c++ reference to function pointer dynamically

Question

I have one application in which following task are to be done

1.) UI application will send command code (integer value).
2.) DLL interface(in c++) will get that integer value and execute corresponding command function.

commands name and command code are maintained as

#define PING 50

there will be 500 commands and applying SWITCH CASE will not sound good. so i decided to implement function pointer in my code as below

   #include "stdafx.h"

    #include<iostream>
    #define PING 20

    using namespace std;
    //extern const int PING = 10; 
    void ping()
    { 
                    cout<<"ping command executed";
    }


    void get_status(void)
    {


    cout<<"Get_status called"<<endl;

    }

    class ToDoCommands
    {
            public:
                void getCommand( void (*CommandToCall)() );                         
    };



    void ToDoCommands::getCommand( void (*CommandToCall)())
    {


        void (*CommandToCall1)();

        CommandToCall1  = CommandToCall;

        CommandToCall1();

    }

    int main()
    {
            int code;
            ToDoCommands obj;
            cout<<"enter command code";
            cin>>code;  // if UI send 50 then Ping function get executed as #define PING 50

            obj.getCommand(ping);   // here m passing ping manually..
            //obj.getCommand(get_status);

                return 0;
    }

how can i pass command name corresponding to command code in

obj.getCommand(ping);

Are the command values defined, or can you pick whichever numbers you like? — Wutz
– Wutz, Commented Sep 17, 2012 at 10:31

Sergey Kalinichenko · Accepted Answer · 2012-09-17 13:06:43Z

3

You are almost there: make a std::map of std::string to function pointer, initialize it with data pairing a string name to a corresponding function pointer, and then use that map at runtime to pick the correct pointer based on the string parameter passed in.

#include <iostream>
#include <string>
#include <map>

using namespace std;

void ping() {
    cout << "ping" << endl;
}
void test() {
    cout << "test" << endl;
}
int main() {
    map<string,void(*)()> m;
    m["ping"] = ping;
    m["test"] = test;
    // I am using hard-coded constants below.
    // In your case, strings will come from command line args
    m["test"]();
    m["ping"]();
    return 0;
}

Link to a demo with std::map.

Here is how you can do it without a map (it will be slower because of the linear search, but you can fix it by ordering names alphabetically and using binary search).

#include <iostream>
#include <cstring>

using namespace std;

void ping() {
    cout << "ping" << endl;
}
void test() {
    cout << "test" << endl;
}
typedef void (*fptr_t)();
int main() {
    const fptr_t fptrs[] = {test, ping};
    const char *names[] = {"test", "ping"};
    const char *fname = "test";
    for (int i = 0 ; i != 2 ; i++) {
        if (!strcmp(fname, names[i])) {
            fptrs[i]();
            break;
        }
    }
    return 0;
}

Link to a demo with arrays.

edited Sep 17, 2012 at 13:06

answered Sep 17, 2012 at 10:32

Sergey Kalinichenko

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9 Comments

Sergey Kalinichenko Over a year ago

@james What do you mean? You use strings already, there's nothing wrong with adding a map. Nobody will be able to tell that you use STL in your dll.

Balwinder SIngh Over a year ago

this dll will be linked with C# and IOs application and also it wont work under linux environment...so i need arrays instead of map

Sergey Kalinichenko Over a year ago

@james Needing arrays instead of a map is not a problem. Can you at least use std::pair?

Frerich Raabe Over a year ago

@james: Is it possible that you don't want to use the STL in functions exported from your DLL because STL does some memory allocations/deallocations (a no-no across DLL boundaries)? If so, you could have the STL as an implementation detail of your DLL and simply expose a void invoke(const char *fn) function.

Sergey Kalinichenko Over a year ago

@NiravMadhani C++ has no reflection capabilities: once the code is compiled, all the names are thrown away.

|

Gant · Accepted Answer · 2012-09-17 10:46:28Z

0

Declare an array of function pointers. Where you treat the index as your "code". For example:

void foo(){
    printf("foo\n");
}

void bar(){
    printf("bar\n");
}

int main(void)
{
    void (*code_to_function[100])();
    int code;

    code_to_function[0] = foo;
    code_to_function[1] = bar;
    printf("Enter code: ");
    scanf("%d", &code);
    code_to_function[code]();

    return 0;
}

Please note that for this rudimentary example, inputting integer code other than 0 and 1 will result in a segfault.

answered Sep 17, 2012 at 10:46

Gant

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Comments

Kamran Amini · Accepted Answer · 2012-09-17 10:48:27Z

I should say @dasblinkenlight is right but if you don't want to use std::map you should implement a map yourself. This can be buggy and not a optimized way, but if you don't want to use STL, it seems you should implement it yourself.

You can use 2 arrays with corresponding indices. One of them is a char * array and another one is function pointers. They are better to be encapsulated in a class named something like MyMap.

class MyMap {
  public:
    ...

    inline void add(char *name, (void (*ptr)(void)) ) {
      names_[currIndex_] = name; // Or stcpy
      ptrs_[currIndex_] = ptr;
      currIndex_++;
    }

    inline (void(*)(void)) get(char *name) {
      int foundIndex = -1;
      for (int i = 0; i < currIndex_; i++) {
        // Find matching index
      }
      if (foundIndex_ >= 0) {
        return ptrs_[foundIndex_];
      }
      return NULL;
    }

  private:
    int currIndex_;
    char *names_[10];
    (void (*ptrs_[10])(void));
};

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