在 C++ 中创建未知派生类的实例

发布于 2024-11-19 12:07:49 字数 354 浏览 3 评论 0原文

假设我有一个指向某个基类的指针，并且我想创建该对象的派生类的新实例。我该怎么做？

class Base
{
    // virtual
};

class Derived : Base
{
    // ...
};


void someFunction(Base *b)
{
    Base *newInstance = new Derived(); // but here I don't know how I can get the Derived class type from *b
}

void test()
{
    Derived *d = new Derived();
    someFunction(d);
}

原文

let's say I have a pointer to some base class and I want to create a new instance of this object's derived class. How can I do this?

class Base
{
    // virtual
};

class Derived : Base
{
    // ...
};


void someFunction(Base *b)
{
    Base *newInstance = new Derived(); // but here I don't know how I can get the Derived class type from *b
}

void test()
{
    Derived *d = new Derived();
    someFunction(d);
}

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醉南桥 2024-11-26 12:07:49

克隆

struct Base {
   virtual Base* clone() { return new Base(*this); }
};

struct Derived : Base {
   virtual Base* clone() { return new Derived(*this); }
};


void someFunction(Base* b) {
   Base* newInstance = b->clone();
}

int main() {
   Derived* d = new Derived();
   someFunction(d);
}

这是一个非常典型的模式。

创建新对象

struct Base {
   virtual Base* create_blank() { return new Base; }
};

struct Derived : Base {
   virtual Base* create_blank() { return new Derived; }
};


void someFunction(Base* b) {
   Base* newInstance = b->create_blank();
}

int main() {
   Derived* d = new Derived();
   someFunction(d);
}

虽然我不认为这是典型的事情；在我看来，它有点代码味道。您确定需要它吗？

Cloning

struct Base {
   virtual Base* clone() { return new Base(*this); }
};

struct Derived : Base {
   virtual Base* clone() { return new Derived(*this); }
};


void someFunction(Base* b) {
   Base* newInstance = b->clone();
}

int main() {
   Derived* d = new Derived();
   someFunction(d);
}

This is a pretty typical pattern.

Creating new objects

struct Base {
   virtual Base* create_blank() { return new Base; }
};

struct Derived : Base {
   virtual Base* create_blank() { return new Derived; }
};


void someFunction(Base* b) {
   Base* newInstance = b->create_blank();
}

int main() {
   Derived* d = new Derived();
   someFunction(d);
}

Though I don't think that this a typical thing to do; it looks to me like a bit of a code smell. Are you sure that you need it?

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