C++ standard 3.4.1:
.4:
A name used in global scope, outside of any function, class or
user-declared namespace, shall be declared before its use in global
scope.
This is why global variables and functions cannot be used before an afore declaration.
.5:
A name used in a user-declared namespace outside of the definition of
any function or class shall be declared before its use in that
namespace or before its use in a namespace enclosing its namespace.
same thing just written again as the .4 paragraph explictely restricted its saying to "global", this paragraph now says "by the way, its true as well in namespeces folks..."
.7:
A name used in the definition of a class X outside of a member
function body or nested class definition29 shall be declared in one of
the following ways: — before its use in class X or be a member of a
base class of X (10.2), or — if X is a nested class of class Y (9.7),
before the definition of X in Y, or shall be a member of a base class
of Y (this lookup applies in turn to Y ’s enclosing classes, starting
with the innermost enclosing class),30 or — if X is a local class
(9.8) or is a nested class of a local class, before the definition of
class X in a block enclosing the definition of class X, or — if X is a
member of namespace N, or is a nested class of a class that is a
member of N, or is a local class or a nested class within a local
class of a function that is a member of N, before the definition of
class X in namespace N or in one of N ’s enclosing namespaces.
I think this speaks of all the code that does not stand in cpu executed code (eg declarative code).
and finally the interesting part:
3.3.7 Class scope [basic.scope.class]
1 The following rules describe the scope of names declared in classes.
1) The potential scope of a
name declared in a class consists not only of the declarative region
following the name’s point of declaration, but also of all function
bodies, brace-or-equal-initializers of non-static data members, and
default arguments in that class (including such things in nested
classes).
2) A name N used in a class S shall refer to the same
declaration in its context and when re-evaluated in the completed
scope of S. No diagnostic is required for a violation of this rule.
3)
If reordering member declarations in a class yields an alternate valid
program under (1) and (2), the program is ill-formed, no diagnostic is
required.
particularly, by the last point they use a negative manner to define that "any ordering is possible" because if re-ordering would change lookup then there is a problem. its a negative way of saying "you can reorder anything and its ok, it doesnt change anything".
effectively saying, in a class, the declaration is looked-up in a two-phase compilation fashion.