复制派生类的构造函数，更改基类

Question

Let's assume I have following situation:

class NamedObject{
public:
    NamedObject(const std::string &name):name_{std::move(name)}{}
private: 
    const std::string name_;
}

class Person: public NamedObject{
public:
    Person(const std::string &name, int age): NamedObject(name), age_{age}{}
private:
    int age_;
}

I want to create a "copy constructor" where I copy all members from Person but change the name (for whatever reason).

class Person: public NamedObject{
public:
    Person(const std::string &name, int age): NamedObject(name), age_{age}{}
    Person(const std::string &newName, const Person &other): NamedObject(name){
        age_ = other.age;
    }
private:
    int age_;
}

Now assume I have not only one attribute like age but many attributes and they are changed a lot during development. Is it somehow easily possible to create a function like Person(const std::string &newName, const Person &other) without manually copying all attributes like age_ = other.age; . The idea is that during development I do not have to always recall to change this constructor if I add a new attribute.

Please note that I cannot simply change the name because it is const .

Answer 1

You could define an assignment operator on NamedObject that does nothing. Then you would be able to use *this = other in the "copy constructor":

class NamedObject{
public:
    NamedObject(const std::string &name):name_{name}{}
    NamedObject &operator= (const NamedObject &) {
        return *this;
    }
    const std::string name_;
};


class Person: public NamedObject{
public:
    Person(const std::string &name, int age): NamedObject(name), age_{age}{}
    Person(const std::string &newName, const Person &other): NamedObject(newName){
        *this = other;
    }
    int age_;
};

Live example: https://onlinegdb.com/rJ4J5oy3M

Why it works:

C++ will automatically define a copy assignment operator for eligible classes (check the following link for details on what eligible means). From cppreference :

If the implicitly-declared copy assignment operator is neither deleted nor trivial, it is defined (that is, a function body is generated and compiled) by the compiler if odr-used. For union types, the implicitly-defined copy assignment copies the object representation (as by std::memmove). For non-union class types (class and struct), the operator performs member-wise copy assignment of the object's bases and non-static members, in their initialization order, using built-in assignment for the scalars and copy assignment operator for class types.

Thus, for your example the implicitly defined copy assignment operator (the one that is called when we do *this = other; ) will be defined as follows:

• it will use the copy assignment operator of NamedObject to copy the base class (which we defined to do nothing, so it won't copy name_ ),
• it will use the copy assignment operator for age_ and any additional member you add to the class.