我有一个设计模式,其中有一个对象生成器(MorselGenerator及其子),其任何实例总是生成相同的确切类型的对象少量食物和它的孩子),但类型检查员不会让我对这些生成的物体中的两个或多个进行任何操作,相信它们可能会有所不同。斯卡拉类型推断未能注意到这些类型是相同的,无论它们是什么
我怎样才能让这个过去的类型检查?
trait Morsel
{
type M <: Morsel
def calories : Float
def + (v : M) : M
}
trait MorselGenerator
{
type Mg <: Morsel
def generateMorsel : Mg
}
class HotDog(c : Float, l : Float, w : Float) extends Morsel
{
type M = HotDog
val calories : Float = c
val length : Float = l
val width : Float = w
def + (v : HotDog) : HotDog = new HotDog(v.calories + calories, v.length + length, v.width + width)
}
class HotDogGenerator extends MorselGenerator
{
type Mg = HotDog
def generateMorsel : HotDog = new HotDog(500.0f, 3.14159f, 445.1f)
}
object Factory
{
def main (args : Array[String])
{
val hdGen = new HotDogGenerator()
println(eatTwo(hdGen))
}
def eatTwo (mGen : MorselGenerator)
{
val v0 : mGen.Mg = mGen.generateMorsel
val v1 : mGen.Mg = mGen.generateMorsel
v0 + v1 /// ERROR HERE
}
}
编译器生成以下编译错误
Generator.scala:43: error: type mismatch;
found : v1.type (with underlying type mGen.Mg)
required: v0.M
v0 + v1 /// ERROR HERE
^one error found
更新
这里是C++代码,那就是我想要做的更多或更少的等价物。请注意,eatTwo函数是完全多态的,并且没有提及特定派生类型的Morsel或MorselGenerator。
#include <stdlib.h>
#include <stdio.h>
template <class M> class Morsel
{
public:
Morsel(float c) : calories(c) {}
float calories;
virtual M operator + (const M& rhs) const = 0;
};
template <class M> class MorselGenerator
{
public:
virtual M * generateMorsel() const = 0;
};
class HotDog : public Morsel<HotDog>
{
public:
HotDog(float c, float l, float w) : Morsel<HotDog>(c), length(l), width(w) {}
float length, width;
HotDog operator + (const HotDog& rhs) const
{ return HotDog(calories+rhs.calories, length+rhs.length, width+rhs.width); }
};
class HotDogGenerator : public MorselGenerator<HotDog>
{
HotDog * generateMorsel() const { return new HotDog(500.0f, 3.14159f, 445.1f); }
};
///////////////////////////////////////////////
template <class MorselType> float eatTwo (const MorselGenerator<MorselType>& mGen)
{
MorselType * m0 = mGen.generateMorsel();
MorselType * m1 = mGen.generateMorsel();
float sum = ((*m0) + (*m1)).calories;
delete m0; delete m1;
return sum;
}
int main()
{
MorselGenerator<HotDog> * morselStream = new HotDogGenerator();
printf("Calories Ingested: %.2f\n", eatTwo(*morselStream));
delete morselStream;
}
也许这将有助于:http://stackoverflow.com/questions/9198562/scala-self-type-and-this-type-in-collections-issue – tuxSlayer 2012-02-22 20:01:46