Why don't methods defined in classes in C++ need forward declarations when referring to methods that only show up later in the class?

Question

Why is this a compilation error:

void f() {
    g();
}

void g() {

}

but not this:

class X {
    void a() {
        b();
    }

    void b() {
        
    }
};

I was under the assumption the compiler would (at least, generally) read code from top-down, left-to-right, and that was the reason that in the 1st piece of code we'd need to define a void g() forward declaration before f() to make the code compile. But that logic doesn't seem to apply to classes -- why?

Answer 1

C dates from 1972; C++ dates from 1985. C++ inherited the forward declaration requirement and the design of header files from C, and kept that behavior for compatibility, but was able to improve things for classes, with C didn't have.