class Base { } class Derived : Base { } public static void Run(IEnumerable<Base> bases) { } IEnumerable<Derived> derivedBases = new Derived[] { new Derived(), new Derived() }; Run(derivedBases);
類似的程式碼在C++就失敗了…
Covariance Example lang: c#
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class Animal { public void Feed() {} } class Frog : Animal { } static void DoSomethingToAFrog(Action<Frog> action, Frog frog) { action(frog); } Action<Animal> feed = animal => { animal.Feed(); }; DoSomethingToAFrog(feed, new Frog());
跟數學裡面Set comprehension的表達方法很像,假設我們想要找出直角三角形中,周長小於50的三元組的數量。 (假設 a <= b <= c 且 a ^ 2 + b ^ 2 = c ^ 2)。
如果是以往的作法,大概會像這樣
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for (int a = 1; a <= 10; a++) #define sqr(x) (x * x) typedef tuple<int, int, int> tuple3; std::vector<tuple3> ans; for (int a = 1; a <= 50; a++) for (int b = a; b <= 50; b++) for (int c = b; c <= 50; c++) if ((sqr(a) + sqr(b) == sqr(c)) && ((a + b + c) < 50)) ans.push_back(make_tuple(a, b, c));