参数中的C ++协方差 [英] C++ covariance in parameters

问题描述

我想知道为什么C ++不支持参数中的协方差，例如下面的示例，或者如果有一种方法来实现它？

I wanted to know why C++ does not support co-variance in parameters like in example below or if there is a way to achieve it?

class base {
public:
virtual base* func(base * ptr) { return new base(); }
};

class derived : public base {
 public:
 virtual derived* func(derived * ptr) override { return new derived(); } //not allowed
};

推荐答案

返回类型是允许的，因为 derived 继承自 base ，但是函数参数不能工作 - 不是所有 base 实例也将是派生。在 func 被指向 base 的指针调用的情况下应该发生的，参数不是 derived ？最派生的实现不可调用。

The return type is permissible since derived inherits from base, but the function parameter can't work - not all base instances will be a derived also. What's supposed to happen in the cases where func is called on a pointer to base with a parameter that's not a derived? The most derived implementation isn't callable.

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