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<TITLE>5: Hiding the Implementation</TITLE>
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Thinking in C++, 2nd ed. Volume 1</FONT></H2></FONT>
<H3><FONT FACE="Verdana">©2000 by Bruce Eckel</FONT></H3></FONT>
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</P></DIV><A NAME="_Toc472654835"></A><FONT FACE = "Verdana"><H1 ALIGN="LEFT">
5: Hiding the Implementation</H1></FONT>
<A NAME="Heading208"></A><FONT FACE = "Verdana"><H1 ALIGN="LEFT">
</H1></FONT>
<DIV ALIGN="LEFT"><P><FONT FACE="Verdana" SIZE=4>A typical C library contains a
<B>struct</B> and some </FONT><BR><FONT FACE="Verdana" SIZE=4>associated
functions to act on that <B>struct</B>. So far,
</FONT><BR><FONT FACE="Verdana" SIZE=4>you've seen how C++ takes functions that
are <I>conceptually</I> associated and makes them <I>literally</I> associated by
</FONT><BR></P></DIV>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">putting the function declarations inside
the scope of the <B>struct</B>, changing the way functions are called for the
<B>struct</B>, eliminating the passing of the structure address as the first
argument, and adding a new type name to the program (so you don’t have to
create a <B>typedef</B> for the <B>struct</B> tag).</FONT><BR></P></DIV>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">These are all convenient – they
help you organize your code and make it easier to write and read. However, there
are other important issues when making libraries easier in C++, especially the
issues of safety and control. This chapter looks at the subject of boundaries in
structures.</FONT><A NAME="_Toc312373837"></A><A NAME="_Toc472654836"></A><BR></P></DIV>
<A NAME="Heading209"></A><FONT FACE = "Verdana"><H2 ALIGN="LEFT">
Setting limits</H2></FONT>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">In any relationship it’s important
to have boundaries that are respected by all parties involved. When you create a
library, you establish a relationship with the <I>client
programmer<A NAME="Index1179"></A><A NAME="Index1180"></A></I> who uses that
library to build an application or another library.</FONT><BR></P></DIV>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">In a C
<A NAME="Index1181"></A><A NAME="Index1182"></A><B>struct</B>, as with most
things in C, there are no rules. Client programmers can do anything they want
with that <B>struct</B>, and there’s no way to force any particular
behaviors. For example, even though you saw in the last chapter the importance
of the functions named <B>initialize( )</B> and <B>cleanup( )</B>, the
client programmer has the option not to call those functions. (We’ll look
at a better approach in the next chapter.) And even though you would really
prefer that the client programmer not directly manipulate some of the members of
your <B>struct</B>, in C there’s no way to prevent it. Everything’s
naked to the world.</FONT><BR></P></DIV>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">There are two reasons for controlling
access
<A NAME="Index1183"></A><A NAME="Index1184"></A><A NAME="Index1185"></A><A NAME="Index1186"></A>to
members. The first is to keep the client programmer’s hands off tools they
shouldn’t touch, tools that are necessary for the internal machinations of
the data type, but not part of the interface the client programmer needs to
solve their particular problems. This is actually a service to client
programmers because they can easily see what’s important to them and what
they can ignore.</FONT><BR></P></DIV>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">The second reason for access control is
to allow the library designer to change the internal workings of the structure
without worrying about how it will affect the client programmer. In the
<B>Stack</B> example in the last chapter, you might want to allocate the storage
in big chunks, for speed, rather than creating new storage each time an element
is added. If the interface and implementation are clearly separated and
protected, you can accomplish this and require only a relink by the client
programmer.<A NAME="Index1187"></A><A NAME="Index1188"></A><A NAME="Index1189"></A></FONT><A NAME="_Toc312373838"></A><A NAME="_Toc472654837"></A><BR></P></DIV>
<A NAME="Heading210"></A><FONT FACE = "Verdana"><H2 ALIGN="LEFT">
C++ access control</H2></FONT>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">C++ introduces three new keywords to set
the boundaries in a structure: <B>public</B>, <B>private</B>, and
<B>protected</B>. Their use and meaning are remarkably straightforward. These
<I>access specifiers</I>
<A NAME="Index1190"></A><A NAME="Index1191"></A><A NAME="Index1192"></A>are used
only in a structure declaration, and they change the boundary for all the
declarations that follow them. Whenever you use an access specifier, it must be
followed by a colon.</FONT><BR></P></DIV>
<DIV ALIGN="LEFT"><P><A NAME="Index1193"></A><FONT FACE="Georgia"><B>public</B>
<A NAME="Index1194"></A>means all member declarations that follow are available
to everyone. <B>public</B> members are like <B>struct</B> members. For example,
the following <B>struct</B> declarations are identical:</FONT><BR></P></DIV>
<BLOCKQUOTE><FONT SIZE = "+1"><PRE><font color=#009900>//: C05:Public.cpp</font>
<font color=#009900>// Public is just like C's struct</font>
<font color=#0000ff>struct</font> A {
<font color=#0000ff>int</font> i;
<font color=#0000ff>char</font> j;
<font color=#0000ff>float</font> f;
<font color=#0000ff>void</font> func();
};
<font color=#0000ff>void</font> A::func() {}
<font color=#0000ff>struct</font> B {
