因为您没有默认构造函数，编译器会抱怨它无法创建对象 primary ，你应该在 secondary 构造函数中添加一个参数/给它一个默认值：

Because you don't have a default constructor the compiler complains that it can't create an object for primary, you should either add a parameter to the secondary constructor / give it a default value:

class secondary : public primary
{
    public:
        secondary(int x);
        virtual ~secondary();
    protected:
    private:
};

然后调用基类构造函数：

And then call the base class constructor:

secondary::secondary(int x) : primary(x)
{
    //ctor
}

或者：

secondary::secondary() : primary(5)
{
    //ctor
}

或者只为主要添加默认构造函数：

Or just add a default constructor for primary:

class primary
{
    public:
        primary(int x);
        primary() : primary_x(0) {}
        virtual ~primary();
    protected:
    private:
        int primary_x;
};