Using an instance of a class inside another class

Question

Here is a simplified version of what I'm trying to do

class Firstclass
    {
    public:
    Firstclass(int x)
        {
        //do things with x;
        }
    };

class Secondclass
    {
    public:
    Secondclass()
        {
        Firstclass a(10);
        }

    void func()
        {
        //Do things with a
        }

    private:
    Firstclass a;
    };

So I have a class (Firstclass) with a constructor that takes an int argument. Now I'd like to create an instance of that class inside the constructor of another class (Secondclass).

The lines

private:
Firstclass a;

if what I'd do if a were just a variable instead of a class: mention it first such that I can use it elsewhere (in the function func() for instance). This doesn't seem to work with classes, because the compiler doesn't understand what the constructor of Secondclass is supposed to do.

How do I do this correctly?

Answer 1

Initialize it through the member-initializer list:

Secondclass() : a(10) { }

This is required because Firstclass doesn't have a default constructor. Also, in-class initialization with parameters gets ambiguated with a function declaration, so you can do it in the class body. In C++11, this is resolved with aggregate-initialization:

Firstclass a{10};