使用基类指针调用基类的值构造函数的语法是什么？

Question

基类有一个接受参数的值构造函数。它没有默认构造函数。

下面使用了一个base 类指针，它设置为等于堆上某个derived 类对象的地址。这显式调用了derived 的值构造函数。有没有办法在这一行显式调用base 的值构造函数？我理解另一种选择是您可以让派生类构造函数调用基类构造函数。

base *ptr = new derived(args);

Answer 1

您可以在您的行中显式调用base 的值构造函数，如下所示（但请注意参数列表不同）

   #include<iostream>

    class Base{

    public:
        Base(int arg){std::cout<<"\nBase constructor";}
        Base()=default;
        virtual ~Base(){}
    };

    class Derived:public Base{
    public:
        Derived(double arg){
            std::cout<<"\nDerived constructor";
        }
        using Base::Base;//This is the key for the answer

    };


    int main(){

        Base* derivedPtr1 {new Derived{0.2}};//calls derived ctor
        Base* derivedPtr2 {new Derived{2}};//calls base ctor
        delete derivedPtr2;
        delete derivedPtr1;

    }

没有默认ctor的代码

#include<iostream>

class Base{

public:
    Base(int arg){std::cout<<"\nBase constructor";}
    virtual ~Base(){}

};

class Derived:public Base{
int k{};
public:
    Derived(double arg):Base(2){
        std::cout<<"\nDerived constructor";
    }
    using Base::Base;
    Derived()=default;
    ~Derived(){std::cout<<"\nDerived destructor";}

};


int main(){

    Base* derivedPtr1 {new Derived{0.2}};
    Base* derivedPtr2 {new Derived{2}};
    delete derivedPtr2;
    delete derivedPtr1;

}