Example Unity Script

gistfile1.cs

using UnityEngine;
using System.Collections;
// We need this one for importing our IOS functions
using System.Runtime.InteropServices;

public class TestPlugin : MonoBehaviour
{
// Use this #if so that if you run this code on a different platform, you won't get errors.
#if UNITY_IPHONE
[DllImport ("__Internal")]
private static extern int _pow2(int x);

// For the most part, your imports match the function defined in the iOS code, except char* is replaced with string here so you get a C# string.    
[DllImport ("__Internal")]
private static extern string _helloWorldString();

[DllImport ("__Internal")]
private static extern string  _combineStrings(string cString1, string cString2);

#endif

// Now make methods that you can provide the iOS functionality

static int Pow2(int x)
{
    int pow = 0;
    // We check for UNITY_IPHONE again so we don't try this if it isn't iOS platform.
#if UNITY_IPHONE
    // Now we check that it's actually an iOS device/simulator, not the Unity Player. You only get plugins on the actual device or iOS Simulator.
    if (Application.platform == RuntimePlatform.IPhonePlayer)
    {
        pow = _pow2(x);
    }
#endif
    // TODO:  You could test for Android, PC, Mac, Web, etc and do something with a plugin for them here.

    return pow;
}

static string HelloWorldString()
{
    string helloWorld = "";
    // We check for UNITY_IPHONE again so we don't try this if it isn't iOS platform.
#if UNITY_IPHONE
    // Now we check that it's actually an iOS device/simulator, not the Unity Player. You only get plugins on the actual device or iOS Simulator.
    if (Application.platform == RuntimePlatform.IPhonePlayer)
    {
        helloWorld = _helloWorldString();
    }
#endif
    // TODO:  You could test for Android, PC, Mac, Web, etc and do something with a plugin for them here.

    return helloWorld;
}

static string CombineStrings(string string1, string string2)
{
    string combinedString = "";
    // We check for UNITY_IPHONE again so we don't try this if it isn't iOS platform.
#if UNITY_IPHONE
    // Now we check that it's actually an iOS device/simulator, not the Unity Player. You only get plugins on the actual device or iOS Simulator.
    if (Application.platform == RuntimePlatform.IPhonePlayer)
    {
        combinedString = _combineStrings(string1, string2);
    }
#endif
    // TODO:  You could test for Android, PC, Mac, Web, etc and do something with a plugin for them here.

    return combinedString;
}