<font color=#0000ff>public</font>:
<font color=#0000ff>int</font> i;
<font color=#0000ff>char</font> j;
<font color=#0000ff>float</font> f;
<font color=#0000ff>void</font> func();
};
<font color=#0000ff>void</font> B::func() {}
<font color=#0000ff>int</font> main() {
A a; B b;
a.i = b.i = 1;
a.j = b.j = 'c';
a.f = b.f = 3.14159;
a.func();
b.func();
} <font color=#009900>///:~</font></PRE></FONT></BLOCKQUOTE>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">The
<A NAME="Index1195"></A><B>private</B> <A NAME="Index1196"></A>keyword, on the
other hand, means that no one can access that member except you, the creator of
the type, inside function members of that type. <B>private</B> is a brick wall
between you and the client programmer; if someone tries to access a
<B>private</B> member, they’ll get a compile-time error. In <B>struct
B</B> in the example above, you may want to make portions of the representation
(that is, the data members) hidden, accessible only to you:</FONT><BR></P></DIV>
<BLOCKQUOTE><FONT SIZE = "+1"><PRE><font color=#009900>//: C05:Private.cpp</font>
<font color=#009900>// Setting the boundary</font>
<font color=#0000ff>struct</font> B {
<font color=#0000ff>private</font>:
<font color=#0000ff>char</font> j;
<font color=#0000ff>float</font> f;
<font color=#0000ff>public</font>:
<font color=#0000ff>int</font> i;
<font color=#0000ff>void</font> func();
};
<font color=#0000ff>void</font> B::func() {
i = 0;
j = '0';
f = 0.0;
};
<font color=#0000ff>int</font> main() {
B b;
b.i = 1; <font color=#009900>// OK, public</font>
<font color=#009900>//! b.j = '1'; // Illegal, private</font>
<font color=#009900>//! b.f = 1.0; // Illegal, private</font>
} <font color=#009900>///:~</font></PRE></FONT></BLOCKQUOTE>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">Although <B>func( )</B> can access
any member of <B>B</B> (because <B>func( )</B> is a member of <B>B</B>,
thus automatically granting it permission), an ordinary global function like
<B>main( )</B> cannot. Of course, neither can member functions of other
structures. Only the functions that are clearly stated in the structure
declaration (the “contract”) can have access to <B>private</B>
members.</FONT><BR></P></DIV>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">There is no required order for access
specifiers<A NAME="Index1197"></A><A NAME="Index1198"></A><A NAME="Index1199"></A>,
and they may appear more than once. They affect all the members declared after
them and before the next access
specifier.</FONT><A NAME="_Toc312373839"></A><A NAME="_Toc472654838"></A><BR></P></DIV>
<A NAME="Heading211"></A><FONT FACE = "Verdana"><H3 ALIGN="LEFT">
protected</H3></FONT>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">The last access specifier is
<A NAME="Index1200"></A><B>protected<A NAME="Index1201"></A></B>.
<B>protected</B> acts just like <B>private</B>, with one exception that we
can’t really talk about right now: “Inherited” structures
(which cannot access <B>private </B>members) are granted access to
<B>protected</B> members. This will become clearer in Chapter 14 when
inheritance is introduced. For current purposes, consider <B>protected</B> to
be just like
<B>private</B>.</FONT><A NAME="_Toc312373840"></A><A NAME="_Toc472654839"></A><BR></P></DIV>
<A NAME="Heading212"></A><FONT FACE = "Verdana"><H2 ALIGN="LEFT">
Friends</H2></FONT>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">What if you want to explicitly grant
access to a function that isn’t a member of the current structure? This is
accomplished by declaring that function a <A NAME="Index1202"></A><B>friend</B>
<A NAME="Index1203"></A><I>inside</I> the structure declaration. It’s
important that the <B>friend</B> declaration occurs inside the structure
declaration because you (and the compiler) must be able to read the structure
declaration and see every rule about the size and behavior of that data type.
And a very important rule in any relationship is, “Who can access my
private implementation?”</FONT><BR></P></DIV>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">The class controls which code has access
to its members. There’s no magic way to “break in” from the
outside if you aren’t a <B>friend</B>; you can’t declare a new class
and say, “Hi, I’m a friend of <B>Bob</B>!” and expect to see
the <B>private</B> and <B>protected</B> members of <B>Bob</B>.</FONT><BR></P></DIV>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">You can declare a global function as a
<B>friend<A NAME="Index1204"></A><A NAME="Index1205"></A><A NAME="Index1206"></A></B>,
and you can also declare a member function of another
structure<A NAME="Index1207"></A><A NAME="Index1208"></A><A NAME="Index1209"></A>,
or even an entire structure, as a <B>friend</B>. Here’s an example
<A NAME="Index1210"></A><A NAME="Index1211"></A>:</FONT><BR></P></DIV>
<BLOCKQUOTE><FONT SIZE = "+1"><PRE><font color=#009900>//: C05:Friend.cpp</font>
<font color=#009900>// Friend allows special access</font>
<font color=#009900>// Declaration (incomplete type specification):</font>
<font color=#0000ff>struct</font> X;
<font color=#0000ff>struct</font> Y {
<font color=#0000ff>void</font> f(X*);
};
<font color=#0000ff>struct</font> X { <font color=#009900>// Definition</font>
<font color=#0000ff>private</font>:
<font color=#0000ff>int</font> i;
<font color=#0000ff>public</font>:
<font color=#0000ff>void</font> initialize();
<font color=#0000ff>friend</font> <font color=#0000ff>void</font> g(X*, <font color=#0000ff>int</font>); <font color=#009900>// Global friend</font>
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