嵌套类、继承和C++中的共享指针

Nested classes, inheritance and shared pointers in C++

本文关键字：共享 指针 C++ 继承 嵌套      更新时间：2023-10-16

下面的程序显然在返回表达式中失败了：

#include <memory>
class Base {
public:
class Nested {
public:
int c;
};
};
class A : public Base {
public:
class Nested : public Base::Nested {
public:
int c = 1;
};
};
class B : public Base {
public:
class Nested : public Base::Nested {
public:
int c = 2;
};
};
int main() {
std::shared_ptr<Base> X = std::make_shared<A>();
return X::Nested.c;
};

如何获取 X 的Nested.c值？

换句话说，我有一个基类(Base(和两个派生类(A和B(。每个派生类都有一个嵌套类(嵌套(。我想从实例X调用Nested.c，该实例被动态选择为派生类之一。

可能只是对嵌套类的误解。嵌套类不会神奇地将成员添加到其父类。您需要手动执行此操作，例如：

class Base {
public:
class Nested {
public:
int c;
};
Nested nested; // member of nested class Base::Nested.
};

请记住，Base::Nested、A::Nested和B::Nested都是不同的类。虽然它们看起来很相似，但它们根本没有关系。

也许以下是您想要的： 
#include <memory>
class Base {
private:
class Nested {
public:
int c;
};
Nested nested; // member of nested class Base::Nested.
public:
virtual int getC() const { return this->nested.c; }
};
class A : public Base {
private:
class Nested {
public:
int c = 1;
};
Nested nested; // member of nested class A::Nested.
public:
int getC() const override { return this->nested.c; }
};
class B : public Base {
private:
class Nested {
public:
int c = 2;
};
Nested nested; // member of nested class B::Nested.
public:
int getC() const override { return this->nested.c; }
};
int main() {
std::shared_ptr<Base> X = std::make_shared<B>();
return (*X).getC();
};

每个类都有自己的嵌套类的成员，并使用虚拟 getter 返回c。

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