CODEWITHSUNDEEP
c#unity-game-engine
Victor Cash
Victor Cash

Reputation: 57

Static var lost its value after exit a method

I'm using unity 5.5, and I met this problem.

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class WhatTheHell : MonoBehaviour
{
    public static int testVal;

    void Awake()
    {
        SetVal(testVal);
        Debug.Log(testVal);
    }

    void SetVal(int val)
    {
        val = 10;
    }
}

The debug result is 0 insted of 10. why?

Upvotes: 1

Views: 129

Answers (4)

Brett Reinhard
Brett Reinhard

Reputation: 382

To avoid other mistakes of this type I would encourage you to read this: https://www.cs.umd.edu/class/sum2003/cmsc311/Notes/Mips/stack.html the

Upvotes: 0

MrVoid
MrVoid

Reputation: 709

The problem is that int is a value type and not a reference type.

When you pass 'testVal' to 'SetVal()', it make a copy of the value of 'testVal' into 'val' just for the method scope.

If you use the keyword ref, 'val' argument will be handle as a reference type

void SetVal(ref int val)

SetVal(ref testVal);

More about value and reference types here : https://msdn.microsoft.com/en-us/library/9t0za5es.aspx

Upvotes: 2

Faraz Ahmed
Faraz Ahmed

Reputation: 1607

you can perform this operation as multiple ways by making Global variable , ref Keyword or simple return int from function. as describe

Global Variable

public static int testVal=0;

void Awake()
{
    SetVal();
    Debug.Log(testVal); // print 10
}

void SetVal()
{
    testVal = 10;
}

ref Keyword

public static int testVal=0;

void Awake()
{
    SetVal(ref testVal);
    Debug.Log(testVal); // print 10
}

void SetVal(ref int testVal)
{
    testVal = 10;
}

return int

public int testVal=0;

void Awake()
{
    testVal = SetVal();
    Debug.Log(testVal); // print 10
}

int SetVal()
{
    return 10;
}

Upvotes: 3

sujith karivelil
sujith karivelil

Reputation: 29006

Here you are defining the testVal as static so it will be available in all methods inside the class(you can access them outside the class as well through the class name that is WhatTheHell.testVal). So actually there is no need for passing the variable in this case.

Then you are passing the variable testVal to the SetVal() method as pass by value, so it will pass the value only not the actual variable. that is why the change is not reflected the actual variable.

The following code will work as you expected:

public static int testVal=0;

void Awake()
{
    Debug.Log(testVal); // print 0
    SetVal();
    Debug.Log(testVal); // print 10
}

void SetVal()
{
    testVal = 10;
}

For more detailed explanation and example you can take a look into the Story of Pass By Value and Pass By Reference in C# by Ehsan Sajjad.

Upvotes: 2

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