Token pasting

[John E]

Suppose I want to call a function based on the value of some variable - e.g.

if (some_var == 1) {
      helper1_func ();
} else if (some_var == 2) {
      helper2_func ();
}

Obviously that's quite simple but if some_var can have 20 different values it'd soon start to get pretty convoluted. Is there a cleverer way to do this (e.g. with token pasting or something like that?)

[VictorN]

Can you use switch statement?

[John E]

Yeah, I could use switch or even an array of function pointers but I'm just wondering if there's some cleverer way

I've a vague feeling I once saw something like this done with token pasting but I could easily be wrong about that...

[laserlight]

I would think that using the preprocessor for this is a hack compared to just using an array of function pointers.

[John E]

Oops, good point.! The value of some_var won't even be known until run time so I must be getting mixed up with something else...

[wolle]

if there's some cleverer way

If you need a mapping then a clever way is to use an std::unordered_set/map. It's fast (O(1)) and flexible (selections can be added/removed also at runtime).

Another clever way is to use Object Oriented programming (OO). OO uses so called dynamic polymorphism to avoid the kind of selections you describe in your original post. In principle "arrays of function pointers" are used and in languages with OO support they're part of the language. In C++ they're known as virtual function tables (vtables) and a function call that's directed via a vtable is called a dynamic dispatch.

There are perfectly legitimate reasons to use explicit mappings in OO of course but generally you avoid them by OO design (and all polymorphic mappings involved will take place automagically behind the curtain). So if you tend to have lots of if-else chains, switches and jump tables in your code to express logical flow, consider an OO design.

[Marc G]

Using macros works. Usually macros are frowned upon, but in certain cases they can be useful.
Another option, maybe you can use the command pattern.
See also: https://stackoverflow.com/questions/...ode-dispatcher

[2kaud]

You can also use a lambda rather than a function - so can capture variables by ref if needed. Consider the code below (which is some test code for something slightly different but the general point is still there)

#include <iostream>
#include <utility>
#include <map>
#include <functional>
using namespace std;

enum class Color {Red, Blue};
enum class Size {Small, Medium, Large};

void process(Color c, Size s)
{
	using cskey = pair<Color, Size>;
	using csfunc = function<void()>;
	using csmap = map<cskey, csfunc>;

	static const csfunc rs = []() {cout << "rs\n"; };
	static const csfunc rm = []() {cout << "rm\n"; };
	static const csfunc rl = []() {cout << "rl\n"; };
	static const csfunc bs = []() {cout << "bs\n"; };
	static const csfunc bm = []() {cout << "bm\n"; };
	static const csfunc bl = []() {cout << "bl\n"; };

	static const csmap mapping {{{Color::Red, Size::Small}, rs}, {{Color::Red, Size::Medium}, rm}, {{Color::Red, Size::Large}, rl},
							{{Color::Blue, Size::Small}, bs}, {{Color::Blue, Size::Medium}, bm}, {{Color::Blue, Size::Large}, bl}};

	if (auto mit = mapping.find({c, s}); mit != mapping.end())
		mit->second();
	else
		throw (runtime_error("Bad key"));
}

int main()
{
	try {
		process(Color::Blue, Size::Small);
	}
	catch (const runtime_error& re) {
		cout << re.what() << endl;
	}
}