Declaring as a class field instead of global variable

Question

Look at this very basic C++ code:

class Class1 {

};

Class1 c1;

class Class2
{
public:
    Class2(Class1 &c) 
    {

    }
};

// Class2 abcd(c1);    // OK outside declaration

class Class3
{
public:
    Class2 abcd(c1); // Declaration of abcd as field -> error: unknown type name 'c1'
};

There is something i do not understand about abcd declaration: It works if i declare it as a global variable. But i have a compiler error if i declare it as a field inside Class3.

Answer 1

The default member initializer (since C++11) for non-static data member could be only used with brace or equals initializer, but not parentheses initializer.

Through a default member initializer, which is simply a brace or equals initializer included in the member declaration, which is used if the member is omitted in the member initializer list

If a member has a default member initializer and also appears in the member initialization list in a constructor, the default member initializer is ignored.

So you can

class Class3
{
public:
    Class2 abcd{c1};          // brace initializer
    Class2 abcd = Class2(c1); // equals initializer
};