Creating a public variable from another class (C )-CodePudding

If you have two classes, class a and class b, could you create a variable in class a from class b? main.cpp

class A {
    public:
        A() {}
};

class B {
    public:
        B() {
            test = A();
            test.<variable name> = <variable value>;
        }
};

The code above is just an example. It will probably cause an error.

"variable name" doesn't exist in class A. Is there a way to create this variable for class A in the constructor for class B?

CodePudding user response：

No, C is not Javascript. Types are strict and, after you define a type, there's no way to modify it.

You can, however, create a local type in a function:

class B {
public:
    B() {
        struct A_extended : A {
            int i;
        };
        auto test = A_extended();
        test.i = 1;
    }
};

CodePudding user response：

What you need is to provide proper constructors, member variables (and possibly getters). Or you need to add a setter to a and use m_a.set_value(42).

#include <iostream>

class A
{
public:
    // constructor for A with a value. 
    // made explicit to avoid implicit type conversions from int to A
    explicit A(int value) : 
        m_value{ value }
    {
    };

    int get_value() const noexcept
    {
        return m_value;
    }

private:
    int m_value;
};

class B
{
public:
    B(int value) :
        m_a{ value } // <-- this will pass value on to the constructor of A
    {
    };

    const A& get_A() const noexcept // not good design, for educational purposes only
    {
        return m_a;
    }

private:
    A m_a;
};


int main()
{
    B b{ 42 };

    std::cout << b.get_A().get_value();

    return 0;
}