如何在另一个类中创建一个类的参数化构造函数作为数据成员？

Question

In below example,

class Car
{
        private:
                int sides;

        public:
                Car()
                {
                        cout<<"\ndefault called constructor";
                }
                Car(int nsides)
                {
                        cout<<"\ncalled constructor";
                        sides=nsides;
                }

};

class Auto
{
        private:
                Car ch;
        public:
        Auto(int a) : Car(a)
        {

                //Car test(5);
        }
};

int main()
{
        Auto at(5);
        return 0;

}

After referring below links :-

create objects in object passing variables through constructor

http://www.cplusplus.com/forum/beginner/9746/

I tried to write the same and execute it.unfortunately I am getting following compiler error :-

check.cpp: In constructor ‘Auto::Auto(int)’:
check.cpp:44: error: type ‘Car’ is not a direct base of ‘Auto’

If solution mentioned in the given links are correct then what wrong in my code ? My next query is ...why only parametrized constructor() throws compiler if try to initialize it without using initialization list.
This will throw compiler error :-

class Auto
{
        private:
                Car ch(5);
        public:
        Auto(int a)
        {

        }
};

But this does not :-

class Auto
{
        private:
                Car ch;
        public:
        Auto(int a)
        {

        }
};

Please help me in understanding this behaviour.
Thanks in advance.

Answer 1

In your example you are specifying by your constructor Auto(int a) : Car(a) that Auto is derived from Car, and that's what the compiler complains about.

To initialize your Car object (inside of Auto), do this Auto(int a) : ch(a) . You put the type instead of the member's name.

About your second question, in-class member initialization is a new feature brought by C++11. You may use it by adding the parameter -std=c++11 to your compiler (GCC or Clang, msvc doesn't support it). See this question . In your case you can use it as chris pointed out :

class Auto {
// ...
Car ch{5};
int someVal = 5;
// ...
};