' flag for ld, or __declspec(dllexport) (hope that was right - haven't used if for a long time) on windows.

Answer 4

Maybe a pointer to this function is more adequate. It allows you to easily chose between functions to call:

#include<iostream>
using namespace std;

class modify_field
{
    public:
        std::string modify_1(std::string str)
        {
            return str;
        }
        std::string modify_2(std::string str)
        {
            return str + str;
        }
};

int main()
{
    string (modify_field::* fun_ptr) (string) = &modify_field::modify_1;
    modify_field m;
    cout << (m.*fun_ptr)("test") << endl;
    fun_ptr = &modify_field::modify_2;
    cout << (m.*fun_ptr)("test") << endl;
}

Answer 5

If the functions have same signature you can create a map of string to std::function objects. See more about std::function here: std::function

Answer 6

If you placed all the functions that you would like to lookup by name in a set of source files that you then generate a DLL with, you could use the dlopen() and dlsym() functions to find the entry point.

An example of how to do this can be found in the man pages of dlsym() on your machine, or you could refer to this link.

This only works well on functions that are unmangled (i.e. with C++ you may have some issues). You may either have to define these entry points as "extern C" to prevent mangling, or come up with some other mechanism to guess the mangled name for a C++ entry point. I have never done the latter so there are bound to be some nuances there that I'm oblivious to.

If you are using Windows (wasn't clear what OS you are on), you should lookup the documentation of GetProcAddress() which is available online here.

A good tutorial on the subject in general, and this one does talk about how to do all this with C++ can be found here.