How to make efficient C++ jump table? - c++

Question

I'm beginner to C++ and I have implemented the following simple jump table, but was wondering if I'm doing it the right way. Is there anyway I can improve the following code?
The following code is using a dictionary (I'm from a C# background) to store functions' pointers.
#include <cstdio>
#include <iostream>
#include <string>
#include <unordered_map>
using namespace std;
void Zero() { printf("Zero\n"); }
void One() { printf("One\n"); }
void Two() { printf("Two\n"); }
void Three() { printf("Three\n"); }
string prompt()
{
printf("Enter number from 0 to 3 or q to quit:\n");
string line;
getline(cin, line);
return line;
}
int main(int argc, const char * argv[]) {
unordered_map<string, void(*)()> map;
map["0"] = Zero;
map["1"] = One;
map["2"] = Two;
map["3"] = Three;
while (true) {
string c = prompt();
if (c == "q") break;
map[c]();
}
return 0;
}

Answer 1

How about a switch statement?
switch (c) {
case 0:
printf("Zero\n"); break;
case 1:
printf("One\n"); break;
case 2:
printf("Two\n"); break;
case 3:
printf("Three\n"); break;
default:
break;
}

Answer 2

There's not much you can do to make your code "faster" without going for the switch solution which breaks the original idea of having an array of functions. If you only gonna use 'characters' such as '0' => '9', 'a' => 'z' you could dodge the memory allocation needed for the string, and you could also initialize your map with an initializer_list, and you could also make such array const static if that's viable.
Here goes my "optimized" code if it helps.
inline char prompt() //this function will probably 900% be inlined even if you don't specify the inlike keyword
{
printf("Enter number from 0 to 3 or q to quit:\n");
char v;
while (!(std::cin >> v)); //Just to make sure we get valid input
return v;
}
int main()
{
static const std::unordered_map<char, void(*)()> mymap =
{
{ '0' , Zero },
{ '1' , One },
{ '2' , Two },
{ '3' , Three }
};
while(1)
{
auto it = mymap.find(prompt());
// Without this check, your program will crash if input is invalid.
if (it != mymap.end())
{
it->second();
break;
}
}
return 0;
}

Answer 3

Pleas provide more details for your case of efficiency. Do you mean memory/cpu cycles/pass-through?
According to your code:
it's not error prone (use auto it = map.find(key); function for searching and check output it != map.end() value, so no new elements will be created)
it's good enough for string key type
you case easily make more flexible by replacing function pointer with std::function<void()>
It terms of more low-level control you can you custom hash function and custom hash tables implementations.
On some data it may be usefull to consider std::map or sorted std::vector as an option.

Answer 4

As static lookup is fast, This will perform very good irrespective of compiler. Jump table differs from compiler to compiler. I would use following code, May be some people will object to this as global are bad. But before commenting, please evaluate this
string prompt()
{
printf("Enter number from 0 to 3 or q to quit:\n");
string line;
getline(cin, line);
return line;
}
enum Choice = {ZERO = 0, ONE, TWO, THREE};
static char *choice_str[] = {
"Zero",
"One",
"Two",
"Three"
};
int main(int argc, const char * argv[]) {
while (true) {
string c = prompt();
if (c == "q")
{
break;
}
else {
assert(atoi(c) >= Choice::ZERO && atoi(c) <=Choice::THREE);
printf("%s\n", choice_str[atoi(c)]);
}
}