Question

我正在使用XNA框架制作游戏，因此我使用了许多对矢量进行操作的函数。（尤其是Vector2（64位结构））。困扰我的是大多数方法都是用ref和out参数定义的。这是一个例子：

void Min(ref Vector2 value1, ref Vector2 value2, out Vector2 result)

我看起来有点奇怪。还有另一个Min更明显

public static Vector2 Min(Vector2 value1, Vector2 value2);

基本上，几乎所有函数都有ref和out s的重载。类似，其他APIs。

这种设计有什么好处？ XNA针对性能进行了优化，可能是结果吗？说，Quaternion需要128b，其中ref更少。

编辑：

这是一个测试代码：

public class Game1 : Microsoft.Xna.Framework.Game
{
    GraphicsDeviceManager graphics;
    SpriteBatch spriteBatch;

    private Vector2 vec1 = new Vector2(1, 2);
    private Vector2 vec2 = new Vector2(2, 3);
    private Vector2 min;
    private string timeRefOut1;
    private string timeRefOut2;
    private SpriteFont font;

    public Game1()
    {
        graphics = new GraphicsDeviceManager(this);
        Content.RootDirectory = "Content";

        refOut1();
        refOut2();
    }

    private Vector2 refOut1()
    {
        Vector2 min = Vector2.Min(vec1, vec2);
        return min;
    }

    private Vector2 refOut2()
    {
        Vector2.Min(ref vec1, ref vec2, out min);
        return min;
    }

    protected override void Initialize()
    {
        const int len = 100000000;
        Stopwatch stopWatch = new Stopwatch();
        stopWatch.Start();
        for (int i = 0; i < len; i++)
        {
            refOut1();
        }
        stopWatch.Stop();

        timeRefOut1 = stopWatch.ElapsedMilliseconds.ToString();

        stopWatch.Reset();
        stopWatch.Start();
        for (int i = 0; i < len; i++)
        {
            refOut2();
        }
        stopWatch.Stop();

        timeRefOut2 = stopWatch.ElapsedMilliseconds.ToString();

        base.Initialize();
    }

    protected override void LoadContent()
    {
        spriteBatch = new SpriteBatch(GraphicsDevice);
        font = Content.Load<SpriteFont>("SpriteFont1");
    }

    protected override void Update(GameTime gameTime)
    {
        if (GamePad.GetState(PlayerIndex.One).Buttons.Back == ButtonState.Pressed)
            this.Exit();

        base.Update(gameTime);
    }

    protected override void Draw(GameTime gameTime)
    {
        GraphicsDevice.Clear(Color.CornflowerBlue);

        spriteBatch.Begin();
        spriteBatch.DrawString(font, timeRefOut1, new Vector2(200, 200), Color.White);
        spriteBatch.DrawString(font, timeRefOut2, new Vector2(200, 300), Color.White);
        spriteBatch.End();

