c++ - 在模板派生类中使用协变返回类型覆盖虚拟函数

Question

我想重写模板派生类中的虚函数。但是，我想使用派生类作为返回类型。这是相应的代码:

class Abstract {
  public:
    virtual Abstract* allocate() const = 0;
};
template <typename Derived>
class Base : public Abstract {
  public:
    Derived* allocate() const override {
      return new Derived;
    }
};
class Concrete : public Base<Concrete> {
  public:
};
int main() {
  Concrete c;
  delete c.allocate();
}

不幸的是，我的编译器无法识别 Derived实际上是从 Abstract派生的，并且失败并显示以下错误消息。

mwe.cpp: In instantiation of ‘class Base<Concrete>’:
mwe.cpp:12:25:   required from here
mwe.cpp:8:14: error: invalid covariant return type for ‘Derived* Base<Derived>::allocate() const [with Derived = Concrete]’
     Derived* allocate() const override {
              ^~~~~~~~
mwe.cpp:3:23: note: overridden function is ‘virtual Abstract* Abstract::allocate() const’
 virtual Abstract* allocate() const = 0;
                       ^~~~~~~~

将 allocate函数移到 Concrete类中可以解决此问题，但是在创建多个具体类时会导致代码重复。有没有办法使编译器知道 Derived实际上是从 Abstract派生的？

Answer 1

您可以模拟协变返回类型:
这里的重点是Reintroduce类。没有它，allocate将覆盖虚函数。有了它，它隐藏了继承的函数。

#include <iostream>

class Abstract {
  public:
    virtual Abstract* allocate() const = 0;
};

namespace details{
    class Reintroduce {};

    template <typename Derived>
    class AllocImpl : public Abstract
    {
    public:
        Abstract* allocate() const override {
            return new Derived;
        }
    };
}


template <typename Derived>
class Base : public details::AllocImpl<Derived> 
{
  public:
    Derived* allocate(details::Reintroduce = {}) const 
    {
      return static_cast<Derived*>(details::AllocImpl<Derived>::allocate());
    }
};

class Concrete : public Base<Concrete> {
  public:
};


int main() {
  Concrete c;
  delete c.allocate();
}

如果我们可以不用 AllocImpl来实现它，那就太好了，但这会使调用变得模棱两可。