Sams Teach Yourself C++ in 21 Days

x 20 y 20

Note that class Paneon lines 7–19 is nested inside the namespace Window, which

is on lines 3–20. This is the reason you have to qualify the name Panewith the

Window::qualifier.

The static variable count, which is declared in Paneon line 16, is defined as usual.

Within the function Pane::size()on lines 26–32, notice that MAX_Xand MAX_Yare fully

qualified. This is because Paneis in scope; otherwise, the compiler issues an error diag-

nostic. This also holds true for the function Pane::move().

Also interesting is the qualification of Pane::xand Pane::yinside both function defini-

tions. Why is this needed? Well, if the function Pane::move()were written like this, you

would have a problem:

void Window::Pane::move( int x, int y )

{

if( x < Window::MAX_X && x > 0 )

x = x ;

if( y < Window::MAX_Y && y > 0 )

y = y ;

Platform::move( x, y ) ;

}

Can you spot the issue? You probably won’t get much of an answer from your compiler;

some don’t issue any kind of diagnostic message at all.

The source of the problem is the function’s arguments. Arguments xand yhide the pri-

vate xand yinstance variables declared within class Pane. Effectively, the statements

assign both xand yto itself:

x = x ;

y = y ;

The usingKeyword ............................................................................................

Theusingkeyword is used for both the usingdirective and the usingdeclaration.

The syntax of the usingkeyword determines whether the context is a directive or a

declaration.

The usingDirective........................................................................................

Theusingdirective effectively exposes all names declared in a namespace to be in the

current scope. You can refer to the names without qualifying them with their respective

namespace name. The following example shows the usingdirective:

648 Day 18

OUTPUT

ANALYSIS