C++ Function Map - c++

Question

How will I call the functions?
Lets say I have 3 display functions
void disp1()
{
std::cout<<"\n Disp1 ";
}
void disp2()
{
std::cout<<"\n Disp2 ";
}
void disp3()
{
std::cout<<"\n Disp3 ";
}
I want to create a map of these functions and call them according to the key.
fMap["d1"]
should run that disp1 function. How will I go about this;
If the function has return type other than void I am able to get the value but what about the statements that I need to print. Like the ones above;

Answer 1

You can do this with std::map and std::function:
#include <map>
#include <string>
#include <iostream>
#include <functional>
void disp1()
{
std::cout<<"\n Disp1 ";
}
void disp2()
{
std::cout<<"\n Disp2 ";
}
void disp3()
{
std::cout<<"\n Disp3 ";
}
int main()
{
// add the functions as an initializer-list
std::map<std::string, std::function<void()>> fMap
{
{"d1", disp1}
, {"d2", disp2}
, {"d3", disp3}
};
// or add them one at a time
fMap["d1"] = disp1;
fMap["d2"] = disp2;
fMap["d3"] = disp3;
fMap["d1"](); // call them using ()
fMap["d2"]();
fMap["d3"]();
}

Answer 2

To store the lookup index, you would use std::map. This comprises key-value pairs (mymap[ key ] = value). C++ allows you to store a pointer to a function, here I use the using keyword to create a type name, func_t, of the type of fingerprint our function will have: void (*)(void) (this is how you say "pointer to a function which takes void and returns void).
#include <iostream>
#include <string>
#include <map>
// the functions we intend to map
void disp1()
{
std::cout<<"Disp1\n";
}
void disp2()
{
std::cout<<"Disp2\n";
}
void disp3()
{
std::cout<<"Disp3\n";
}
int main() {
// create a new type, func_t, which describes a pointer
// to a void function that takes no parameters (void).
using func_t = void (*)(void);
// declare a map from a string key to a func_t value,
// and initialize it with a mapping of f1->disp1, f2->disp2
// and f3->disp3
std::map<std::string, func_t> functionMap = {
{ "f1", disp1 }, { "f2", disp2 }, { "f3", disp3 }
};
// declare a string for reading input
std::string input;
// loop until there is no more input on std::cin.
while (std::cin.good()) {
// prompt
std::cout << "Which disp (f1, f2, f3)? ";
// fetch the next line of text from cin, without the \n
std::getline(std::cin, input);
// if the input is empty we ran out of input or the user
// input a blank line. either way, stop.
if (input.empty())
break;
std::cout << "You chose " << input << "\n";
// look for the key in the map. if the key is not found,
// it will equal the special iterator functionMap.end()
auto it = functionMap.find(input);
// If it's not functionMap.end then we have a valid iterator.
// it->first is the key, it->second is the function pointer.
if (it != functionMap.end()) {
// to use a function pointer, just add the () with any
// arguments after the variable name.
// remember, it->second is the function pointer.
it->second();
} else {
std::cout << "Invalid entry.\n";
}
}
}
Live demo: http://ideone.com/4Xlow1

Answer 3

The answer to your first question is to use std::function in your map.
std::map<std::string, std::function<void>> fMap {
{"d1", disp1},
{"d2", disp2},
{"d3", disp3}
};
Your second question about having a return value I would need to think about.

Answer 4

Solved with the help of #Galik's comment
#include <iostream>
#include <functional>
#include <algorithm>
#include <map>
typedef std::function<void()> func_t;
typedef std::map<std::string, func_t> func_t_map;
void disp1()
{
std::cout<<"\n Display 1 "<<std::endl;
return;
}
void disp2()
{
std::cout<<"\n Display 2 "<<std::endl;
return;
}
void disp3()
{
std::cout<<"\n Display 3 "<<std::endl;
return;
}
void disp4()
{
std::cout<<"\n Display 4 "<<std::endl;
return;
}
int main()
{
func_t_map fMap;
fMap["d1"] = disp1;
fMap["d2"] = disp2;
fMap["d3"] = disp3;
fMap["d4"] = disp4;
fMap["d2"]();
return 0;
}