Passing class to constructor, when no such constructor exists. Why does it work? [duplicate]

Question

I'm having a class that looks like:

class A {
public:
    A(float v) 
    { 
        A::v = v; 
    }
    float v;
    float set(float v) 
    {
        A::v = v;
        return v;
    }
    float get(float v) 
    {
        return A::v;
    }
};

Then I instantiate 2 objects of class A:

A* a = new A(1.0);
A* b = new A(*a);

Why are there no errors when my Class A doesn't have a constructor which takes a class A?

Answer 1

The compiler generates a copy constructor for you:

If no user-defined copy constructors are provided for a class type (struct, class, or union), the compiler will always declare a copy constructor as a non-explicit inline public member of its class.

You can make the copy constructor and assignment deleted and make the compiler not declare move assignment and constructor by declaring one of move constructor or assignment as deleted:

A(A&&) = delete; // Makes the class non-copyable and non-moveable.