Question

Following code does compile whereas name “aNumber” is not declared before use.

class A
{
    A()
        :aNumber(100)
    {
    }
    void foo()
    {
         aNumber = 0;

    }
    int aNumber;
};

If above code compiles then why not following :-

A.
class Dummy
{
    void foo(INT);
    typedef int INT;
};

B.Default initialization by member variable :-

class Dummy
{
    void foo(int y = x);
    int x;
};

Answer 1

You’re comparing apples and oranges. The following code is OK:

class A {
    A() :aNumber(100) { INT bNumber = aNumber; }
    void foo() { aNumber = INT(42); }
    void bar(int bNumber = INT(1)) { aNumber = bNumber; }
    int aNumber;
    typedef int INT;
};

The problem is not what you declare, but where the declarations are used. Method definitions, even it they appear lexically inside a class, are compiled as if they are declared inside the class, but defined outside and after the class declaration, and so are default arguments.