Question

Example:

H

class MyClass {
    int x,y,z;
public:
    MyClass(int,int,int);
    void X();
    void Y();
    void Z();
};

CPP

class MyClass {
    int x,y,z;
public:
    MyClass(int x,int y,int z) {
        this->x=x;
        this->y=y;
        this->z=z;
    }
    void X() {
        printf("x = %d;\n",x);
    }
    void Y() {
        printf("y = %d;\n",y);
    }
    void Z() {
        printf("z = %d;\n",z);
    }
};

Make it C#-like.
Don’t include the header, re-declare the class in the CPP but with method-bodies.
When file include the header then he gets the extern fields\methods and etc from CPP.

It’s legal? I can’t predict problems from it. There is?

Answer 1

This falls into the realm of the One Definition Rule. In particular, a requirement that is put on multiple definitions across several TUs of a single program for the same class is:

[…] — each definition of D shall consist of the same sequence of tokens […]

(Paragraph 5 of 3.2 One definition rule [basic.def.odr])

So even if you ‘fixed’ the first version to declare the member functions inline to match the second version (where providing a definition of the members implicitly declares them inline) you would still run afoul of this rule: the function bodies are additional tokens that appear in the one but not in the other.