        // TODO: Add your drawing code here

        base.Draw(gameTime);
    }
}

结果：

refOut1 2200
refOut2 1400

Win 7 64bit，.Net 4. XNA 4.0

也是IL代码

.method public hidebysig static void  Min(valuetype Microsoft.Xna.Framework.Vector2& value1,
                                          valuetype Microsoft.Xna.Framework.Vector2& value2,
                                          [out] valuetype Microsoft.Xna.Framework.Vector2& result) cil managed
{
  // Code size       69 (0x45)
  .maxstack  3
  IL_0000:  ldarg.2
  IL_0001:  ldarg.0
  IL_0002:  ldfld      float32 Microsoft.Xna.Framework.Vector2::X
  IL_0007:  ldarg.1
  IL_0008:  ldfld      float32 Microsoft.Xna.Framework.Vector2::X
  IL_000d:  blt.s      IL_0017
  IL_000f:  ldarg.1
  IL_0010:  ldfld      float32 Microsoft.Xna.Framework.Vector2::X
  IL_0015:  br.s       IL_001d
  IL_0017:  ldarg.0
  IL_0018:  ldfld      float32 Microsoft.Xna.Framework.Vector2::X
  IL_001d:  stfld      float32 Microsoft.Xna.Framework.Vector2::X
  IL_0022:  ldarg.2
  IL_0023:  ldarg.0
  IL_0024:  ldfld      float32 Microsoft.Xna.Framework.Vector2::Y
  IL_0029:  ldarg.1
  IL_002a:  ldfld      float32 Microsoft.Xna.Framework.Vector2::Y
  IL_002f:  blt.s      IL_0039
  IL_0031:  ldarg.1
  IL_0032:  ldfld      float32 Microsoft.Xna.Framework.Vector2::Y
  IL_0037:  br.s       IL_003f
  IL_0039:  ldarg.0
  IL_003a:  ldfld      float32 Microsoft.Xna.Framework.Vector2::Y
  IL_003f:  stfld      float32 Microsoft.Xna.Framework.Vector2::Y
  IL_0044:  ret
} // end of method Vector2::Min

和

.method public hidebysig static valuetype Microsoft.Xna.Framework.Vector2 
        Min(valuetype Microsoft.Xna.Framework.Vector2 value1,
            valuetype Microsoft.Xna.Framework.Vector2 value2) cil managed
{
  // Code size       80 (0x50)
  .maxstack  3
  .locals init (valuetype Microsoft.Xna.Framework.Vector2 V_0)
  IL_0000:  ldloca.s   V_0
  IL_0002:  ldarga.s   value1
  IL_0004:  ldfld      float32 Microsoft.Xna.Framework.Vector2::X
  IL_0009:  ldarga.s   value2
  IL_000b:  ldfld      float32 Microsoft.Xna.Framework.Vector2::X
  IL_0010:  blt.s      IL_001b
  IL_0012:  ldarga.s   value2
  IL_0014:  ldfld      float32 Microsoft.Xna.Framework.Vector2::X
  IL_0019:  br.s       IL_0022
  IL_001b:  ldarga.s   value1
  IL_001d:  ldfld      float32 Microsoft.Xna.Framework.Vector2::X
  IL_0022:  stfld      float32 Microsoft.Xna.Framework.Vector2::X
  IL_0027:  ldloca.s   V_0
  IL_0029:  ldarga.s   value1
  IL_002b:  ldfld      float32 Microsoft.Xna.Framework.Vector2::Y
  IL_0030:  ldarga.s   value2
  IL_0032:  ldfld      float32 Microsoft.Xna.Framework.Vector2::Y
  IL_0037:  blt.s      IL_0042
  IL_0039:  ldarga.s   value2
  IL_003b:  ldfld      float32 Microsoft.Xna.Framework.Vector2::Y
  IL_0040:  br.s       IL_0049
  IL_0042:  ldarga.s   value1
  IL_0044:  ldfld      float32 Microsoft.Xna.Framework.Vector2::Y
  IL_0049:  stfld      float32 Microsoft.Xna.Framework.Vector2::Y
  IL_004e:  ldloc.0
  IL_004f:  ret
} // end of method Vector2::Min

似乎开销是由temp Vector引起的。我也试过1GHz WP 7.5设备：

1979
1677

一个数量级的迭代次数的刻度数。

Answer 1

Vector2是一个结构体，这意味着当它作为值返回时，将返回一个副本，而不是返回对现有结构的引用。通过使用ref / out参数，您可以避免此副本，以便在Min方法中创建的Vector是result变量中的确切向量。

这是通常不鼓励的那些微优化之一，但在游戏世界中它经常做得足够，并且在性能足够重要的环境中，值得稍微不那么可读的选项。

Answer 2

Servy提到的性能效率之外的另一个区别是能够拥有多个“返回”值：而不是按常规方式返回它们，将它们列为ref / var参数。

使用out / ref与return相比有什么好处？

2 个答案: