Why can I use an as-of-yet undeclared member variable in a member function?

Question

For example:

struct X{
X():a{10} {}
void foo() { a = 10; }

private:
int a;
};

Why does this compile when variable a hasn't been declared yet?

Answer 1

The compiler basically does two passes over the class or structure definition. One for the structure/class to parse and handle declarations of members, then one pass for the inline functions.