如何返回派生类型? [英] How to return derived type?

问题描述

我有一个Validator类和从其派生的类； 当我尝试返回指向派生类的指针时，方法将返回基类(Validator)而不是派生类.

I have a Validator class and derived classes from it; When i'm trying to return pointer to derived class then method return base class(Validator) instead of Derived.

class Validator
{
public:
    std::string m_name = "BaseValidator";

    static const std::map<std::string, Validator *> validators();

    static Validator *getByName(std::string &name);
};


const std::map<std::string, Validator*> Validator::validators()
{
    std::map<std::string, Validator*> result;
    //RequiredValidator is derived
    result["required"] = new RequiredValidator();
    return result;
}

Validator* Validator::getByName(std::string &name)
{
    auto g_validators = Validator::validators();
    auto validator = g_validators.find(name);
    if(validator != g_validators.end()){
        std::cout << "getByName: " << validator->second->m_name << std::endl;
        return validator->second;
    }else{
        std::cerr << "Unknow type of validator: " << name << std::endl;
    }
    return nullptr;
}

//output BaseValidator but i need RequiredValidator


class RequiredValidator : public Validator
{
public:
    std::string m_name = "RequiredValidator";
};

推荐答案

它正在返回派生实例，但是由于validator是Validator*，因此您正在查看Validator的m_name成员，不是RequiredValidator之一.
(尽管名称相同，但它们是不同的变量.没有虚拟变量".)

It is returning a derived instance, but since validator is a Validator*, you're looking at the m_name member of Validator, not the one of RequiredValidator.
(Despite having the same name, they are distinct variables. There are no "virtual variables".)

有两种选择；

• 您可以拥有一个虚拟的getName函数，并在每个子类中覆盖它.

• You can have a virtual getName function and override it in every subclass.

在派生类中设置 base m_name，例如，通过将该名称作为基本构造函数的参数.

Set the base m_name in derived classes, for instance by making the name a parameter of the base constructor.

示例:

class Validator
{
public:
    Validator(const std::string& name = "BaseValidator") : m_name(name) {};
    // ...
};

class RequiredValidator : public Validator
{
public:
    RequiredValidator() : Validator("RequiredValidator") {}
    // ...
};

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