float? (with question mark), what it is?

I`m trying to do trajectory path for my game, and I found useful script to do that, but for first time I see use of float? in the code. I thought it is some code mistake on website, but apparently VS reconize this, but I can`t find any documentation about that. What does it do? Here is code sample:

Code (CSharp):

1.     void RotateGun()
2.     {
3.         //Get the 2 angles
4.         float? highAngle = 0f;
5.         float? lowAngle = 0f;
6.  
7.         CalculateAngleToHitTarget(out highAngle, out lowAngle);
8.  
9.         //Artillery
10.         float angle = (float)highAngle;
11.         //Regular gun
12.         //float angle = (float)lowAngle;
13.  
14.         //If we are within range
15.         if (angle != null)
16.         {
17.             //Rotate the gun
18.             //The equation we use assumes that if we are rotating the gun up from the
19.             //pointing "forward" position, the angle increase from 0, but our gun's angles
20.             //decreases from 360 degress when we are rotating up
21.             gunObj.localEulerAngles = new Vector3(360f - angle, 0f, 0f);
22.  
23.             //Rotate the turret towards the target
24.             transform.LookAt(targetObj);
25.             transform.eulerAngles = new Vector3(0f, transform.rotation.eulerAngles.y, 0f);
26.         }
27.     }
28.  
29.     //Which angle do we need to hit the target?
30.     //Returns 0, 1, or 2 angles depending on if we are within range
31.     void CalculateAngleToHitTarget(out float? theta1, out float? theta2)
32.     {
33.         //Initial speed
34.         float v = bulletSpeed;
35.  
36.         Vector3 targetVec = targetObj.position - gunObj.position;
37.  
38.         //Vertical distance
39.         float y = targetVec.y;
40.  
41.         //Reset y so we can get the horizontal distance x
42.         targetVec.y = 0f;
43.  
44.         //Horizontal distance
45.         float x = targetVec.magnitude;
46.  
47.         //Gravity
48.         float g = 9.81f;
49.  
50.  
51.         //Calculate the angles
52.  
53.         float vSqr = v * v;
54.  
55.         float underTheRoot = (vSqr * vSqr) - g * (g * x * x + 2 * y * vSqr);
56.  
57.         //Check if we are within range
58.         if (underTheRoot >= 0f)
59.         {
60.             float rightSide = Mathf.Sqrt(underTheRoot);
61.  
62.             float top1 = vSqr + rightSide;
63.             float top2 = vSqr - rightSide;
64.  
65.             float bottom = g * x;
66.  
67.             theta1 = Mathf.Atan2(top1, bottom) * Mathf.Rad2Deg;
68.             theta2 = Mathf.Atan2(top2, bottom) * Mathf.Rad2Deg;
69.         }
70.         else
71.         {
72.             theta1 = null;
73.             theta2 = null;
74.         }
75.     }

 

its float that can be also null:
https://www.dotnetperls.com/nullable-int

 

If you want even more information C# has a System defined Struct called Nullable<T>
https://msdn.microsoft.com/en-us/library/b3h38hb0.aspx

Code (CSharp):

1. float? highAngle;

is just shorthand for

Code (CSharp):

1. Nullable<float> highAngle = new Nullable<float>();

In general in C# anything that is a reference type variable (like classes) are nullable inherently and can't use the ? syntax

Value type variables (floats, int, structs) can't be null so you need to use ? to make them able to equal a null. (this is different than equaling 0).

 

Ah, and I was searching for this! I mean nullable float, and I didn`t knew it was so simple! So it means that this code has flaw - angle isn`t declared as nullable - so angle != null won`t work.

Thank You!!!

 

If you rewrite CalculateAngleToHitTarget to return true/false to indicate if it could find an angle instead of putting null in the out parameters, you can drop the whole nullable thing.

 

You are right, I also never worked with custom "out" parameters, didn`t thought about using function return with out parameter, thanks!

 

You probably haven't seen it much because making a struct nullable isn't very popular in C#. There is seldom a need for it, and in many cases it makes program logic harder to follow.

 

Its rarely useful for returning things. Like @Baste said just using bool to return true/false works just as well as setting the parameter to null. However, I have found it handy when you taking a struct as an input parameter. You might have a struct with say 6 variables in it that the you fill in and pass to the function, but sometimes you just want to use the default ones so you can just pass in a null. Then the function checks for null and uses all the default values.